All posts by Pale Red Dot

Living in Twilight: An Overview of our Closest and Smallest Stellar Neighbors

by Sergio Dieterich, Carnegie Institution for Science

When members of our research group go observing at Cerro Tololo Inter-American Observatory in the Chilean Andes we spend most of our time in a cozy heated control room. Modern astronomical observing is done mostly by monitoring computer screens and entering commands to tell the telescope where to point next. If we have to put on our winter jackets and climb the flight of stairs to where the telescope is—under the open dome—it is because something went wrong and we are frantically trying to fix the problem and minimize the loss of precious telescope time. There is one exception. Our group’s tradition dictates that when we are training a new student, and the season and time of night is just right, we will go up to the dome and have our new colleague look through the telescope’s eyepiece. Photography does not do justice to the sight that emerges: a bright ruby red speck of light floats seemingly in front of a vast ocean of fainter and whiter stars. That red speck is Proxima Centauri, the closest star to us other than the Sun, the subject of the Pale Red Dot project, and a typical low mass star. Stars like Proxima Centauri, or just Proxima for short, are amongst the smallest but also the most common types of stars in the Galaxy. Let’s take a few minutes to understand our smallest and closest stellar neighbors a little better.

Imagine for a moment that we drop a large ceramic dinner plate on a hard kitchen floor. The plate shatters into many, many, pieces, of all different sizes. We then look down and examine the results of our carelessness. Our attention is first drawn to the handful of large fragments. After a more careful look we see that for every one of those large ceramic fragments there are dozens, if not hundreds, of much smaller pieces. Further, we soon realize that if we have any hope of reconstructing the original plate or figuring out what happened we cannot simply ignore those smaller pieces and sweep them under the rug. This unfortunate kitchen accident is a rough analogy to the stellar formation process, and it sheds some light on how the Milky Way Galaxy ended up with the stellar population we observe today. Stars are formed when clouds of interstellar gas and dust, called giant molecular clouds, are somehow perturbed—causing the cloud to start collapsing under its own gravitational pull. Several points in the collapsing cloud achieve higher and higher density, and therefore exert an even greater gravitational force. Over the course of hundreds of thousands of years these high density regions consume enough gas and become compact enough to form stellar embryos, or protostars. When the protostar’s core becomes hot enough to ignite and sustain nuclear fusion, a star is born. In a manner similar to what happens with our shattering plate, but for different physical reasons, the result of this cloud collapse mechanism heavily favors the production of stars whose masses are anywhere from about 60% to about only 8% of our Sun’s mass. When fully formed and contracted these are tiny stars, with the majority having radii between 20% and only 10% our Sun’s radius. The smallest are very close in size (but not in mass or density!) to the planet Jupiter. What these small stars lack in terms of size they make up for in their sheer numbers. Indeed, out of the 366 stars whose accurately measured distances place them within 32.6 light-years (or 10 parsecs, in astronomical lingo) of our Solar System, 275 belong to this type. These objects are commonly known as red dwarfs, or M dwarfs, in the stellar classification system used by professional astronomers. Using the fair assumption that our solar neighborhood is typical of much of the Milky Way Galaxy, that means that about 75% of the stars in our galaxy are M dwarfs. The M dwarf class is sometimes subdivided, with stars having about 20% or less the mass of our Sun being called Very Low Mass, or VLM stars. Proxima is in the upper mass range of the VLM stars.

Figure 1: A graphical representation of all known stars within 32.6 light-years (10 parsecs) of Earth. Stars of each category in the stellar classification system are represented by filled circles with sizes proportional to the star's size and colors that approximate their true colors. The Sun, a G type star, is represented by one of the yellow circles. The M dwarfs are themselves subdivided into two hues of red and 3 different sizes to represent the diversity within the M class. M dwarfs vastly outnumber all other types. The very small dots at the center represent stellar remnants that have exhausted their nuclear fuel and are called white dwarfs. The 8 planets of the Solar System are also plotted for size comparison, with Mercury and Mars too small to be noticeable. Updated counts are available at www.recons.org. Courtesy of Todd J. Henry / RECONS.
Figure 1: A graphical representation of all known stars within 32.6 light-years (10 parsecs) of Earth. Stars of each category in the stellar classification system are represented by filled circles with sizes proportional to the star’s size and colors that approximate their true colors. The Sun, a G type star, is represented by one of the yellow circles. The M dwarfs are themselves subdivided into two hues of red and 3 different sizes to represent the diversity within the M class. M dwarfs vastly outnumber all other types. The very small dots at the center represent stellar remnants that have exhausted their nuclear fuel and are called white dwarfs. The 8 planets of the Solar System are also plotted for size comparison, with Mercury and Mars too small to be noticeable. Updated counts are available at www.recons.org. Courtesy of Todd J. Henry / RECONS.

What are red dwarfs like as stars, and how does their energy output compare to our Sun’s? These stars are incredibly faint, and not even Proxima can be seen with the naked eye despite its proximity of only 4.25 light-years. To put this distance in context, the best estimates for the diameter of the Milky Way Galaxy place it at anywhere between 100,000 to 180,000 light-years; if our galaxy were a city 10 km across Proxima would be so close to us as to be knocking on our front door! And yet stars that are intrinsically more luminous can be seen with the naked eye from distances almost one fifth of the way across the galaxy. If a representative sample of red dwarfs were all placed at the same distance to us as the Sun the brightest ones would shine only about 7 percent as bright as the Sun. Recent research by our group indicates that the faintest of the VLM stars would shine with only about 0.016 percent, or about 1/6,000th , the brightness of our Sun. Proxima has a total energy output about 0.2% that of our Sun.

Red dwarfs are not only faint, but the little light they do emit is also very different from the warm sunlight we enjoy on a Caribbean beach on Earth. The surface of our Sun shines at a temperature of approximately 5,500 degrees Celsius (10,000 F). At that temperature most of the light is emitted in the yellow-green region of the visible light spectrum. It therefore makes sense that the human eye has evolved to be the most sensitive to the yellow-green light that most strongly bathes our planet. Low mass stars have significantly cooler surface temperatures: about 3,500 C (6,400 F) for the hottest red dwarfs and approximately 1,800 C (3,300 F) for the smallest and faintest VLM stars. At these temperatures not only does the star emit considerably less light overall, but the light emitted is also shifted to longer wavelengths, which we perceive as redder colors. The color spectrum of the hottest red dwarfs has its peak at a deep red color that is just at the limit of the detection range of the human eye. For the faintest VLM stars the color spectrum peaks in the near infrared range of the electromagnetic spectrum, well beyond the detection capabilities of the human eye. In both cases the human eye’s enhanced sensitivity to yellow-green light will shift the perceived colors to shorter wavelengths than the peak color emission. A future interstellar voyager who sees a hot red dwarf up close will likely perceive a distinctive orange hue, whereas one of the cooler red dwarfs may appear to be a lively red (Figure 2ab). To make these faintest of faint stars even more unusual, there is evidence to suggest that they have strong surface magnetic fields. These magnetic fields would cause dark spots analogous to sunspots, but they may be more numerous and larger—perhaps covering a substantial portion of the star’s surface.

Figure 2: Artist's conception of a red dwarf star as seen from close proximity. It is thought that the hotter red dwarfs may actually look more orange than red due to the human eye's enhanced sensitivity to yellow light (a), whereas the cooler red dwarfs most likely would appear bright red. Figure credit: Walt Feimer/NASA.
Figure 2: Artist’s conception of a red dwarf star as seen from close proximity. It is thought that the hotter red dwarfs may actually look more orange than red due to the human eye’s enhanced sensitivity to yellow light (a), whereas the cooler red dwarfs most likely would appear bright red. Figure credit: Walt Feimer/NASA.

Astronomers currently think that as many as 1/3 of red dwarfs may harbor rocky planets with compositions similar to Earth’s. Could life evolve on these planets, and what would life around a red dwarf be like? The idea of life evolving on planets around red dwarfs is extremely exciting. If for no other reason, their sheer numbers means that the question of red dwarf habitability has tremendous implications in determining whether we live in a Universe teeming with life or whether life is a sparse occurrence. Despite this huge potential, the notion of life on low mass star systems is not without its challenges.

Because of their lower mass and consequentially weaker gravitational pull, red dwarfs take a very long time to settle into their fully contracted configuration, once they stop accreting material from their parent star forming cloud. Similarly, the comparatively slow rate of nuclear reactions in a low mass star’s core causes these stars to have extremely long lives when compared to more massive stars. Their slow evolution and long lives are both a blessing and a curse for the possibility of life. Once fully formed and contracted, red dwarfs change very little for hundreds of billions of years. The oldest red dwarfs may therefore have provided a stable environment for life for as long as they have existed, roughly 10 billion years based on current estimates for the age of the Galaxy. Compare that with only 4.1 billion years of biological evolution on Earth. Even if evolution around a planet hosting red dwarf happened slower and hit a few dead ends, the final result might still mean a complex and diverse ecosystem. However, the prospect of a prolonged period of stability suitable for biological evolution is only exciting if we assume that the right conditions for life were present to begin with, and that is where a red dwarf’s life in the slow lane becomes a problem. Liquid water is essential for life as we know it on Earth, and liquid water can only exist if the temperature on a planet’s surface allows it. A planet’s temperature is governed primarily by the planet’s orbital distance from its parent star and the star’s intrinsic luminosity. Astronomers call the range of orbital radii allowing the existence of liquid water the ‘habitable zone’ around a star. All stars are significantly brighter during their initial contraction phase, when most of the star’s energy comes from its gravitational collapse and not from nuclear fusion. For red dwarfs this initial period of increased luminosity may last up to 3 billion years, which is well beyond the formation time for planets. Any planet that forms in what will eventually become the star’s habitable zone will be subject to scorching heat during its early life. Calculations suggest that this fiery youth may cause all water to evaporate away, thus effectively sterilizing the planet. A possible way out of this scenario involves the retention of water in minerals called chondrites. If chondrites are present in sufficient amounts in the rocky material that coalesces to form planets, the fully formed planets could have substantial water reserves in their interiors. The water could then be released from the planet’s interior by volcanic activity at later times when the surface temperature is right for liquid water. Whether or not this scenario is likely is an area of active research.

