Ignas Snellen, Leiden Observatory, Leiden University, The Netherlands
How amazing would it be if we found that there exists a planet, with a mass and temperature just like Earth, around our nearest neighbour—Proxima Centauri. How could we find out whether life exists on such a world?
The Pale Red Dot project is all about following up a hint that such a planet may exist. This is extremely exciting. Finding such a planet may actually not be such a surprise. Proxima is a faint red dwarf star, seven times smaller than the Sun. From previous observations, for example using the NASA Kepler satellite targeting planets that transit their host star, it is clear that in general red dwarf stars indeed often have small rocky planets. Therefore, Proxima—our nearest neighbour at a distance of 4.2 light-years—could be expected to also host small planets. Since the luminosity of Proxima is one thousand times smaller than that of the Sun, the energy a planet receives from Proxima is that same amount less than it would receive from the Sun. Therefore a planet in an Earth-like orbit would be freezing cold. Astronomers like to think that to host life a planet needs to be able to sustain liquid water. It is crucial for life as we know it as a solvent; making the complex chemical reactions associated with biological activity possible. Indeed, no life on Earth can function without liquid water. For planets around Proxima it means that their orbital distances need to be significantly smaller, only a fraction of the Earth-Sun distance, to be able to sustain life—corresponding to an orbital period of about two weeks, right in the ballpark of the possible planet chased by Pale Red Dot.
Let us for now assume that the Pale Red Dot observations indeed reveal such a planet. How could we possibly find out there is life on it? Let me first sketch the picture of why this is so difficult. For hundreds of years astronomers have wondered whether there is life on Mars, without a conclusion yet. Over the last fifty years, Mars research has accelerated at an immense pace. NASA and ESA have sent many probes that dig, drill, and can measure the Mars atmospheric constituencies to high precision—and still we do not know. One has to realize that we will never be able to do this for extrasolar planets. The distances are simply too large. Even Proxima, our nearest neighbour star, is more than two-hundred-thousand times further away than Mars. We simply cannot send probes to Proxima on practical human time scales. Therefore, any information we collect from possible planets orbiting Proxima can only come from remote sensing—observing them using our telescopes on and around Earth.
Is it then entirely hopeless that we could find life? Luckily not! Of course, if the situation is like on Mars, where life may be hiding deep under the surface, or on Jupiter’s moon Europa, where simple life may be deep in its ocean, it will indeed be hopeless. We will never be able to recognize it from a distance. However, the situation is very different for Earth. If an alien civilization would point a telescope at Earth and measure its spectrum, it can see clear signs of biological activity. They can see that molecular oxygen is very abundant in our atmosphere, which is only present due to the organisms that produce oxygen as a waste product (which subsequently animals like us use as an energy source). Oxygen is a very reactive gas, which you do not expect to find in an atmosphere like ours. Indeed it took life on Earth more than a billion years before the oceans were saturated enough that the oxygen they produced ended up in the atmosphere. If all life on Earth would cease to exist, all atmospheric oxygen would disappear on a very short time-scale. Therefore we call oxygen a biomarker gas—it can provide evidence for biological activity on an Earth-like planet. Note however that a lack of oxygen does not mean there is no life. It could still hide under a rock like possibly on Mars.
Also, for more than a billion years there was abundant life on Earth without a significant amount of oxygen in the atmosphere. Anyway, at least we can start somewhere.
So how could we detect oxygen on a planet more than four light years away? Characterizing a planetary atmosphere is significantly more difficult than finding the planet, like is being tried now with the Pale Red Dot project. Most of the techniques used to find planets, including the radial velocity method used for Proxima, are indirect methods. They do not identify one single photon from the planet, but instead indirectly infer the presence of a planet—e.g. using the gravitational pull of the planet on the host star, or the moment a planet transits the disk of its host star blocking a small fraction of starlight that can be observed. This can no longer be the case for atmospheric characterization. We really need to identify photons from the planet as a function of wavelength, and in this way build up a planet spectrum. Although this is very difficult, this can be done, but is so far almost exclusively achieved for hot gas giant planets.
Broadly speaking, separating the planet’s light from that of the star can be done in the time, spatial, spectral and polarization domain, or using combinations of these. An example of the time domain is transmission spectroscopy. When a planet, as seen from Earth, moves in front of its host star, a little bit of starlight filters through the planet’s atmosphere, leaving an imprint of absorption from its atmospheric constituencies. A planet can also be eclipsed by its host star, temporarily blocking the planet light, which can be measured. In such a way the thermal emission spectrum or reflected light spectrum can be determined. However, such transit and eclipse observations require that the planet orbit is seen exactly edge-on – which will only be a small probability for the alleged Proxima planet. We have a better chance by trying to separate the planet from the star in the spatial domain. This is called direct imaging, or high-contrast imaging. Using a combination of techniques the starlight can be suppressed by orders of magnitude, leaving the planet signal coming from a slightly different direction unaffected.
Although Proxima is our nearest neighbour, the targeted planet’s orbit will be very close in—only ~1/200,000th of a degree (20 milli-arcseconds) as seen from Earth. The sharpness of a telescope directly scales with its diameter, implying that revealing a planet at such a small angular distance from its host star requires a mirror diameter of at least 25 meters. Even the James Webb Space Telescope, which will be launched in 2018, will not be able to separate this planet from its host star. We would really have to wait until the next generations of ground-based telescopes come online, like the US Thirty Meter Telescope and Giant Magellan Telescope, and the European Extremely Large Telescope. Even with these giants it will be a real challenge—we will have to look through our own atmosphere to detect the atmosphere of the planet. However, we have been practicing just that using giant planets over the last decade, and we are starting to get pretty good at it! We will have to be patient though—do not expect the specific instrumentation needed to detect oxygen to be ready before 2030. A long wait, but the prospects are very exciting!
About the Author: Ignas Snellen is Professor in astronomy at the University of Leiden in the Netherlands. After concluding his PhD research in Leiden in 1997, he worked for three years as a postdoctoral fellow at the Institute of Astronomy in Cambridge (UK), after which he became an astronomy lecturer at the University of Edinburgh (UK). He returned to Leiden University in 2004. Snellen and his group develop observation and data-reduction techniques for ground-based telescopes, particularly geared to be used for the future extremely large telescopes (ELT). He and his group pioneered studies of molecules in exoplanet atmospheres using high resolution spectroscopy in the near infrared.