How to find out if exo-Earths host life

SO CLOSE, yet so far. Gliese 581 g is the first planet discovered that is the right mass and distance from its star for the surface to be awash with liquid water and perhaps life. Chances are we'll never know for sure without an armada of space telescopes, and their future looks uncertain. But a 2014 mission could tell us whether any habitable worlds with better viewing angles have signs of life.

Gliese 581 g is 20 light years from our solar system and three to four times as massive as Earth. The planet is likely to be rocky and lies squarely in the habitable zone around its star, where temperatures are just right for liquid water to exist on its surface.

To find evidence for life we would need to measure the light spectrum of the planet's atmosphere and look for the signature of water vapour, as well as possible by-products of life, such as oxygen and methane.

That would mean launching an expensive array of space telescopes to tease out the faint glow of the planet from the powerful glare of its star. NASA and the European Space Agency were hoping to launch such a mission in 2014, called the Terrestrial Planet Finder (TPF), or Darwin.

But in 2006 NASA backed away from the mission, postponing it indefinitely to free up more funds for human space exploration. Darwin/TPF was dealt another blow this August, when a key panel of US astronomers failed to recommend its construction in the next decade.

All is not lost, however, according to Paul Butler of the Carnegie Institution in Washington DC, co-discoverer of the new planet. He says the ground-based instruments that helped him discover it should soon turn up a flood of worlds in the habitable zones of their stars. "Over the next 10 years, I would be shocked if there weren't many tens of these things," he says.

About 5 to 10 per cent of these should, unlike Gliese 581 g, pass in front of their parent stars as seen from Earth, making it easier to measure their atmospheric spectra, Butler says. The James Webb Space Telescope could make such observations after it is launched in 2014, at least for the nearest stars (New Scientist, 16 May 2009, p 10). We may not have to wait too long before we see signs of a planet with life.