Another interesting aspect of the idea of life in planets orbiting red dwarfs has to do with the extreme proximity of the star’s habitable zone to the star itself. These stars are so faint that planets in their habitable zones would have orbits smaller than the orbit of Mercury in our Solar System. At such small distances the slight difference in the star’s gravitational pull from the planet’s side facing the star to the planet’s far side causes a phenomenon called tidal locking. In a tidally locked planet the same side of the planet always faces the star, causing it to be much hotter than the side that is perpetually facing away from the star. The Earth-Moon system is a good example of a tidally locked satellite. The habitable conditions in a tidally locked planet may be confined to a narrow ring shaped region where the illuminated side meets the dark side of the planet. This habitable region would be in perpetual twilight, with the star shining low in the horizon. Such low illumination conditions may seem rather depressing to us humans, but low light levels peaking at redder wavelengths are the norm around red dwarfs, and it is quite possible that any existing life form in these otherworldly environments may have evolved to use infrared light in much the same way we utilize the bright yellow-green light of our parent star. Perhaps venturing too close to the planet’s illuminated side would cause these creatures to get a “star burn” from red light in much the same way we get can get a sunburn from the small portion of our Sun’s energy that is emitted as ultraviolet light.

Finally, a treatment of low mass stars would not be complete without making a connection to their lower mass cousins, the substellar brown dwarfs. Looking back to our shattered plate analogy of star formation, the cloud collapse process that produces stars with a wide range of different masses can also produce objects whose mass is too small to create the conditions necessary for sustainable core nuclear fusion. These objects are called brown dwarfs. Brown dwarfs look much like their VLM star counterparts in their youth because during that phase gravitational contraction releases a large amount of energy for both stars and brown dwarfs. However, once brown dwarfs are fully contracted they keep cooling down over the course of billions of years. For much of the red dwarf range of temperatures and colors it is difficult to tell whether a given object is a young brown dwarf or a VLM star of any age. Recent research by my collaborators and I indicates that the stellar sequence comes to an end when we reach objects with surface temperatures of about 1,800 C (3,300 F) and luminosities of roughly 1/6,000th that of our Sun (interested in the technical details? read the paper here). We came to this conclusion by performing the observations necessary to estimate the radius of a sample of 63 objects thought to lie close to the end of the stellar sequence. We then noted that for temperatures higher than 1,800 C the objects cover a wide range of radii, including the radii expected for old and fully contracted stars. At cooler temperatures we encountered larger radii that can only be explained if the objects in question are young brown dwarfs that are not yet fully contracted (Figure 3).

Figure 3: Temperature-Radius diagram for VLM stars. In keeping with astronomical tradition, the temperature axis is plotted with decreasing values from left to right. The star 2MASS J0523-1403 marks the temperature at which the minimum radius is reached, meaning that objects like 2MASS J0523-1403 are old and fully contracted. At cooler temperatures only larger objects are present, indicating that they are relatively young and not yet fully contracted brown dwarfs. A few objects are marked as unresolved binaries, meaning that we are seeing the light from two closely spaced stars and therefore the radius we calculate for those stars is.
Figure 3: Temperature-Radius diagram for VLM stars. In keeping with astronomical tradition, the temperature axis is plotted with decreasing values from left to right. The star 2MASS J0523-1403 marks the temperature at which the minimum radius is reached, meaning that objects like 2MASS J0523-1403 are old and fully contracted. At cooler temperatures only larger objects are present, indicating that they are relatively young and not yet fully contracted brown dwarfs. A few objects are marked as unresolved binaries, meaning that we are seeing the light from two closely spaced stars and therefore the radius we calculate for those stars is inaccurate.

The temperature we obtained for the end of the stellar sequence is substantially higher than that predicted by theoretical models, and we are now trying to pinpoint the root causes of this discrepancy. As a part of this research we have found what we believe to be the smallest known star to date and also a representative of the smallest possible stars. This star is called 2MASS J0523-1403, and shines faintly in the constellation Lepus the hare, under the feet of Orion the hunter. (Figure 4). 2MASS J0523-1403 has a radius of only percent the radius of our Sun. That radius makes 2MASS J0523-1403 about 15 percent smaller than the planet Jupiter. Indeed, perhaps coincidentally, the size we calculate for 2MASS J05234-1403 is within 1 percent of the size of the planet Saturn. Therefore while we can say that VLM stars in general have sizes comparable to Jupiter, we can go one step further and say that the smallest stars are Saturn sized. In making these comparisons we must be careful not to confuse volume and mass. While these stars have the volume of giant planets their mass is theoretically predicted to be anywhere from 70 to 80 times the mass of Jupiter, making them incredibly dense. In fact, it is the quantum mechanical limit on the allowed upper density that causes brown dwarfs to stop contracting before nuclear fusion ignites.

Figure 4: The smallest known star, 2MASS J0523-1403, as seen with the Cerro Tololo 0.9 meter telescope is shown using a color scheme that approximates its true color.
Figure 4: The smallest known star, 2MASS J0523-1403, as seen with the Cerro Tololo 0.9 meter telescope, is shown using a color scheme that approximates its true color.

Over the last few decades our knowledge of red dwarfs has gone from simply knowing that they exist, to realizing just how numerous they are, and finally to being able to characterize them and assess their suitability as hosts for habitable planets. This progress is in part due to advances in observational astronomy, such as the substitution of blue sensitive photographic film to red sensitive digital CCD detectors and infrared detectors. Those advances in sensitivity and data management were then utilized to conduct large all-sky surveys that revealed a multitude of new red dwarfs and gave astronomers the unprecedented ability to study them not only as individual objects but also as a population. We now have a good understanding of how red dwarfs contribute to the overall stellar population of the Galaxy and are gaining greater understanding of their promises and challenges as hosts of livable planets. The history of astronomy has taught us that we cannot predict what the next discovery will be and how it will change our understanding of things. It could well be that after thorough study we may realize that the roughly 75 percent of the stars in the Galaxy that we call red dwarfs are not suitable as hosts of living planets. That alone would let us know that life in the Universe might be a bit more special than previously thought and how fortunate we are to have a home on planet Earth. On the opposing view, we know from our experience on Earth that evolution usually finds a way to make life flourish in the most extreme and odd environments. If life forming mechanisms are able to overcome the challenges we discussed here, plus many others that we have not yet even imagined, it is quite possible that our solar neighborhood abounds with beings of unimaginable forms thriving under the soft red twilight of their tiny parent star.

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About the author. Sergio (Serge) Dieterich is an observational astronomer who studies the properties of the smallest stars in the solar neighborhood, and the differences and similarities between these stars and their lower mass substellar cousins, the brown dwarfs. He is particularly interested in how stellar structure, and evolution processes happening deep within the core of a star or brown dwarf, are related to the colors and spectroscopic features of the surface of the star, which is the only part probed by telescope observations. Serge also specializes in the technique of astrometry, which measures minute changes in the relative position of a star in the sky to determine the star’s distance from Earth, as well as any orbital motion the star may have about an unseen companion. Serge was born in Porto Alegre, Brazil, and moved to Miami, Florida, just before starting high school. He has a B.A. in physics from Johns Hopkins University, an M.S. in physics from Georgia State University, and recently obtained his Ph.D. in astronomy also from Georgia State. After college and before starting graduate school Serge taught high school physics and middle school physical science for two years. He currently holds a National Science Foundation postdoctoral fellowship at the Department of Terrestrial Magnetism from the Carnegie Institution for Science. In addition to cutting edge research, he is also developing contents for high school level students about astronomy and stars.

How a Star Can Hide its Earths

Autor: Xavier Dumusque, Observatory of Geneva

Let’s consider that we want to find a planet extremely similar to Earth, meaning that it’s orbiting a star similar to the Sun, with a mass equal to Earth and an orbital period of one year. Let’s also consider that we want to detect this object by measuring the gravitational effect it induces on its host star. I know what you are telling yourself; “No way, the Sun is so massive it doesn’t move!”. You are right, the Sun is extremely massive, in fact 300,000 times more than the Earth, but let’s stick to physics here. The laws of gravity (thank you Newton!) tell us that all object with mass will interact with each other, so the Sun should move. But the question is by how much? Plugging the numbers in, we arrive at a maximum displacement of the Sun’s center of 500 km over a 6-month period. This means that its displacement will be 1,500 times smaller than its radius, and that the maximum velocity the Sun will reach will be only 0.3 km/h (or the velocity of turtle going out for a walk). I agree with you that this is extremely small, but still, it moves!

Displacement of the Sun’s center induced by the gravitational pull of all the planets in the Solar System as a function of time. The Sun moves by about a solar radius, and  the major contributor to this displacement is the most massive planet of the Solar System, Jupiter (CC, Carl Smith’s derivative work)

Let’s now imagine that we build the perfect instrument to measure the tiny effect induced by an Earth twin on its host star. This instrument should therefore be capable of detecting a velocity of 0.3 km/h on a star that is a hundred thousand billion kilometers away. I do not want to go into the details here, but let’s use an analogy to get a feeling of the difficulty we are facing. Imagine that this perfect instrument is a ruler, and you want to measure the width of an object with the same precision that is required to detect an Earth twin. The required precision is 10,000 times smaller than the smallest graduation of the ruler. Not so easy right? With good eyes, you might probably get down to 1/3rd or 1/4th of the graduation, but 1 in 10,000! Today, the best instruments we use are capable of a precision of 1 in 1,000 (see HARPS and HARPS-N). We are therefore capable of detecting a planet ten times more massive than Earth if the host star is similar to the Sun and if its orbital period is one year. At the University of Geneva, where I am working, scientists are now developing a new instrument, called ESPRESSO, that will have the precision required to detect Earth twins.

Let’s now imagine that one year from now ESPRESSO can be used (this is the real timeline), and we start observing several stars to search for Earth-twins. To be confident in a detection, we need to observe at least one full orbital period of a planet, or one year in this case. If these Earth-twins exist, and we are confident there should be a lot of them out there, we should detect a Holy Grail planet before 2020. But—wait a minute!—several things can go wrong here, and I want to just highlight the biggest problem we have nowadays. This big problem is the stars.

Let me try to explain to you how stars can mess everything up. Everything starts with the Doppler effect. A fancy name that physicist like to use, but if you didn’t study physics, you probably do not know what this means, or you heard the name in high school and now it’s forgotten forever. But most of you have already encountered the Doppler effect in real life. One day you were probably walking down the street, when suddenly an ambulance passed by. You could hear the vehicle from far away with its strident siren—you focused on the sound created and could easily hear the pitch of the siren, but once the ambulance passed by you, the pitch changed. Did the driver push a button at the same moment he was passing by you? Probably not. To be sure, you asked other people in the street if they also had the same impression (well, in real life people would have wondered, “who is this weirdo?”, but this is a mental experiment; you can be as weird as you want). And yes, they all confirmed that this phenomenon happened at the moment the ambulance passed them by—confirming that the driver was not playing around. What happened is simply that before overtaking you, the ambulance was moving toward you; whilst after, it was moving away from you. And because sound-waves progress through the air with a limited speed, the difference between the velocity of the ambulance before and after passing by you creates this difference in tonality.

The radial velocity method to detect exoplanet is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. When the star moves towards us, its spectrum is blueshifted, while it is redshifted when it moves away from us. By regularly looking at the spectrum of a star - and so, measure its velocity - one can see if it moves periodically due to the influence of a companion. Image credits : ESO
The radial velocity method to detect exoplanet is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. When the star moves towards us, its spectrum is blueshifted, while it is redshifted when it moves away from us. By regularly looking at the spectrum of a star – and so, measure its velocity – one can see if it moves periodically due to the influence of a companion. Image credits : ESO

Now you know what the Doppler effect is, but what does the ambulance have to do with what we are speaking about here—stars and planets? Well, stars emit light, and because light also has a limited speed (thanks Albert Einstein!), a similar effect will occur. Without entering into too many details, objects emitting light that are moving towards you will appear bluer (or blue-shifted), and objects moving away from you will appear redder (or red-shifted). This Doppler effect is at the origin of the radial-velocity technique used to detect planets. If a star is moving towards you, then away, and continues to do so in a periodic way, this motion is most probably induced by a planet orbiting the star. Another famous example of the use of the Doppler effect in astrophysics is the measurement of the Universe’s expansion. Looking at all the galaxies surrounding us in the Universe, we observe that their light is redder than it should be, therefore all the galaxies in the Universe are moving away from each other; the conclusion being that the Universe is expanding.

I told you that the biggest obstacle to the detection of Earth-twins is the host stars themselves. So let’s come back to this problem. Stars are formed by the contraction of giant molecular clouds, therefore by applying the concept of momentum conservation, you arrive at the conclusion that the stars are rotating around their center, like an ice-skater bringing his arms towards his chest to accelerate his spin. Given that the Sun has a 25 day rotation period and a radius of about 500,000 km, you can do the math and calculate that the rotation velocity of the Sun at its surface is 7,200 km/h. Therefore, looking in detail at the Sun, you will see that the light coming from the approaching limb is bluer than it should be, and the light from the receding limb is redder; do you remember the Doppler effect? So, does this mean that we see half Sun moving forwards and half moving backwards because of the rotation? Yes, it’s exactly what it means, but as the blue and the red shifts are equivalent, the average velocity is zero. This makes sense as the Sun only rotates around its axis, and does not move towards or away from you.

Now, you probably know that the Sun often has dark spots on its surface, so-called sunspots. These sunspots are caused by strong magnetic fields present inside the Sun, that sometimes emerge at the surface. Because they are dark, sunspots can be seen as a mask occulting, or blocking, part of the stellar disc. Therefore, they distort the red- and blue-shift balance; the Sun will appear a little redder (or bluer) and you could mistakenly conclude that it is moving. Considering a large spot on the Sun, that is around 0.1% of the surface area, and a maximum rotational velocity of 7,200 km/h, we arrive at the conclusion that such a sunspot induces a radial velocity effect of 7.2 km/h, which is an order of magnitude larger than the 0.3 km/h required to detect Earth-twins.

Doppler velocity map of the Sun as observed by the MDI instrument on board the SOHO satellite (left image). A black dot was introduced to simulate a sunspot as observed in the solar surface (see  a real sunspot observed by SOHO on the right). The Sun’s rotation produces equivalent blue- and red-shifted hemispheres, but this balance can be broken by a sunspot. On the left image for example, the black dot masks part of the blue shift of the star, so the final flux of the whole star will appear redder than it should be (Credit: SOHO/MDI).

In conclusion, even with an instrument reaching the precision required to detect Earth-twins, perturbing signals induced by stars, such as the effect of sunspots, will significantly complicate their detection. We have been aware of the sunspot problem for nearly 20 years now, and have discovered other stellar effects more recently. Many scientists are trying to understand better these perturbations, and are looking into new techniques to correct for them. I am one of them, and I am convinced that we will manage to solve this critical problem of stellar signals in the coming years.

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Dr. Xavier Dumusque.

About the author.

Dr. Xavier Dumusque’s expertise is planet detection taking into account stellar intrinsic signals. Xavier studied Astrophysics at the University of Geneva where he also obtained his PhD in 2012, in collaboration with the University of Porto. After two postdocs at the Harvard-Smithsonian Center for Astrophysics (USA), he came back to the Observatory of Geneva where he is currently working. He is the first author of the Nature article announcing an Earth mass planet orbiting Alpha Centauri B (2012) and of an article presenting the discovery of the Mega-Earth planet orbiting Kepler-10 (2014). Xavier is actively involved in the development of a solar telescope that will help characterize and understand the origin of perturbing signals in the Sun to develop new state-of-the-art techniques to mitigate their impact on the detectability of Earth-twins orbiting other stars. Among the awards he has obtained we highlight the Schläfli Prize for outstanding thesis (Swiss Academy of Science, 2014), the Yale Center for Astronomy and Astrophysics Postdoctoral Prize fellowship (2015), and the Branco Weiss fellowship (2015).

The Doppler Method and Proxima Centauri

by Hugh R. A. Jones, Centre for Astrophysical Research at University of Hertfordshire

Proxima Centauri is the closest star to the Sun, hence its name. Nonetheless, gravitationally speaking, it belongs to a triple system in which Proxima Centauri orbits a central binary system at a distance of 10 thousand times the Earth–Sun distance. The members of the inner binary are designated as Alpha Centauri A and B. They orbit one another with a distance 20 times that of the Earth–Sun distance. Both Alpha Centauri A and B are rather similar to our Sun. The components of a multiple star system are named by adding uppercase letters to the name of the star. Alpha Centauri A is the brightest component, Alpha Centauri B is the slightly fainter second star and Alpha Centauri C is the much fainter Proxima Centauri. Currently Alpha Centauri A and B are rather close together on the sky and present as the 2nd brightest night-time object in the Southern sky—after Canopus which is a hotter, more distant star. Proxima Centauri was not discovered until 1915 in part because its luminosity is only 0.1% that of the Sun. Despite being next door (astronomically speaking) it was not easily spotted near its brighter neighbours. Naturally people have long speculated the possibilities for the closest possible places for life beyond the Solar System. In 2012 a radial velocity or Doppler wobble search of Alpha Centauri B revealed the signal of an Earth-mass planet in a three day orbit. However, several subsequent studies have analysed exactly the same data and not confirmed the claimed signal. The 2012 discovery depended on a model accounting for the activity of Alpha Centauri B just as one might have to account for the effects of solar rotation, activity and the sunspot cycle if one were interested in detecting the Earth next to the Sun. A number of scientists have been working hard on accounting for the cycles of stellar activity but so far the evidence for an Earth-mass planet around Alpha Centauri B is unconfirmed.

One might ask why we and the 2012 study of Alpha Centauri B use the Doppler wobble technique at all when so many planets have been found by the Kepler space telescope. It is important to realize that detection by Kepler requires that the planet blocks the light from the star so while transit surveys have been hugely successful they can only deliver objects around the small fraction of stars whose alignment happens to give rise to a transit signal. On the other hand Doppler wobble or radial velocity signals can potentially be discerned for all stars with planets, unless they have face-on orbits. In principle, Proxima Centauri presents a better opportunity to search for planets because its mass and radius are only 10% that of Alpha Centauri B. The lower mass of Proxima Centauri means that the same mass planet in orbit could be proportionally easier to spot. Of course this will depend on the details! For our search, we measure the Doppler wobble induced by the planet on its host star through their gravitational pull on each other. In the case of Alpha Centauri B the claimed signal was 51 centimetres per second (1.8 km/hr), about the speed of a baby crawling and corresponding to a mass close to that of the Earth. However, if we found such a signal around the much lighter star Proxima Centauri it would correspond to an even lighter planet.

Example of measurements (in red) overplotted on the expected Doppler signal caused by an exoplanet on the Star. Changes in the velocity of the Sun-like star 51 Peg used by M. Mayor and D. Queloz to infer the presence of a gas-giant planet in a short period orbit around the star.
Example of measurements (in red) overplotted on the expected Doppler signal caused by an exoplanet on the Star. Changes in the velocity of the Sun-like star 51 Peg used by M. Mayor and D. Queloz to infer the presence of a gas-giant planet in a short period orbit around the star.

Previous work on Proxima gives us a constraint that any signals around Proxima will likely be no more than 10 Earth-masses. Proxima Centauri’s mass—of about a tenth that of the Sun—means the mass ratio between Proxima and any planet in orbit will be at least 3000. To put this is context think of the forces on you when you spin an object on a string around your body. In the case of an Olympic hammer thrower the hammer twirls around their gyrating body with the string keeping them together. This analogy with the hammer thrower as the star and the hammer as the planet serves to illustrate that although the planet does most of the moving a star with a planet in orbit around it will be tugged slightly to and fro as the planet orbits, and these subtle movements of the star show up as subtle shifts in the color of the star’s light we see from Earth.

The men’s Olympic hammer throwing event involves a 7.3kg steel ball attached to a wire with a handle. Since the ball is 7.3kg this takes some strength and technique to throw so hammer throwers tend to be fairly large and strong. The force on the hammer thrower from the hammer is quite substantial because the mass of the hammer is high relative to the thrower. When Yuriy Sedykh set his world record for the hammer throw he was some 110 kg—a mass ratio of 15. The substantial force experienced by the athlete is not a particularly close analogy though, since a planet around Proxima Centauri would have a mass ratio of at least 3000. A better analogy needs something with a bit less mass. A golf ball weighing 45g serves as a better analogy but to get a mass ratio of 3000 we need the hammer thrower to be a bit heavier. The average sumo wrestler weighs in at 150kg so an appropriate analogy would be a sumo wrestler whizzing a golf ball around them. Or alternatively the force felt on a toddler (8.1 kg) swinging a ping pong ball (mass 2.7g) around.

Hammer thrower Mike Mai practices at Fort Lewis, July 1. Mai finished third at the U.S. National Championships and will soon compete at the World Track and Field Championships in Berlin, Germany. Image credits : Phil Sussman.
Hammer thrower Mike Mai practices at Fort Lewis, July 1. Mai finished third at the U.S. National Championships and will soon compete at the World Track and Field Championships in Berlin, Germany. Image credits : Phil Sussman.

When we look for the Doppler wobble of stars due to unseen planets we are actually looking to detect the small change in the light that results from the periodic stretching and compressing of the light from Proxima Centauri due to the motion induced on it by the planet. It might also be instructive to think about the Doppler effect in terms of other phenomena one experiences. Listen while a fast car moves past you or stand on the platform while a high speed train whizzes through the station. In these cases it is only possible to perceive these changes in sound when they are moving fast. While a vehicle moving at 100km/h (about 30 m/s) is only moving about ten times faster than Proxima Centauri might be moving, the change that our ears perceive is happening over much less than a second. However, we do not expect a planet in orbit around Proxima Centauri to make the change in period over less than a few days to appear to move from being slightly blue-shifted to slightly red-shifted. If we think in terms of sound waves the frequency ratio between two adjacent notes on a piano is approximately 1.06 which happens to be equivalent to the world record hammer throw ratio . The smallest shift in sound waves we can perceive is approximately 3.6 Hz, which at middle C corresponds to a rather modest ratio of 75. Thus even the smallest changes we can perceive in the frequency of sound are not really close to being representative of the level of effect we are trying to measure.

A star orbited by an exoplanet wobbles around the center of mass of the system as viewed in line with the orbital plane of the system. Image credits : Reyk
A star orbited by an exoplanet wobbles around the center-of-mass of the system as viewed in line with the orbital plane of the system. Image credits : Reyk

We are fortunate that modern technology, along with data handling ingenuity built up by successive generations of astronomers and instrument builders, allows us to reliably measure the frequency or wavelength of light over long periods of time. Rather than gazing into the sky and enjoying the mysterious wonder of all the far away specks of light; if we have access to a telescope and a sensitive digital camera we can collect enough light from the star that we can disperse the light so that rather than looking at a broad range of wavelengths—white light as we perceive it—we seek to measure the subtle changes in light from a star as a function of wavelength. This is equivalent to putting the starlight through a very powerful prism which enables us to split the white light into the full rainbow of colors. In practice this is achieved by putting the star light onto an echelle grating. This is a piece of glass (like a microscope slide) which has hundreds of lines ruled on it per millimetre. This enables the light to be dispersed a lot more than a prism, which in turn enables the resolution of individual atomic species and molecules that undergo particular transitions at precise energies. Since Proxima Centauri is comparatively nearby and space is very empty it can be seen that when a spectrum is taken, the atomic and molecular lines which we see correspond to the expected intensities for Proxima Centauri’s temperature; and move together as it moves relative to us and so we can be sure that our instrument is observing the atmosphere of Proxima Centauri.

The radial velocity method to detect exoplanet is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. When the star moves towards us, its spectrum is blueshifted, while it is redshifted when it moves away from us. By regularly looking at the spectrum of a star - and so, measure its velocity - one can see if it moves periodically due to the influence of a companion. Image credits : ESO
The radial velocity method to detect exoplanets is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from the (unseen) exoplanet as it orbits the star. When the star moves towards us, its spectrum is blue-shifted, while it is red-shifted when it moves away from us. By regularly looking at the spectrum of a star—and by so, measuring its velocity—one can see if it moves periodically due to the influence of a companion. Image credits : ESO

Another key ingredient of many required to precisely record Doppler wobble is a stable reference source for the spectrograph . In the case of our experiment the instrument HARPS (High Accuracy Radial velocity Planet Searcher) has a Thorium Argon lamp as its reference source. A standard light bulb works when a voltage is applied to heat a tungsten filament to glow and emit light. In a reference source lamp the voltages applied are larger so that the electrons in the atoms are ionized. Suitable elements are chosen, which are those that have large number of different energy states. When excited electrons in the atom move between the many possible energy states, they emit photons of light, which can then be detected as a relative forest of emission lines across a broad wavelength range.

On the left, we can see emission lines of a Thorium Argon lamp (extracted from http://tdc-www.harvard.edu/instruments/tres/). On the right a small chunk of the spectrum of Proxima Centauri is shown for comparison (Image credits : G.Anglada-Escude). Sharp features like the ThAr lines and the hundreds of narrow absorption features in Proxima's spectrum are ideal to measure accurate Doppler velocities.
On the left, we can see emission lines of a Thorium Argon lamp (extracted from http://tdc-www.harvard.edu/instruments/tres/). On the right a small chunk of the spectrum of Proxima Centauri is shown for comparison (Image credits : G. Anglada-Escude). Sharp features like the ThAr lines and the hundreds of narrow absorption features in Proxima’s spectrum are ideal to measure accurate Doppler velocities.

The heavy element Thorium and the buffer gas of Argon are the best combination found so far. Whenever we take a spectrum of Proxima Centauri we compare it to the Thorium Argon lamp reference spectrum to determine how much the lines in Proxima Centauri’s spectrum have moved. All the time the temperature and the pressure of the Thorium Argon lamp, and the spectrograph as a whole, are carefully controlled in order that nothing changes. The idea is that nothing in the experimental procedure should be altered from one measurement of Proxima Centauri to the next. This means that we can use the recorded data to infer the motion of Proxima Centauri and thus any Doppler wobble that it has due to any planets in orbit around it.

Hugh R. A. Jones

About the author. Professor Hugh Jones is the Director of Bayfordbury Observatory at the University of Hertfordshire and teaches astrophysics and maths to undergraduates. He is also Director of Research for the University’s Science and Technology Research Institute, one of three multi-disciplinary research facilities in the University. His research is focused on our stellar neighbourhood and has led to the discovery of a number of extra-solar planets using the Doppler wobble method including the first long-period circular one, the 100th discovered and the one with the most eccentric orbit. He received his BSc in Physics from the University of Leeds, followed by an MSc in astrophysics from the University of Alberta in 1990. After working at Blackwell Scientific and setting up an educational electronics company (MadLab) he moved into academia gaining his PhD in astrophysics from the University of Edinburgh in 1995. He then moved to the University of Tokyo as a European Commission research fellow to work with Professor Takashi Tsuji on the model atmospheres of cool dwarf stars working on the inclusion of dust in model atmospheres finding the first empirical evidence for dust in cool dwarf stars. At this time he co-founded the Anglo-Australian Planet Search using the Doppler wobble technique. In 1997, he moved to Liverpool John Moores University and in 2000 a position at the University of Liverpool being closely involved with founding and running a joint Physis degree, a suite of ten online distance learning courses and the the Liverpool Robotic Telescope. He moved to Hertfordshire in 2004, led the Institute of Physics accreditation of the Physics degree and set up the inaugural European Week of Astronomy and Space Science. Hertfordshire is now host to a number of researchers involved in the discovery and characterisation of nearby stars and their companions. The group is unusual in finding planets by a number of different complementary methods and is particularly active in the systematic discovery of the coolest stars including most of the coolest known examples.

Intensifying the Proxima Centauri Planet Hunt

By Paul Gilster, author of Centauri Dreams

There will always be a ‘proxima’—a star that is closest to our own—but it won’t always be Proxima Centauri, which in tens of thousands of years will doubtless revert to a different name, perhaps Alpha Centauri C or some other designation. We live in a dynamical universe, one in which the red dwarf Ross 248 will (in forty thousand years or so) be the new ‘proxima.’ We can even anticipate stars being much closer than Proxima Centauri is today. Go 1.4 million years into the future and GL 710 will move within 50000 AU (an Astronomical Unit, or AU, being roughly the distance between the Earth and the Sun). In time’s other direction, the bright Alpha Centauri system of today would not have been visible to the naked eye 3 million years ago.

700px-Near-stars-past-future-en.svg
Nearest stars in a time range between 20,000 years in the past and 80,000 years in the future.

In this ongoing celestial dance, the closest star will always captivate a technological society looking into life elsewhere and pondering strategies for sending probes across the interstellar gulf. The nearest star is a natural magnet for exoplanet hunters, as is the entire Alpha Centauri system, which comprises Centauri A and B and, if it is indeed gravitationally bound, as seems likely, Proxima itself. What good news that the Pale Red Dot project is now planning a two­-month observing campaign to search for potential Earth-analogs around Proxima Centauri using HARPS, the High Accuracy Radial velocity Planet Searcher spectrograph at the ESO La Silla 3.6m telescope. Nightly monitoring began on January 18th.

Discovered in 1915, by the Scottish astronomer Robert Innes, Proxima Centauri has been kindling imaginations ever since. For science fiction writer Robert Heinlein, it was the inevitable destination of the starship Vanguard, which carried crews that lived and died aboard the ‘generation ship’ in two 1940’s short stories that became his novel Orphans of the Sky. Murray Leinster had earlier claimed the star as our primary target in his 1935 story “Proxima Centauri.” And while Centauri B has recently gotten the lion’s share of attention with the still unconfirmed and now doubtful declaration of a Centauri Bb planetary candidate, Proxima Centauri has had a recent run of study that has helped define the parameters of the planet search.

To Find a Transiting World

proxima_small
Proxima Centauri.

Some 4.218 light years away from the Sun, this red dwarf star would be obscure even from a planet around Centauri A or B. Separated from them by 15,000 AU, Proxima is small and dim enough that it might take any Alpha Centauri astronomers some time to realize it was close, making the call only once its large proper motion became obvious. A naked eye object, yes, but at magnitude 3.7, it would hardly dominate the sky. Yet it might exert quite an effect on the two larger stars, with Greg Laughlin and Jeremy Wertheimer (UC­Santa Cruz) recently speculating that it could have a role in dislodging comets from the circumbinary disk that presumably surrounds both stars, hence delivering water to their planets.

Whether planets exist around Proxima itself remains an open question. To answer it, various modes of exoplanet detection are being brought into play, the most recent being a transit search by David Kipping’s (CfA) using the Canadian Space Agency’s MOST (Microvariability & Oscillations of STars) space telescope. Begun in the summer of 2014, the project took 13 days of data that year and an additional 30 in 2015. Results are to be announced by the summer of 2016. A small and inexpensive instrument, MOST is best known as the telescope that found transits of 55 Cancri e, making its primary the first naked eye star found with a transiting planet.

A transit detection, tracing the dip in starlight as a planet passed in front of the star as seen from MOST, would put the space telescope in the history books. Transit studies have advantages when it comes to small stars like Proxima Centauri. Proxima’s size is roughly one-tenth that of our Sun. Any habitable planet around it should produce a relatively deep transit signature in the star’s light curve, because the size of the planet in relation to the star is significant as opposed to small worlds around much larger G­- or F-­class stars. For the same reason, the likelihood of a transit alignment is enhanced.

A Planet through Gravity’s Lens

Gravitational microlensing also offers up prospects for tracking down Proxima planets, as noted in 2013 by Kailash Sahu (Space Telescope Science Institute), who realized that a star with such high angular motion across the sky might frequently occult a more distant object. In microlensing, the nearer object creates a ‘lensing’ of the background source as light flows along curved spacetime, an effect predicted by Einstein. An occultation of a distant star by Proxima might allow one or more planets to be revealed as they create their own lensing effect following the occultation by Proxima Centauri itself, slightly brightening the image of the background star.

Sahu found two occultation events, the first being passage in front of a 20th-­magnitude background star in October of 2014, the second an occultation of a 19.5­-magnitude star in February of 2016. Using both, it should be possible to measure Proxima’s mass to an accuracy of five percent. The Hubble Space Telescope, the European Southern Observatory’s Very Large Telescope (Chile) and ESA’s Gaia space telescope are all capable of measuring down to 0.2 milliarcseconds, while the displacement of the two background stars induced by Proxima’s mass is estimated at 0.5 milliarcseconds and 1.5 milliarcseconds respectively.

Probing Stellar ‘Wobbles’

Gravitational microlensing may or may not yield a Proxima Centauri planet, but the star has also been subjected to several radial velocity studies, in which we look for and analyze a characteristic stellar motion. This signal manifests as an extremely faint Doppler shift caused by the effect of an orbiting planet as the star moves slightly further away from us, then closer again. We can track this apparent ‘wobble’ with exquisitely sensitive spectrographs, as Michael Endl (UT­Austin) and Martin Kürster (Max­Planck­Institut für Astronomie) have done for Proxima Centauri using seven years of data from the UVES spectrograph at the Very Large Telescope in Paranal (Chile).

detectability
Upper limit of planetary masses that could have been detected orbiting Proxima Centauri based on observations of M. Endl and M. Kürster. The habitable zone is shown in as a green zone. Image from Endl & Kürster, A&A, 488, 1149.

No planet has been detected, but we’re only part way into the game, for we are beginning to see what kind of planets we can exclude from the realm of possibility. Endl and Kürster find no planet with Neptune’s mass or above, for instance, out to about 1 AU from the star. We can also make a statement about ‘super­-Earths’—rocky worlds more massive than our own—the researchers find no such worlds larger than 8.5 Earth masses in orbits of less than 100 days.

We are not, then, excluding the possibility of planets, but only beginning to declare what we have not yet found. Scientists consider a star’s habitable zone to be the region where liquid water could exist on the surface of a planet. In Proxima Centauri’s case, that zone should reach between 0.022 and 0.054 AU, corresponding to orbits between 3.6 and 13.8 days. The Proxima investigations have yet to find anything in this window, but so far the most we can say is that super­-­­Earths of 2­3 times the mass of the Earth in circular orbits have been ruled out.

With these limits in mind, it’s worth noting an astrometric study, led by G. Fritz Benedict (McDonald Observatory) in the 1990s, used the Hubble telescope to scrutinize the precise position of Proxima Centauri in the sky. In conjunction with a 2013 astrometric study by Lurie (Research Consortium on Nearby Stars), the results produced no planet. These studies indicate that Proxima can have no planet with a mass greater than Jupiter in orbits from 0.14 to 12.6 years.

What Pale Red Dot Might Find

The Pale Red Dot campaign’s radial velocity studies sharpen our focus on a target that is rife with possibilities. What about the prospects for life if we do locate a planet within the Proxima Centauri habitable zone? Here we have two issues to contend with. Like many younger M-­dwarfs, Proxima is prone to sudden, violent flares, producing sudden changes in brightness to Earth observers and cascades of deadly particles for any life forms on a planet. This may or not create an evolutionary niche as creatures adapt themselves over time to the incoming sleet of energetic particles; how such adaptations would succeed can only be speculated about.

Just as significant is the prospect of a planet in the habitable zone being so close to the parent star that it becomes tidally locked, forever putting the same face forward to its star. In a world like this, where the star does not move in the sky, we have permanent night on one presumably very cold side, and permanent day on the other. Fortunately, models developed by Jérémy Leconte (University of Toronto) and colleagues suggest that the presence of an atmosphere can largely overcome this difficulty by distributing hot and cold air so as to moderate temperatures around the planet.

Moreover, 3­D weather simulations by Jun Yang and Dorian Abbot (both of the University of Chicago) and Nicholas Cowan (Northwestern University) show that the side of a tidally locked planet facing the star would develop highly reflective clouds at the ‘sub­stellar’ region directly below the star’s position in the sky. Such cloud coverage could stabilize the atmosphere and produce a cooling effect that bodes well for temperate regions on the day side. There is even the prospect in recent work by Xavier Delfosse (IPAG, Grenoble) that close-­in habitable worlds may be captured into a spin­orbital resonance, but not necessarily into synchronous rotation. The possibility of life on red dwarf planets thus remains open.

Red dwarfs like Proxima Centauri are thought to comprise up to 80 percent of the stars in our galaxy, giving us tens of billions of planets likely to be in the habitable zone of their host stars. Some 100 are relatively close to the Sun, but Proxima retains pride of place as the nearest star to our own. At 4.2 light years, it is a destination we may one day be able to cross using technologies like beamed sails driven by laser or microwaves, but even at a tenth of the speed of light, any probes will take four decades to reach their destination. What could impel us to press ahead is the discovery of a potentially habitable world, a prospect all scientists working on the exoplanet hunt would applaud. The enticing presence of the K-­class Centauri B and solar-like G­-class Centauri A, just 15,000 AU further, is all the more reason we may one day make the crossing.

gilster
Paul Gilster. Photo credit: Paul Gilster.

About the author. Paul Gilster writes and edits Centauri Dreams (http://www.centauri-dreams.org), tracking ongoing developments in interstellar research from propulsion to exoplanet studies and SETI. A full time writer for the last thirty-five years, he is the author of Centauri Dreams: Imagining and Planning for Interstellar Flight (Copernicus, 2004) and Digital Literacy (John Wiley & Sons, 1997). He is also one of the founders of the Tau Zero Foundation and now serves as its lead journalist. This organization grew out of work begun in NASA’s Breakthrough Propulsion Physics program, and now seeks philanthropic funding to support research into advanced propulsion concepts for interstellar missions. Gilster has contributed to numerous technology and business publications, and has published essays, feature stories, reviews and fiction both in and out of the space and technology arena.

Stars!

by Prof. Stefan Dreizler from Georg-August-Universität Göttingen – Institute for Astrophysics

A star is a sphere of very hot gas (called plasma) confined by its own gravity. Our Sun is the nearest star, astronomers therefore use it as a standard. Compared to our Earth, the Sun is more than 300,000 times more massive and its diameter is a factor of 100 larger. Compared to Earth and other planets like Jupiter, stars produce energy from nuclear fusion processes, which results in the emission of light over a broad range of wavelengths. The maximum of this emission, for example of our Sun, is in the region of the electromagnetic spectrum visible to our eyes, but the Sun also emits significant levels of ultraviolet and infrared radiation. These nuclear processes require very high temperatures (millions of degrees) and very high pressures in the central regions of the stars. Straight forward estimates show that a minimum mass of a bit less than a tenth of a solar mass or equivalent to 80 times the mass of Jupiter is required to reach the temperature and pressure to operate stable nuclear fusion processes over a very long time; in the case of our Sun comparable to the age of the Universe.

Objects below this critical mass of 80 Jupiter masses are called brown dwarfs. They can sustain some nuclear processes but only for a rather short time. They can be therefore seen as failed stars. Below a mass of about 13 Jupiter masses, not even short term nuclear processes are possible, objects below this mass limit are planets. It should be noted that this differentiation between planets and brown dwarfs is only one possibility—others are in use as well.

How do Stars Form?

