Exo-forecast: it’s foggy out there. For the first time, water vapour has been detected in the atmosphere of a Neptune-sized planet outside the solar system.
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.
The observation is a “rung on the ladder” down to being able to study the atmospheres of Earth-like exoplanets, says Eliza Kempton of Grinnell College in Iowa.
It isn’t clear why other exo-Neptunes’ atmospheres have been so hard to probe. High clouds or a very compressed atmosphere of heavier molecules such as carbon dioxide might be to blame for the lack of signal. “It has been an annoyance,” says Kempton. “We kept observing these planets that should be interesting and we’ve see nothing.”
Journal reference: Nature, DOI: 10.1038/nature13785
More on these topics: