© G. Bacon STSCI / ESA / NASAThe next generation of space telescopes will be capable of detecting "biosignatures" in the light from planets orbiting other stars
Any clues about life on these exoplanets will have to come from the tiny fraction of the parent star's light that interacts with the planet on its journey towards Earth. The Hubble and Spitzer space telescopes have both detected gases such as carbon dioxide and water vapour in the atmospheres of a handful of gas-giant exoplanets as they pass in front of their parent stars. The gas molecules absorb light at characteristic wavelengths, and this shows up as dark lines in the spectrum of the starlight which has been filtered through the planet's atmosphere. But seeing evidence of life - so-called biosignatures - in the spectrum of worlds small enough to be rocky like Earth is beyond the sensitivity of these instruments.
One potential biosignature is oxygen, which is abundant in Earth's atmosphere because it is produced by plants and photosynthesising microbes.
At a symposium on the search for life beyond Earth held last week in Baltimore, Maryland, delegates heard how NASA's infrared James Webb Space Telescope (JWST) could discern signs of oxygen present in the atmospheres of Earth-like planets around the nearest handful of stars, if such worlds are present and happen to "transit" in front of their parent.
Using a computer simulation based on the Earth and its atmosphere, Lisa Kaltenegger of Harvard University and Wesley Traub of NASA's Jet Propulsion Laboratory in Pasadena, California, calculated the strength of the oxygen signature in the starlight passing through the atmosphere of an Earth-like exoplanet. "If we're really lucky", says Kaltenegger, there's a transiting Earth-like planet around one of the nearest stars, where the JWST would be able to detect this biosignature.
NASA's proposed Terrestrial Planet Finder, which could launch in the 2020s, would be powerful enough to spot oxygen-rich planets in systems much farther from Earth, even when they are not passing in front of their parent stars, because it would be able to see light reflected from the planet's surface. Because the Terrestrial Planet Finder would have so many more planetary systems in its sights, this would offer the best chance of spotting oxygen-rich worlds, Kaltenegger says.
Oxygen alone does not prove that life is present, however, especially on a planet close to its parent star. Energy streaming in from the star would evaporate any water, and then break down the resulting water vapour into oxygen and hydrogen. The hydrogen would leak away into space, leaving an oxygen-rich atmosphere.
On a planet further away from its parent, an abundance of oxygen would be harder to explain without life, says Jim Kasting of Pennsylvania State University in University Park. "Oxygen can be a good biosignature," Kasting says, as long as you are careful to identify the circumstances.
Another possible biosignature might be found in the light reflected off living matter. Bill Sparks of the Space Telescope Science Institute in Baltimore, Maryland, measured the polarisation of light bounced off leaves and cyanobacteria. He found about 1 per cent of the light becomes "circularly polarised", which could be detected by one of the giant ground-based telescopes with mirrors 30 metres or more across, which are expected to come into service in the next decade, Sparks told the Baltimore symposium.
The physicist and astrobiologist Paul Davies of Arizona State University in Tempe says that any life found in an alien solar system would be of great significance, because it would have to have arisen independently of ours. "The probability of a rock being knocked off Earth and conveying a terrestrial microorganism to another Earth-like planet in another star system is infinitesimal," he says.
How to map an exoplanet's oceans
The red and blue light reflected from an alien world could be used to create maps of its oceans.
Though the existence of liquid water on an exoplanet would not prove that it harbours life, it might at least indicate the planet is habitable. Exoplanets are too far away for us to be able to see their surfaces in detail, but an experiment by Nicholas Cowan of the University of Washington in Seattle and his colleagues suggests it should still be possible to detect oceans on such worlds.
The team used measurements of light from Earth made from 50 million kilometres away by NASA's Deep Impact spacecraft, which showed Earth's apparent colour varying over time. When more water is on the side facing the probe, the planet appears bluer, and when a large landmass like Asia rotates into view it looks redder, team member Drake Deming of NASA's Goddard Space Flight Center in Greenbelt, Maryland, told the Baltimore symposium.
From this, the team was able to construct a crude map of how land and water are distributed on our planet. A telescope like NASA's proposed Terrestrial Planet Finder could do the same for Earth-like exoplanets, they say.
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