Hubble data are often released to the public as soon as they are gathered, and in this case, that policy allowed Tsiaras and his co-workers to perform their study. And because the planet transits, some of that starlight passes through its upper atmosphere en route to telescopes here, picking up and transmitting information about the cocktail of gases in K2-18 b’s air.Įight different times between 20, a team led by Björn Benneke of the University of Montreal measured K2-18 b’s transitory atmospheric shimmer with the Hubble Space Telescope, as well as with Kepler and the Spitzer Space Telescope. That star shines with less than 3 percent the luminosity of our own sun, but because K2-18 b orbits so closely to it, the planet receives just 5 percent more starlight than our own. Found by Kepler in 2015, the world nestles in a 33-day orbit around a dim, cool red dwarf star some 110 light-years away, in the constellation of Leo. K2-18 b is slightly more than twice the size of Earth and nearly nine times as massive, likely with a solid core of rock or ice surrounded by an oppressively thick envelope of hydrogen-and other gases, apparently including water vapor. Pinning down just how much water (as well as other gases, such as methane, carbon dioxide and ammonia) is there will require more broadband observations using future space facilities such as NASA’s James Webb Space Telescope, the European Space Agency’s Atmospheric Remote-Sensing Infrared Exoplanet Large-Survey (ARIEL) telescope and a nascent generation of extremely large ground-based telescopes. Tsiaras and his colleagues suggest the water vapor could make up anywhere between a hundredth of a percent to half of K2-18 b’s atmosphere. The Hubble data do not speak with significance about the volume of water on K2-18 b-in the planet’s upper atmosphere, either a whiff of moisture or an ocean’s worth would express a similar signal. 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. “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. 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. 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. 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.Ī 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. In short, transiting worlds have proved to be the keystones in the burgeoning search for Earth’s cosmic twins. 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. 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.
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