Category Archives: Expert Opinions

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.

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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

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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).

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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.

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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.

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.