Water found on Neptune-sized world

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.

Gases such as water vapor in a planet’s atmosphere leave their mark by absorbing specific frequencies of light. When HAT-P-11b comes between Earth and its star, the planet’s atmosphere filters out some of the starlight. The astronomers detected water by observing infrared light that disappeared each time the planet passed between Earth and its host, an orange dwarf about 122 light-years away in the constellation Cygnus.

The data also revealed a relatively clear atmosphere that is rich in hydrogen. The abundance of hydrogen jibes with theories of planet formation, in which gas giants form around a rocky or icy core that quickly attracts an atmosphere by pulling hydrogen out of the gaseous disk encircling an infant star.