Previously, astronomers had found water in the atmospheres of so-called hot Jupiters, which are as massive or larger than Jupiter and orbit scorchingly close to their stars. But similar efforts for smaller planets had been stymied by hard-to-interpret data.
Now, Jonathan Fraine of the University of Maryland, College Park, and colleagues have used the Hubble and Spitzer space telescopes to study HAT-P-11b, a planet 123 light-years away in the constellation Cygnus. With a radius four times that of Earth, it’s the smallest yet to have its atmosphere probed.
In addition to water vapour, they found a lot of hydrogen, and some oxygen and heavier elements.
“It does look like a Neptune clone, only hotter,” says planetary scientist Julianne Moses of the Space Science Institute in Seabrook, Texas. This molecular profile suggests that the planet formed far from its star, then moved closer.