Last December astron omers excitedly announced the discovery of a planet called Kepler 22b. Located 620 light-years away, it is the first planet found by NASA's Kepler space telescope to reside in its star’s habitable zone—a region that can support liquid water, a key requirement for life on Earth. One NASA artist took the news a step further with the illustration above, depicting an inviting blue, cloud-streaked world like our own.
Unfortunately, Kepler cannot provide that kind of detail on the more than 2,300 likely planets it has discovered around other stars. It cannot tell us if the planets are rocky, have oxygen in their atmosphere, or hold liquid water on their surface. In other words, it cannot tell us what we really want to know: Is there life beyond Earth?
In the mid-1990s, when the first exoplanets around other stars were being discovered, NASA engineers proposed missions that would attempt to address that question. One was the Terrestrial Planet Finder, a $1 billion-plus space telescope that would capture light from individual planets; spectroscopic analysis (separating the light into its different wavelengths to determine the composition of the planet’s atmosphere) would then be used to look for life-friendly molecules like water vapor, oxygen, and methane.
