Astronomers have discovered that a Neptune-sized planet orbiting another star has an atmosphere containing water and clouds. While thousands of exoplanets have been discovered to date, little is known about their atmospheres, especially for bodies smaller than Jupiter. However the composition of a planet's atmosphere can provide valuable clues as to how the exoplanet formed. The atmospheres of Neptune-mass worlds could have arisen from many different sources, resulting in a wide range of possible atmospheric compositions. Using four recent observations from the Hubble Space Telescope and two previous observations from the Spitzer Space Telescope, Hannah R. Wakeford and colleagues now report that the Neptune-sized exoplanet HAT-P-26b contains clear signs of water in its atmosphere. Measuring the abundance of atmospheric water allows researchers to infer the proportion of elements in the atmosphere heavier than hydrogen and helium, a value astronomers refer to as the metallicity; here, it was lower than expected, less than that of Neptune and Uranus, for example. That suggests HAT-P-26b obtained its gaseous envelope late in its formation process, with no substantial pollution by impacting debris afterwards. These findings will help provide a better understanding of how atmospheric composition varies between exoplanets with different masses, and will help constrain models of planet formation.

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