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.

But NASA ran into budgetary problems, leading it to table funding for the Terrestrial Planet Finder and other such life-seeking projects. As a result, there was no obvious follow-up mission in the works when Kepler launched in 2009.

“The fiscal realities are frustrating,” says Mark Swain, an astrophysicist at the NASA Jet Propulsion Laboratory in Pasadena, California. “But at the same time, it’s an exciting challenge to figure out the best way to follow up all these amazing discoveries.”

As funding has dried up, the excitement over exoplanets has only grown stronger. Kepler’s results are compelling in the sheer number and variety of planetary finds. Other recent studies suggest that our galaxy, the Milky Way, contains more than 100 billion planets. Tens of billions of those, in turn, could have the right temperature for liquid water. The bounty of potentially habitable planets has astronomers scrambling for ways to revive the spirit of the Terrestrial Planet Finder, but on a shoestring budget.

Low-Hanging Fruit
The trick to keeping costs down is focusing on planets around relatively bright, nearby stars—the easiest ones to detect. Swain is principal investigator of the Fast Infrared Exoplanet Spectroscopy Survey Explorer (Finesse), a proposed 30-inch space telescope that would probe more than 200 planets around nearby stars to learn about their atmospheres and how they formed. The telescope would examine each star to see how its light changes as a planet passes in front of and behind it. By seeing which wavelengths are absorbed as the starlight passes through the planet’s atmosphere, astronomers could determine whether 
the atmosphere contains water, carbon monoxide, methane, and carbon dioxide.

Alternatively, an MIT-led group of astronomers is developing the Transiting Exoplanet Survey Satellite, or TESS, a spacecraft containing an array of telescopes that would survey the entire sky, looking for exoplanets in the habitable zone around the nearest and brightest stars. TESS would not do spectroscopy itself; that job would be left to observatories such as the Hubble Space Telescope or the upcoming James Webb Space Telescope—which will be larger and more powerful than Hubble and have a spectrograph tailored to the study of planetary atmospheres.