Stars form out of very cold gas and dust clouds, called molecular clouds, which become gravitationally unstable and therefore collapse under their own gravity. The cause of the instability can vary from density waves in the Galaxy, close-by stellar explosions or even galaxy-galaxy collisions! These molecular clouds contain gas of typically many ten thousands of solar masses and have diameters in the range of tens of light years—on the order of a few ten thousand times larger than our solar system. During the collapse, the cloud may fragment, i.e. a single cloud forms a large number of stars in the range of hundred thousand stars. The resulting stars have a good chance to stay mutually gravitationally bound for a very long time. We find such clusters of stars, called globular clusters, in our own Galaxy as well as in other galaxies. Less populated clusters are visible as open clusters, with the Pleiades as a prominent example, but dissolve rather quickly contributing their stars to the Galaxy. Nevertheless, multiple systems—containing two, three or even more stars—are very common. Proxima Centauri for example is a member in a triple system.

This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light-years from Earth, M80 contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Image credits : NASA, The Hubble Heritage Team, STScI, AURA
This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light-years from Earth, M80 contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Image credits : NASA, The Hubble Heritage Team, STScI, AURA

The fragmentation and contraction process proceeds through several stages until the fragment, at that stage called a protostar, is not much larger than our Sun. The contraction leads to a continuous transformation of potential energy into kinetic energy resulting in significant heating of the gas. The star is born when the interior is hot and dense enough to sustain steady nuclear fusion which then brings the star into a stable equilibrium. On astronomical time-scales, the formation process is short, i.e. on the order of a million years. Star formation is self-terminating, the first stars forming in such a cloud start to heat the molecular cloud which quickly leads to dissolution of the cloud.

 In one of the most detailed astronomical images ever produced, NASA/ESA's Hubble Space Telescope captured an unprecedented look at the Orion Nebula. ... This extensive study took 105 Hubble orbits to complete. All imaging instruments aboard the telescope were used simultaneously to study Orion. The Advanced Camera mosaic covers approximately the apparent angular size of the full moon. Image credits : NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team
In one of the most detailed astronomical images ever produced, NASA/ESA’s Hubble Space Telescope captured an unprecedented look at the Orion Nebula. … This extensive study took 105 Hubble orbits to complete. All imaging instruments aboard the telescope were used simultaneously to study Orion. The Advanced Camera mosaic covers approximately the apparent angular size of the full moon. Image credits : NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team

A rather near-by star forming region is the Orion nebula 1,300 light years away. With a small telescope, four prominent stars become visible which started to heat up the surrounding gas visible though the very characteristic light emission. Parts of the original cold molecular cloud gas are also visible as dark clouds. A closer look with the Hubble Space Telescope reveals a large number of newly formed stars.

The stellar composition reflects the material composition of our Universe, by number, hydrogen makes up about 90% of gas atoms, followed by helium with just less than 10%. All other elements, like carbon, oxygen, or iron, contribute less than 1% by number. We will see below that the amount of other elements is an indicator of the population the star belongs to, i.e. whether it has formed earlier in the life time of the Galaxy or more recently.

Since more than 99% of the stellar material is hydrogen and helium in a nearly identical composition from star to star, the properties of the newly formed stars are determined by its mass, which covers a wide range. The smallest objects formed are brown dwarfs; the most massive can have masses of the order of one hundred solar masses. The distribution of masses is quite universal, making the low-mass stars between a tenth and a half of the solar mass the most abundant ones. Proxima Centauri belongs to this category of low-mass stars. In that respect, it would be a better astronomical standard than our Sun.

Stellar Evolution and the HertzsprungRussell Diagram

As mentioned above, stars produce energy from nuclear reactions. Since hydrogen is the most abundant element and the amount of energy per atom gained in a fusion reaction is the largest, the fusion of hydrogen to helium is by far the longest lasting energy resource for stars. For example, our Sun can sustain this process for about 10 Billion years—the Sun is half-way through this stage in its evolution. Low-mass stars like Proxima Centauri have less hydrogen to be transformed, however, they emit less energy (light) per unit mass and can therefore sustain the hydrogen fusion (in the astronomers slang called “burning”) for much longer. On the other hand more massive stars than our Sun have a shorter life time since they consume their hydrogen more efficiently.

Hertzsprung–Russel diagram identifying many well known stars in the Milky Way galaxy. Image credits : ESO,
Hertzsprung–Russel diagram identifying many well known stars in the Milky Way galaxy. Image credits : ESO,

This leads us to the basic properties observable for stars, their luminosity, i.e. the total amount of emitted light per second, and the surface temperature. These two quantities are often displayed in a diagram, called the Hertzsprung–Russell diagram, which is one of the most important representations of stars in astronomy. The stars in their phase of hydrogen burning form the so called main-sequence. This is a sequence with the least massive stars having low temperatures and low luminosities, and temperature and luminosity increasing with mass. The surface temperature determines the color of the star; cooler stars appear red, hotter stars look blue. The surface temperature also determines the appearance of the stellar spectra. The emitted light of a star is not continuously distributed over the wavelengths as for example in a classical light bulb. The solar spectrum (the spectrum produced by the Sun) is characterized by millions of absorption lines indicating the presence of the chemical elements. Cooler stars like Proxima Centauri are characterized by the absorption lines of many molecules. These spectra are like finger prints and astronomers can learn a lot about the physical properties of stars by analyzing their spectra. In the context of planets around other stars, the spectral lines, or more precisely their shift in wavelength towards the red or the blue, is a measurement of the stars’ velocity. This is the basis of the Doppler Velocity Method which Pale Red Dot will be using to detect a planet orbiting Proxima Centauri.

In the Hertzsprung–Russell diagram displayed here, the main stellar properties are indicated. For historical reasons, the stars are categorized in so called spectral classes, labeled with the capital letters indicated on the horizontal axis (OBAFGKM, O star are bluer, M stars are redder). Among others, Proxima Centauri‘s position in the Hertzsprung-Russell diagram is on the right bottom, indicating that it is a low-mass red star of spectral type M, with a surface temperature of the order of 3,000 degree Kelvin and a luminosity of about a thousandth of the solar luminosity.

In the Hertzsprung–Russell–Diagram, other branches of stars are also visible. These positions in the diagram are populated by stars which finished their phase of hydrogen burning. For the sake of a compact description, most of the details of the further stages of stellar evolution are left out here. Basically, stars expand after the nuclear burning of hydrogen has ceased. This transforms the star into a giant. Most of the stars start a second phase of nuclear processes, now transforming helium into carbon and oxygen. This is less efficient and lasts therefore shorter. For most stars, the exhaustion of helium marks the end of nuclear processes and the star ends up as a white dwarf. Stars which are about ten times more massive then our Sun go through more nuclear processes, which end when a significant fraction of the stellar material is transformed to Iron. The final stage is either a neutron star or a stellar black hole formed in a violent supernova explosion.

The Peaceful End of Stellar Life

The later stages of stellar evolution are interesting for planets and planetary systems in two aspects. First of all, the transition of our Sun into a giant will terminate all life on Earth. The increasing solar radius and the increasing luminosity will bring the mean Earth temperature far above 100 degrees Celsius. The inner planets Mercury and Venus are very likely swallowed by the expanding Sun. When the Sun finally turns into a white dwarf, the Earth on its current orbit would be far too cold to sustain life. Moreover, there are observations indicating that we can observe decaying former planetary systems around white dwarfs which were destabilized due to the evolution of its host star.

Stellar evolution of low-mass (left cycle) and high-mass (right cycle) stars, with examples in italics. The background image is derived from http://imagine.gsfc.nasa.gov/teachers/lessons/xray_spectra/images/life_cycles.jpg by NASA's Goddard Space Flight Center.
Stellar evolution of low-mass (left cycle) and high-mass (right cycle) stars, with examples in italics. The background image is derived from http://imagine.gsfc.nasa.gov/teachers/lessons/xray_spectra/images/life_cycles.jpg by NASA’s Goddard Space Flight Center. Source : Wikipedia.

Secondly, stars lose a large fraction of their mass during the late phase of stellar evolution. Since this material is enriched in heavy elements like carbon, oxygen, silicon, iron, etc. and since the next generation of stars and their planets is formed out of this processed material, later generations of stars have more material to form planets like our Earth which mainly consist of iron and silicon and sustain life which relies on carbon and oxygen.

Stars and their Planets

Planets are a natural by-product of the star formation process. The contraction of the fragments of the star forming molecular cloud, based on angular momentum conservation—a fundamental principle of nature, leads to the formation of a disk around the protostar. These disks are the birth places of planets which form either in a bottom-up process from the growth of dust particles to larger and larger objects, finally of planetary size, or in the top-down, direct gravitational collapse of a fragmenting disk. Many details of these processes are still debated, but the basic principles seem to be clear. While this may indicate that all stars should have planets, planets might be lost, for example due to close stellar encounters, leaving stars without planets.

The properties of the star determines the distance at which a planet can sustain habitable conditions, i.e. having a temperature suitable for liquid water. This depends on the stellar luminosity which sets the planet’s mean temperature depending on the star–planet distance. A less luminous star such as Proxima Centauri has a habitable zone much closer to the star compared to our Sun, where obviously our Earth is in the habitable zone.

Stellar properties also determine how difficult it is to detect planets around it. Detecting planets around red dwarfs like Proxima Centauri is easier than around Sun-like stars. This is because red dwarfs are less massive (larger wobble due to a planet around them), smaller (larger dip in light when a planet crosses in front of them) and less luminous (higher planet–star contrast). An important aspect for life on other planets is also the stellar activity. Since we can study our Sun from nearby, we know that it is not a uniformly emitting sphere. The turbulent motion of the outer solar layers in connection with the solar magnetic field lead to a variety of phenomena summarized under the keyword activity. This activity determines the amount of high energy radiation and particles a planet is exposed to. This does not only impact possible life directly, it also influences the conditions in the planet’s atmosphere.

Overall we can state that the knowledge of planets around other stars is only as detailed as our knowledge of the host star making stellar astrophysics a key aspect in our exploration of other worlds.

