The Kepler spacecraft is a wonderful asset. Despite its ailing health, it has managed to provide us with 3,000 candidate planets which permit a statistical look at the census of planets in the local Galaxy. We’ve learned planets are actually pretty common – they’re the rule rather than the exception. But for all of its contributions to science, and they have been numerous, the candidate planets Kepler has found tend to orbit stars that are far away. This is because the spacecraft stares at the same patch of sky continuously. If you take any patch of the sky, there will be far more distant stars than near stars in that patch because the volume of space within that patch increases with distance. So while the Kepler results are splendid for estimating the frequency of planets, they are not particularly useful for providing us with targets for follow-up study.
What we need is a mission to identify transiting planets around nearby and/or bright stars. It seems NASA has answered the call for such a mission and has, as of today (April 5, 2013), selected the TESS mission for development and launch in 2017.
The Transiting Exoplanet Survey Satellite (TESS) was conceived to address this problem. It will observe an area of sky 400 times that of Kepler at a time, studying two million stars with brightnesses V < 12, as well as the closest 1,000 M dwarf stars — essentially all red dwarfs within 30 pc. The mission will last two years and will uncover perhaps 2,000 planet candidates, a few hundred of which could be Earth-sized. The number of discoveries could be as much as from Kepler, but around nearby and bright stars where these planets would actually be feasible to study and examine for the possibility of life with transmission spectroscopy. Ground-based spectrographs would have a much easier time confirming the planets with Doppler spectroscopy.
It’s important to note that TESS will not stare at the same part of the sky for an extended time to detect Earth-analogues as Kepler does. TESS will move from one part of sky to another, observing each part for only a few months, so the orbital periods of discovered planets will be on the order of weeks – perfect for habitable planets around M dwarfs, but less so for habitable planets around G or K dwarfs. TESS’s wide, shallow approach to finding planets nicely complements Kepler‘s narrow, deep search for planets.
The study of transiting planets around M dwarfs is a worthwhile pursuit. M Dwarfs emit most of their energy in the infrared, where absorption lines of water and carbon dioxide reside and are prominent. The planets will necessarily be in shorter period orbits. This is hugely exciting for extrasolar planet science. It’s quite possible that the first extrasolar biosphere, and perhaps even the first extrasolar planet that is visited by a human spacecraft in the distant future, will be discovered by TESS. It is my opinion that this spacecraft is much more exciting than even Kepler.