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. The good news is that observatories now being planned could have a shot. Yet it is hardly a lock.
The next generation of giant, ground-based telescopes 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 in Astronomy & Astrophysics. The two groups of scientists calculated what possible biomarkers might be detectable with the planned European Extremely Large Telescope (E-ELT) (above), which would dwarf the twin Keck telescopes on Mauna Kea in Hawaii that are now on the cutting edge of astronomy.
Living organisms on Earth 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 could provide strong indications of the presence of life there.
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Astronomers using the world's best telescopes have already identified specific atoms and molecules in the atmospheres of massive, 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 world. The giant observatory may also be able to identify water, which is thought to be important but not sufficient for life as well as ozone (O3), a molecule closely related to oxygen gas. “When we are sure there is ozone, we could be pretty sure that there is oxygen in the atmosphere,” explains astrophysicist Pascal Hedelt of the German Aerospace Center and the Laboratory of Astrophysics of Bordeaux in France, who is the lead author of the Astronomy & Astrophysics study. Methane might also be detectable in some of the scenarios explored by Hedelt's group.
The connection between chemistry and life is not always straightforward, however, and the detection of oxygen, methane or some other biologically relevant molecule will require careful interpretation. Venus has an ozone layer, and Mars, according to research that is somewhat controversial in the planetary science community, releases occasional plumes of methane. Yet no solid evidence indicates that either planet hosts any microbes. “Only finding oxygen in principle is not enough,” says Ignas Snellen of Leiden University in the Netherlands, who led the Astrophysical Journal study. Regarding future exoplanet studies, Snellen cautions, “You really need to characterize the atmosphere as a whole.”