PASSING THROUGH: When a planet transits its star, passing in front of the star from an observer's vantage point, filtered starlight can carry spectral information about molecules in the planet's atmosphere. Here, Venus transits the sun. Image: SDO/NASA
Even as astronomers work toward the hotly anticipated milestone discovery of an Earth-like twin orbiting another star, researchers are already asking what it will take to detect the existence of extraterrestrial life on such a planet.
First, the bad news: No telescope in existence seems to have the observing power to pick out the kinds of molecular signals that would indicate an exoplanet is habitable or even inhabited. On the bright side, observatories now being planned or already under construction could have a shot. But it’s hardly a lock.
The next generation of giant, ground-based telescopes, generically known as extremely large telescopes (ELTs), may be able to tease out biomarker signals from the starlight filtering through exoplanetary atmospheres, according to research recently published in The Astrophysical Journal and forthcoming in the journal Astronomy & Astrophysics. The two groups of scientists calculated what possible biomarkers might be detectable with the European Extremely Large Telescope (E-ELT), a planned observatory with a 39-meter primary mirror that would dwarf the 10-meter twin Keck telescopes now on the cutting edge of astronomy. (The Kecks can breathe easy for now: E-ELT will not come online until the 2020s at the earliest.) The results are cause for cautious optimism: assuming that Earth-like planets are relatively common, the E-ELT or a comparable observatory might be able to identify several molecules important to, or even indicative of, life.
On Earth living organisms leave numerous chemical imprints on the environment via, for instance, the production of oxygen by plants and bacteria, the release of methane during digestion, and the generation and consumption of carbon dioxide in the global carbon cycle. Measurements of those chemical species in an exoplanet’s atmosphere—particularly measurements that indicate a chemical cycle out of static equilibrium—could provide strong indications of the presence of life on that world. “By identifying certain molecules in the atmospheres of the planets, you can have some first proof that life is there,” says astronomer Ignas Snellen of Leiden University in the Netherlands.
Simply discovering extrasolar planets is difficult enough, however. Teasing out subtle chemical signals from the spectra of their atmospheres at such distances is a tremendous challenge. Nevertheless, astronomers using the world’s best telescopes have already identified specific atoms and molecules in the atmospheres of giant, highly irradiated exoplanets. To do the same for smaller planets in cooler orbits—objects from which photons are relatively scarce—will require much bigger telescopes and many years of observations.
With a high-resolution spectrograph to break down the collected light from an exoplanet into its component wavelengths, the E-ELT would in principle be able to spot oxygen gas in the atmosphere of a temperate, Earth-like exoplanet, according to the Astrophysical Journal study. On Earth oxygen predominantly originates from photosynthesis. “If there was no life, if there was no biological activity, this oxygen would not be there,” says Snellen, who led the study. Therefore, the presence of oxygen in an exoplanetary atmosphere would suggest a familiar process at work on a foreign world. “With the next type of telescope that will become available in the next decade, it will be very difficult,” he says. “It will be possible, but very difficult.”