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.”
But the detection of oxygen, the authors note, would be made easier if the planet in question orbited a red dwarf star rather than a larger sunlike star. The reason: smaller, dimmer stars are cooler, which means that habitable planets with liquid water could exist closer to the star. A smaller orbit means that the planet completes a lap around the star more quickly and reveals itself to an Earthly telescope more often. Hence astronomers could examine a planet several times a year and, according to the calculations of Snellen and his colleagues, could build a solid case for oxygen within a decade or so.
The second study arrived at a somewhat sunnier conclusion, both literally and figuratively. Low-resolution spectral analysis, although less conclusive, might allow astronomers to probe Earth-like planets orbiting brighter, more sunlike stars for molecules of biological relevance. The E-ELT, the researchers found, could 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 (O2). “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, lead author of the Astronomy & Astrophysics study. Methane might also be detectable in some of the scenarios explored by Hedelt’s group.
The low-resolution search technique would employ filters that cover a range of wavelengths rather than pinpointing narrow spectral lines to identify molecular constituents of a planet’s atmosphere. The advantage is a boost in the signal-to-noise ratio of the planetary signal. In some cases, though, a single filter might encompass multiple molecular signatures of interest—a filter that singles out wavelengths around 2.7 microns, for instance, would pick up the absorption signature of water vapor and carbon dioxide, but would be unable to discriminate between the two.
That is where NASA’s planned James Webb Space Telescope (JWST) comes in. The Webb, which is scheduled to launch in five years or so, will have a much smaller aperture than an ELT. But the telescope will be situated in deep space and not have to contend with the confounding signals from Earth’s atmosphere. “The problem with the ELT is it’s located on the Earth,” Hedelt says. “An ELT in space would be awesome.” Failing that, the smaller JWST may still complement the giant telescope on the ground: If the Webb could rule out the presence of carbon dioxide in an exoplanetary atmosphere by observing at other wavelengths, the E-ELT could confirm the presence of water with its broadband filters.
Seeing as neither the E-ELT nor other proposed ELTs (such as the Giant Magellan Telescope or the Thirty Meter Telescope) have been built, and the JWST has yet to fly, all of the calculations remain a bit theoretical. But Snellen notes that the method proposed by his group has already proved feasible with smaller telescopes targeting larger planets.
Additionally, the connection between chemistry and life is not always straightforward, 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. But no solid evidence indicates that either planet hosts any microbes. “Only finding oxygen in principle is not enough,” Snellen cautions of future exoplanet studies. “You really need to characterize the atmosphere as a whole.”