About the author. Stefan Dreizler’s expertise is in Stellar physics and observational spectroscopy. He studied physics and obtained his PhD at University of Kiel (1992). After that he worked as a Postdoctoral fellow at University of Erlangen-Nurnberg between 1992 and 1995. He then moved to University of Kield as a Research Assitant (1995 and 1996), and to University of Tuebingen (1997-2000), where he was appointed as Lecturer between 2000-2003. His last move was to University of Goettingen where he is today a Full Professor in Astrophysics. He was Dean of the departmenbt of Physics between 2007 and 2009, and he has been deeply involved in the construction of the Multi Unit Spectroscopic Explorer (MUSE installed at ESO’s VLT), whose mechanical structure was assembled in Goettingen. He is member of the Kuratorium of the Max-Planck-Institut fur Sonnensystemforschung (Solar System Research). He runs astronomy outreach activities with the MONET telescopes (Texas and South Africa), and he is a very active member in the CARMENES project, which recently started operations and will also search for small planets around M-dwarfs in the northern hemisphere.

First spectrum with HARPS. Live from La Silla

Last image of the all sky camera. Sun is rising. See you tomorrow #palereddot !
Last image of the all sky camera. Sun is rising. See you tomorrow #palereddot !

[09:21:45] Alexandre Santerne: Thanks for following us ! End of the night. Time for astronomers observing #PaleRedDot to sleep.

[09:21:03] Guillem Anglada Escudé: anything you want to add to your audience?

[09:18:18] Guillem Anglada Escudé: ok. Yes. I’ll prepare a clean timeline later. Hopefully twitter comes back online so I can pull your text as well

[09:16:05] Alexandre Santerne: seeing during the observation: 1.3” – 1.4″

And the first spectrum!

First spectrum of PaleRedDot from HARPS. Looks promising!
First spectrum of PaleRedDot from HARPS. Looks promising!
Light going into HARPS as measured by its exposuremeter
Light going into HARPS as measured by its exposuremeter

[09:08:14] Guillem Anglada Escudé: yeah. Its good!

[09:08:04] Alexandre Santerne: snr@650nm = 65.3[09:07:46] Alexandre Santerne: exposition ended

The image on the right is Proxima on top of the optical fibre at the telescope focus. The same fibre goes all the way down to the spectrometer, which sits in the basement on top of the bedrock and lots of concrete

Integrating…

Control screen. The image on the right is Proxima centered on the optical fibre at the telescope.
Control screen. The image on the right is Proxima centered on the optical fibre at the telescope.

(not much to do for next 20 min, which is the exposure time…)

[08:49:51] Alexandre Santerne: flux level ~ 30%

[08:49:48] Guillem Anglada Escudé: cool

[08:46:41] Alexandre Santerne: exposure started

[08:46:36] Alexandre Santerne: focus ok

[08:43:07] Alexandre Santerne: checking focus

[08:39:31] Alexandre Santerne: pointing telescope …

UTC 08:38 Ambient of the ESO3.6m control room a few minutes before pointing #PaleRedDot

UTC 08:37. Two minutes to start pointing

UTC 08:32. Twitter is down (?!#@!) but we are still live on website! First spectrum in short. Go Alex!

Follow Alexandre Santerne @eso La Silla as the first spectrum of Proxima out of 60 is obtained.

Proxima rising at the end of the night

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Sunset of Jan 18th from la Silla

sunset_

Pale Blue Dot, Pale Red Dot, Pale Green Dot, …

By Alan Boss, Carnegie Institution for Science

Even Carl Sagan would be astonished by what has transpired in the 20 years since the first reproducible evidence for a giant planet in orbit around a sun-like star was announced in October 1995. The announcement of the discovery of a giant planet in orbit around the near-solar twin 51 Pegasus by Michel Mayor and Didier Queloz, followed by its confirmation a few weeks later by Geoff Marcy and Paul Butler, was completely unexpected, not because 51 Peg b has a mass of about half that of Jupiter, or a circular orbit, but because 51 Peg b orbits its star at a distance just 1/100 that of Jupiter, twenty times closer to 51 Peg than the Earth is to the Sun. Theorists such as myself could not imagine forming a presumably gas giant planet that close to a star, a confined space lacking in the raw materials necessary for forming any giant planet. We also feared that if a giant planet formed at a more reasonable distance, similar to Jupiter’s present orbit, subsequent gravitational interactions between the giant planet and the residual planet-forming disk of gas and dust might result in unchecked inward orbital migration of the giant planet toward the growing central protostar that could only result in the planet being swallowed by the voracious youngster. 51 Peg b proved planet formation theorists to be wrong, and we have been playing catch-up ever since.

Changes in the velocity of the Sun-like star 51 Peg used by M. Mayor and D. Queloz to infer the presence of a planet in a short period orbit around the star.
Changes in the velocity of the Sun-like star 51 Peg were used by M. Mayor and D. Queloz to infer the presence of a planet in a short period orbit around the star. Source : arXiv:astro-ph/0310261

Two months after the announcement of 51 Peg b, Carl Sagan sent letters to George Wetherill and me regarding his claim to have predicted theoretically the formation of a planet similar to 51 Peg b. Sagan had published a paper with a colleague in 1977 that used a simple model of the planet formation process to predict that if a protoplanetary disk happened to have all of its mass concentrated close to the protostar, then a single, massive planet orbiting at 10 times the distance of 51 Peg b might form. Their 1977 paper concluded, however, that such a formation mechanism was “highly questionable”. With the discovery of 51 Peg b, Sagan was ready to drop the “highly questionable” qualifier, and take credit for the first theoretical prediction of an extrasolar planet. Wetherill and I discussed Sagan’s claim, but had several objections of our own: first, whether the initial conditions assumed for the disk by Sagan were at all feasible, and, second, whether the simple model used was up to the task. Detailed computational models of planet formation were Wetherill’s specialty, building on the firm analytical foundation built by Victor Safronov and his colleagues, and Wetherill considered the simple model used in the 1977 paper to be closer to numerology than to proper physics. We politely refrained from supporting Sagan’s claim to theoretical ownership of 51 Peg b.

One year later, Carl Sagan died at the untimely age of 62 of a rare bone marrow disease, a shock to all of us who knew him as the prophet of the search for life beyond Earth. Just as I remember my seventh-grade classroom where I first heard about the assassination of President Kennedy in 1963, I remember the traffic light I was stopped at when a radio news show reported the death of Carl. By the time of his death, the roster of exoplanets discovered by Doppler spectroscopy (see http://home.dtm.ciw.edu/users/boss/planets.html/) had grown from one to seven, five of which were discovered by Butler and Marcy. The list of exoplanet candidates was now growing at the rate of a planet every month. Carl was a visionary prophet who lived long enough to catch a glimpse of the Promised Land beyond Earth, but not long enough to fully comprehend the prevalence of extrasolar planets.

51 Peg b was not in any way the first claimed discovery of an exoplanet. The most famous of these was the gas giant planet thought to orbit around Barnard’s Star, a red dwarf star similar to Proxima Centauri that is our nearest neighbour after the Alpha Centauri AB/Proxima Centauri triple system. Peter van de Kamp announced in 1963 the discovery of this planet, 60% more massive than Jupiter, and with an orbital period twice that of Jupiter’s twelve years. This planet made a lot more sense to the theorists than 51 Peg b, and it was accepted as a real detection. Van de Kamp used the astrometric method to search for the wobbles of the central star caused by an unseen planet, where multiple images are taken over a decade or longer. Ten years later, in 1973 George Gatewood published an independent set of astronomical plates that showed that the wobbles that van de Kamp thought were caused by a planet around Barnard’s star were caused instead by changes in the 24-inch refractor used by van de Kamp and in the photographic emulsions used for the exposures. As of 1973, there were no good examples of planets outside our solar system, leaving theorists to continue to concentrate solely on the puzzles associated with the formation of the our own collection of rocky planets, gas giants, and ice giants.

There were other claims for exoplanet discoveries in the two decades between 1973 and 1995. Gordon Walker and Bruce Campbell started one of the first Doppler spectroscopy searches in 1983, and after twelve years of observing, published their final paper in early 1995, concluding that they had found no firm evidence of planets with masses greater than that of Jupiter. In 1988, they thought they had found evidence for a Jupiter in orbit around Gamma Cephei, but after taking more data, in 1992 they published a retraction of the claim. The case for an exoplanet around Gamma Cephei is still debated (see http://exoplanet.eu/catalog/gamma_cephei_b/).

In 1988 another Doppler detection appeared, that of an object orbiting the star HD114762, discovered by David Latham and Michel Mayor. This object, however, had a minimum mass of about 11 Jupiter masses, perilously close to the critical value of 13.5 Jupiter masses, which separates Brown dwarfs from Jupiters. Brown dwarfs are massive enough to burn deuterium during their early evolution, whereas planets are forbidden to enjoy the energy generated by hydrogen fusion reactions (see http://home.dtm.ciw.edu/users/boss/definition.html/). Alexander Wolszczan and Dale Frail used the most exotic method of all to discover planetary-mass objects: in 1992 they published evidence from precise timing of the radio wave pulses emitted by the pulsar PSR1257+12 of the presence of not one, but two planets with masses of several times that of the Earth. The fact that these objects orbited in the deadly radiation field of a neutron star that presumably resulted from a supernova explosion made for a fascinating discovery, but one that held little interest for those of us who were fixated on searching for potentially habitable Earth-mass planets around solar-type stars.

Artists impression of extrasolar planets in the pulsar, PSR B1257+12. NASA/JPL-Caltech/R. Hurt (SSC) - http://photojournal.jpl.nasa.gov/catalog/PIA08042
Artists impression of extrasolar planets in the pulsar, PSR B1257+12.
NASA/JPL-Caltech/R. Hurt (SSC) – http://photojournal.jpl.nasa.gov/catalog/PIA08042

In 2004, Butler and his colleagues announced the discovery of the first example of a new class of exoplanets: super-Earths. They showed that the M dwarf star Gliese 436 was orbited by a planet with a mass as small as 21 times that of the Earth, a mass that suggested a composition lacking in gas but rich in rock and ice. Doppler spectroscopy surveys have found hundreds of exoplanets and super-Earths in the intervening years, enough so that by 2009, the prediction could be made that roughly 1/3 of all M dwarf stars were orbited by super-Earths. M dwarfs are at most about 1/2 the mass of the Sun, with much lower luminosities, leading to their having habitable zones much closer to their stars than Earth is to the Sun, but this remarkably high estimate of M dwarf exoplanets was a strong encouragement that the same high abundances would turn out to be the case for G dwarf stars like the Sun.

