The smallest, coolest exoplanet known to host water is roughly the size of Neptune, astronomers report in the Sept. 25 Nature. Previously, researchers had found water only on exoplanets that are about the size of Jupiter. The planet HAT-P-11b is just over four times as wide as Earth.
Jonathan Fraine, an astronomer at the University of Maryland in College Park, and colleagues discovered the water after a year and a half of observations with the Hubble, Spitzer and Kepler space telescopes.
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