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Twenty years ago, almost to the day, two competing teams of astronomers independently discovered the first known transiting exoplanet—a world that, viewed from Earth, passed across the face of its star, casting a shadow toward watchful telescopes here. Two decades later, transits have become the lifeblood of exoplanet studies, yielding thousands of worlds via space telescopes such as NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS) missions and allowing researchers not only to gauge a planet’s size and orbit but also its density and bulk composition. In short, transiting worlds have proved to be the keystones in the burgeoning search for Earth’s cosmic twins. Back in 1999, however, the notion that these exoplanetary shadows would be detectable at all was so fantastic that validating it took the separate efforts of two groups.

A similar scenario is now playing out again: Two scientific teams have announced their independent discovery of water—the foundation of biology as we know it—in the atmosphere of a transiting planet dubbed K2-18 b. The planet orbits in the habitable zone of its star, the sweet spot in which starlight may sufficiently warm a world to allow water to pool and flow on its surface. A milestone in the search for alien life, the result portends a near future in which astronomers will use new, advanced telescopes on the ground and in space to more deeply study the most promising planets around our sun’s neighboring stars.

“This is the only planet right now that we know outside the solar system that has the correct temperature to support water, that has an atmosphere and that has water in it,” says Angelos Tsiaras, an astronomer at University College London and lead author of one of the studies, which was published today in Nature Astronomy. Tsiaras and his colleagues used sophisticated computer models to tease out signs of water vapor on K2-18 b from data gathered by the Hubble Space Telescope, making the planet, he says, “the best candidate for habitability” presently known.

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Category: Science