Among the signs researchers might look for: the presence of ozone – a molecule made of three oxygen atoms – and nitrogen oxides in a planet's atmosphere. On Earth, fossil evidence indicates that the first algae capable of photosynthesis, which produces oxygen, emerged some 3.5 billion years ago. On Earth, bacteria also produce copious amounts of nitrogen oxides.
The hunt for signs of life elsewhere in the galaxy is one of the drivers behind Kepler, and the inspiration for TESS. But Kepler isn't looking directly for life. Instead, it is looking for Earth-mass planets orbiting at Earth-like distances around sunlike stars in order to provide an estimate of how common such planets are.
On Thursday, Kepler's science team announced the discovery of three super-Earths in or on the boundary of their stars' habitable zones. The habitable zone is a region around the star far enough away so that a planet doesn't overheat, but close enough so it doesn't freeze either. In principle, a planet orbiting in its star's habitable zone should be able to host liquid water in stable quantities on its surface. Liquid water is seen as essential for organic life.
But the nearest of these new systems is 1,200 light-years away. Although the team speculates that one of the two super-Earths there is a water world and the other likely has a rocky surface, and while both fall into their star's habitable zone, they are too far away and their star is too dim to study with anything more than computer models.
TESS's targets should fall well within the gaze of a new generation of ground- and space-based telescopes. But those telescopes need to know where to look, Dr. Hudgins notes. And that's where TESS comes in.
Like Kepler, TESS is designed to detect planets as they pass in front of their host stars, dimming the starlight by a tiny fraction. If one could look back at the sun from beyond the solar system and watch for the wink an orbiting Earth would impart, the light would vary by just 0.000085 percent, a no-see-um in human terms.