Cue the music: It’s the circle of life, spacecraft style.
Just as the 9-year-old Kepler spacecraft is about to take its last sips of fuel and go dark forever, the brand new Transiting Exoplanet Survey Satellite (TESS) spacecraft is prepped and ready for Monday’s planned launch. TESS will search for planets around some 200,000 nearby stars, and astronomers hope that it will turn up as many as 20,000 new worlds, including about 50 Earth-sized ones.
TESS and Kepler use the same basic planet-hunting method, known as the “transit” technique: gaze at field of stars, log their brightness regularly and precisely, and check for tiny, temporary dips that could signal passing planets.
The difference: Kepler’s search was narrow and deep. For the main phase of its mission, it stared at a small patch of sky and stayed there for about four years, which allowed it to pick up planets in wide orbits around distant stars and build up a healthy census of planetary systems.
The TESS survey, on the other hand, is wide and shallow. It will scan almost the whole sky, but it will focus on stars that are, on average, 10 times closer than Kepler’s targets. If you imagine the galaxy as one giant root beer float, Kepler is the skinny straw that drinks it in top to bottom and TESS is the wide spoon that skims up all the froth at the top.
Why focus on nearby planets? Sorry, would-be space tourists: TESS isn’t scouting spots for luxury exo-hotels. Actually, TESS is teeing up planets for observation by other, bigger telescopes. Those second-act observations will reveal properties of each planet that TESS can’t see on its own.
But before we get too gaga over TESS, let’s take a shoulders-of-giants moment to appreciate how Kepler changed the way we think about worlds beyond the solar system. Thanks to Kepler, new exoplanet discoveries have become positively routine. Planets are introduced to the public hundreds at a time, with only the most spectacular and exotic (a planet with two suns! A planet evaporating before our eyes!) singled out for a modicum of celebrity.
Which was pretty much the point of Kepler: to make each new planet just another statistic, so that planet hunters could start actually doing statistics, answering questions such as: Is our solar system typical? How does a planet’s size relate to its distance from its parent star? How common are planets, anyway? (Very common, it turns out: there are probably more planets than there are stars in our galaxy.)
But many of Kepler’s target stars were very far away, limiting astronomers’ ability to find out what the newly-discovered planets were really like. Which brings us to TESS. Because TESS favors nearby planets, astronomers will be able to do a degree of follow-up that was impossible for most Kepler planets.
The first step will be combining TESS’s transit observations with gravitational wobble data from ground-based telescopes, which will enable astronomers to estimate how dense each new planet is and make a good guess at whether it is rocky or gaseous. Then, using telescopes such as the James Webb Space Telescope, which is projected to launch in 2020, astronomers will be able to read out the composition of the new planets’ atmospheres. They might even be able to spot “biosignatures” — molecules that hint at the presence of life.
Kepler mission engineers expect that their spacecraft will run out of fuel in a matter of months, though they don’t know exactly when. (It turns out that it’s surprisingly hard to measure fuel supply in space: floats like the ones in automobile gas tanks don’t work, and pressure gauges are unreliable.) But maybe, for a brief time, Kepler and TESS will share the sky: eyes wide open, looking for someplace like home.
Kate Becker is a science writer living in Boston. Contact her at spacecrafty.com, or connect via facebook.com/katembecker or twitter.com/kmbecker.
