NASA’s Kepler mission has been a smash hit. It has discovered thousands of probable exoplanets—worlds orbiting stars other than the sun—more than 100 of which have already been vetted and confirmed. Many of those planets are among the smallest, most Earth-size planets known: of the 25 smallest-diameter exoplanets discovered to date, all but one were spotted by Kepler.
There is just one asterisk tacked to Kepler’s immensely productive haul: the planets are hundreds or even thousands of light-years away, often too distant to investigate in any detail.
Enter TESS. The Transiting Exoplanet Survey Satellite, which NASA green-lit April 5 for a 2017 launch, will survey a much larger swath of sky than its predecessor to uncover a new population of nearby exoplanets that scientists can scan with forthcoming telescopes. The TESS mission’s cost is capped at $200 million.
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The spacecraft will examine a patch of sky some 400 times larger than the field of stars Kepler scans, explains TESS principal investigator George Ricker, a Massachusetts Institute of Technology astrophysicist. From a large, looping orbit around Earth, the satellite’s telescopes will scan the galaxy in a series of stripes. “We end up basically painting the sky,” Ricker says. “Altogether we’ll examine about half a million stars.” Thousands of those stars are within 100 light-years of the solar system.
Like Kepler and the European Corot satellite before it, TESS will search for planetary transits: evidenced by a brief dimming of starlight occurring at regular intervals that betray the shadowing presence of an unseen exoplanet.
The trade-off in surveying such a large area of sky is that TESS may miss some potentially habitable planets. Planets that follow Earth-like orbits around sunlike stars complete an orbit roughly once a year, revealing themselves to Kepler for a brief window each time. With its wandering eye, TESS will miss out on many of those planets with longer-duration orbits. “The goal of Kepler is to answer the very basic question of how many sunlike stars have Earth-like planets orbiting about them in the habitable zone,” Ricker says. “In the case of TESS we’re answering a different question. We’re actually trying to identify the planetary systems that are in the solar neighborhood.”
Many of those planetary systems could still contain habitable worlds, however. TESS ought to discover planets orbiting abundant nearby M dwarf stars, which are dimmer and cooler than the sun. A habitable planet orbiting an M dwarf could nuzzle much closer to its star than a truly Earth-like planet orbiting a sunlike star, so it would complete an orbit much more often and would reveal itself to TESS several times.
Ricker estimates that TESS may discover some 500 to 700 Earth-size and so-called super-Earth planets, of which a handful will be potentially habitable. “There's a lot of uncertainty in that estimate. It may be a relatively small number,” he says. “It may be five or six. It could be 10 or 20. But it will probably be in that range.”
Around the time that TESS compiles a list of nearby exoplanets at the end of its two-year baseline mission, astronomers may have a powerful new eye in the sky to examine the newfound worlds in more detail. NASA’s James Webb Space Telescope (JWST), currently slated to launch in 2018, may be able to tease out the signatures of certain molecules in the atmospheres of nearby planets. Ultimately those kinds of chemical signatures could be used to infer the presence of extraterrestrial life on a planet—that tantalizing possibility, however, may exceed JWST’s capabilities.
Regardless, if TESS can indeed locate hundreds of nearby planets, astronomers will have their hands full for the foreseeable future—finding out what those planets are like, what kinds of habitats they might support and, just maybe, flinging some future probe toward one enticing-looking world. “For decades, centuries to come, these are going to be the ones that are the real targets,” Ricker says.