Proving this point would fall to NASA’s first space telescope designed specifically for exoplanet detection, the Kepler Space Telescope (see http://kepler.nasa.gov/). Kepler was the brainchild of William Borucki, who struggled for decades to convince his colleagues (and NASA) of the incredible power of a space telescope for discovering exoplanets by the transit photometry technique. Launched in March 2009, Kepler has more than repaid the America taxpayers who funded its development and operations, having discovered nearly 5,000 exoplanet candidates (at a cost of roughly $100K each) and over 1,000 confirmed planets. Kepler has proven that exoplanets are everywhere, even around G dwarf stars, in startling abundances. Estimates range as high as there being one habitable Earth-like planet for every star in our galaxy.

New Kepler Planet Candidates
Kepler Objects of Interest (many of them are most likely planets) as of July 23, 2015. Credits : NASA Ames/W. Stenzel – Licensed under Public Domain via Commons

As someone who has lived through the ups and downs of the history of the field of planet formation and detection, this revelation never fails to amaze me, and often chokes me up when giving public lectures. I cannot imagine that Carl Sagan would feel otherwise were he to have survived long enough to survey the entirety of this Promised Land. We now dream not just of pale blue dots, but of pale green dots indicative of chlorophyll worlds, of not-too-distant future space telescopes capable of the direct imaging of nearby habitable worlds, telescopes powerful enough to sample the compositions of the atmospheres of these worlds in search of molecules associated with habitable and even inhabited planets. Proxima Centauri is a sterling example of such a nearby star that we will continue to scrutinize in the coming years.

Carl Sagan lived at a time when the optimists among us hoped that maybe one out of a hundred stars might have a planet of some sort in orbit around it. His famous reference to the Earth as a pale blue dot hinted at the likely fragility of life in the Milky Way galaxy, life quite possibly confined to a single refuge in the immense void of an otherwise uncaring and oblivious universe. We now know that nearly every star we can see in the night sky has at least one planet, and that a goodly fraction of those are likely to be rocky worlds orbiting close enough to their suns to be warm and perhaps inhabitable. The search for a habitable world around Proxima Centauri is the natural outgrowth of the explosion in knowledge about exoplanets that human beings have achieved in just the last two decades of the million-odd years of our existence as a unique species on Earth. If Pale Red Dots are in orbit around Proxima, we are confident we will find them, whether they are habitable or not.

NASA Spitzer Telescope Science Update where major findings were announced about planets outside our solar system, known as extrasolar planets. Dr. Alan Boss, staff research astronomer, Department of Terrestrial Magnetism, Carnegie Institution of Washington explains science results during the NASA Science update. Tuesday, March 22, 2005. Photo Credit:
Dr. Alan Boss explains science results during the NASA Science update. Tuesday, March 22, 2005. Photo Credit: “NASA/Bill Ingalls”

About the author. Dr. Alan Boss is a Research Scientist at the Carnegie Institution for Science’s Department of Terrestrial Magnetism. He is an internationally recognized theoretical astrophysicist, whose research interests include the study of star formation, evolution of the solar nebula and other protoplanetary disks, and the formation and search for extrasolar planets. Dr. Boss has served on manifold NASA review panels, and has led both NASA and community working groups on extrasolar planet studies, including Chair of the NASA Astrophysics Subcommittee, Chair of NASA Planetary Systems Science Working Group, Chair of NASA Origins of Solar Systems MOWG, Chair of the IAU Working Group on Extrasolar Planets, President of IAU Commissions 51 and 53, and Chair of the AAAS Section on Astronomy. He received a NASA Group Achievement award in 2008 for his role in the Astrobiology Roadmap and another in 2010 for his role in the SIM Planet Finding Capability Study Team. He is a member and Fellow of several professional organizations including the American Astronomical Society, AGU, AAAS, Meteoritical Society, and the American Academy of Arts and Sciences. He has received numerous NASA and NSF grants, served on many professional committees, and is a Series Editor of the Cambridge Astrobiology Series. He has published two books about the search for planets outside the Solar System, “Looking for Earths: The Race to Find New Solar Systems” in 1998, and “The Crowded Universe: The Search for Living Planets” in 2009. Boss is currently the Chair of the NASA Exoplanet Exploration Program Analysis Group, as well as Chair of NASA’s Exoplanet Technology Assessment Committee and WFIRST/AFTA Coronagraph and Infrared Detectors Technology Assessment Committees.

Observing at ESO I. The ESO guesthouse

By Alexandre Santerne, ESO Photo Ambassador & exoplanet researcher at Instituto de Astrofísica e Ciências do Espaço (Portugal)

General view of the gardens of ESO's guesthouse
General view of the gardens of ESO’s Guesthouse. Image credits : Alexandre Santerne/ESO

The La Silla and Paranal observatories in Chile are two of ESO’s sites where astronomical observations are carried out. They are like our working place, but located about 11 000 km from Europe and require a few days to reach them. When going to Chile as a visiting astronomer[1], the first place we arrive to is the ESO Guesthouse. Located in Santiago de Chile, this is a very quiet and peaceful house where we stay one night to recover between the long flight from Europe (about 14 hours long and 5 hours of jet lag) and the night life at the observatories.

 inner garden at ESO's guest house
View of the inner garden at ESO’s Guesthouse. Image credits : Alexandre Santerne/ESO

The Guesthouse has a dozen of rooms with a beautiful botanical garden and a private swimming pool. The most important place is however the living room where we share breakfast / lunch / dinner or even the pisco sour time[2] with other visiting astronomers from the ESO Member States[3].

ESO guest house living room. Image credits : Alexandre Santerne/ESO
ESO Guesthouse living room. Image credits : Alexandre Santerne/ESO

The discussions mostly focus on science, astronomical observations, and weather conditions as well as the upcoming new facilities developed by ESO, in particular the next generation instruments for the Very Large Telescope[4] and the 39-metre European Extremely Large Telescope[5]. Besides providing fantastic ground-based facilities for observations, ESO is also a great place to meet other astronomers working with ESO’s telescopes.

Notes:
[1]- To decrease mission costs, most observations are carried out in service mode and are performed by ESO staff. However, some observations require to be performed by the astronomers themselves. In this case, we are called “visiting astronomers”.
[2]- The pisco sour is the famous Chilean and Bolivian cocktail, composed mostly of Pisco (a kind of brandy) with lemon juice.
[3]- The ESO member states are Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom http://www.eso.org/public/about-eso/memberstates/
[4]- see http://www.eso.org/public/teles-instr/vlt/vlt-instr/
[5]- see http://www.eso.org/public/teles-instr/e-elt/

Launch!

… so you hope to find a pale red dot?

Yes! We think there might be a small planet orbiting our nearest stellar neighbor -an M-dwarf star called Proxima Centauri-, but it might also be magnetic activity. We will observe Proxima for two months with the planet hunting machine HARPS and two networks of smaller telescopes. Such monitoring should leave little doubt about the nature of the Doppler signal but… wait a second!

Doppler signal ??@#!… what is that? A magnetic M-dwarf star, is it a rock band? For two months? that seems long and boring! Can’t you find planets any other way? Shouldn’t we do it from space? How long before we can go to these planets?…

To answer this and many other questions, palereddot.org will feature articles from prominent scientists worldwide discussing extrasolar planets, the search for life beyond Earth, instruments and plans, and what we think about life, the universe and everything else… 😉

As with all the good things in life, Pale Red Dot will be intense but short. After all the data is collected (end of March), the hard core analysis will begin and the website will necessarily hibernate for a bit. After that,  results will be sent to a peer review journal and only then an (in)glorious announcement will be made. Who knows what will happen! This process might take several months, but we will do our best to keep you informed as well.

Do you want to know if such a planet exists? So do we! So stay tuned…

…so what kind of articles will you publish?

  • Expert insights and Expert opinions are articles from exoplanet pioneers, leaders of space missions and giant telescope instruments, visionaries and all sorts of rising stars in the field of exoplanet and stellar physics research. Expert opinions will always be released on Sundays (excellent to read with pancakes), while Expert insights will come during week days (really well suited for your daily commute).
  • Observatory life articles will feature how the different observatories work and how modern astronomical observations are obtained. Real life pictures and videos of the action behind-the-scenes included! Observatory life articles will be released every Saturday.
  • Project updates will be released every Friday, and will contain the highlights of the week, including the usual complaints of bad weather. We will not get any data if it gets cloudy, so astronomers are genuinely interested in talking about the weather.

So, are you ready to join our live exoplanet hunt?

If you have questions for us, we’d be happy to answer them on Twitter, @Pale_Red_Dot and #PaleRedDot.

Full resolution video and description available at http://www.eso.org/public/announcements/ann16003/

The search begins Jan 2016

Pale Red Dot is a scientific and outreach project about astronomy and the search of extrasolar planets that will launch on Jan 11th 2016. Pale Red Dot is a joint initiative of the scientists involved in the observations, the outreach office of the European Southern Observatory (www.eso.org), and several supporting institutions including; Queen Mary University of London/UK (www.qmul.ac.uk), Instituto de Astrofísica de Andalucía/Spain ( www.iaa.es), Universidad de Chile (http://www.das.uchile.cl), University of Hertfordshire/UK (www.herts.ac.uk), University of Goettingen/Germany (www.uni-goettingen.de), Université de Montpellier (http://www.lupm.univ-montp2.fr/), LCOGT.net (https://lcogt.net/), and BOOTES telescope network (http://bootes.iaa.es/).

In addition to regular updates on the acquisition of the data, the website will feature contributed articles by world leading researchers, and science writers from several countries. More details on the program and outreach activities will be announced soon.

Stay tuned!