Twenty years ago you could count all the known planets in the universe on your fingers and toes and recite all their names from memory. Today you’d probably need a calculator and a spreadsheet: thousands of exoplanets — worlds orbiting other stars — fill our catalogues. Astronomers are now poised to find tens of thousands more.
Most of these strange new worlds are overheated, inhospitable and wildly diverse gas giants — “hot Jupiters” or “warm sub-Neptunes” — but some seem to be oversized versions of our own world — rocky, temperate “super-Earths.” A few almost mirror Earth in basic terms of size, mass and orbit. Statistics strongly suggest that planets must circle every star in the sky and even hint that the nearest Earth twin may be less than a dozen light-years away — practically right next door, in interstellar terms. And yet planet hunters still don’t know for certain whether such doppelgangers exist. When and how they finally find out may depend on decisions made in the near future.
Many of the most exciting discoveries to date have come from NASA’s planet-hunting Kepler mission, but that space telescope’s worlds have tended to be too dim and far away for easy follow-up investigation. Kepler’s latest mission phase, dubbed K2, is now looking for planets around closer, brighter stars. That search won’t hit its stride until 2017, however, when NASA plans to launch Kepler’s successor, the Transiting Exoplanet Survey Satellite (TESS). Like Kepler, TESS will search for shadowy worlds that “transit” across the faces of their suns. TESS will probably find thousands of nearby planets, hundreds of which may be small and rocky. A few of those small, rocky worlds will bask in sufficient starlight from the small, dim “M dwarf” stars they orbit to have liquid water on their surfaces.
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