The atmosphere of K2-18b, twice the size of Earth, could contain rain clouds (artist’s impression).

ESA/Hubble/M. Kornmesser

Two teams of astronomers have for the first time detected water vapor around a small planet orbiting in the habitable zone of a distant star, and they’ve even found hints of rain in its liquid water clouds. The discovery shows that water, considered an essential ingredient for life, exists in the atmospheres of small exoplanets, which astronomers had suspected but never observed.

“It’s superexciting,” says astronomer Nikku Madhusudhan of the University of Cambridge Institute of Astronomy in the United Kingdom, who was not involved in the studies. “No one would have predicted this, even a couple of years ago.”

Water vapor has been detected previously in the hot gaseous atmospheres of giant exoplanets, but finding it around smaller exoplanets has been a challenge. Astronomers detect the atmospheres by analyzing light from the host star as the exoplanet passes in front of it, or transits. If the planet has an atmosphere, some wavelengths of light will be absorbed by the atoms or molecules in the atmosphere, leaving characteristic lines in the star’s spectrum. The technique works best when the planet is large and has an extended, puffy atmosphere because more starlight will pass through it. Even then, only a few telescopes, such as the Hubble Space Telescope, have the sensitivity to detect the faint lines. Astronomers have used Hubble to observe several smaller exoplanets, between the size of Neptune and Earth, but they’ve come up empty.

Enter K2-18b. The nearby planet, which orbits a red dwarf star about 110 light-years from Earth, has been considered a prime candidate to look for liquid water. Although its star is much cooler than the sun, its close-in orbit—just 33 days long—means it receives almost the same amount of heat as Earth does from the sun. Liquid water could be stable at planet’s surface, and it thus sits in the habitable zone of its star. A team of astronomers from the United States and Canada was allotted some observing time on Hubble to study K2-18b over several years and gathered data from eight transits of the planet in front of the star.

“It needs to be confirmed, but our paper says not only is there water vapor, but there are hints in the spectrum of clouds,” says team leader Björn Benneke of the University of Montreal in Canada. The team, which posted its results yesterday on arXiv and has submitted them to The Astronomical Journal, also acquired data from NASA’s Spitzer and Kepler space telescopes and fed them all into a climate model of K2-18b. The most likely interpretation of the model is that the planet has clouds of condensed liquid water.

“There is actually rain on this planet, like on Earth,” Benneke says. “If you were up there in a hot-air balloon you would probably be quite comfortable, as long as you had some sort of breathing equipment.”

That doesn’t mean K2-18b necessarily has an Earth-like surface with oceans and landmasses. K2-18b is roughly twice the diameter of Earth and eight times its mass. Benneke says it’s more like a mini-Neptune, with a thick and dense gas envelope that perhaps contains a rocky or icy core deep in its interior. “This is not a second Earth,” says Angelos Tsiaras, leader of a team from University College London (UCL), which today published its own analysis of the publicly available Hubble data in Nature Astronomy. Both teams agree on the presence of water vapor and the possibility of clouds. “You would expect it to have clouds,” says Giovanna Tinetti of the UCL team.

Even without an Earth-like surface, Benneke says K2-18b could have a water cycle, with rain falling through the atmosphere, evaporating in a dense and warm gaseous layer lower down, only to rise up again and recondense into clouds.

The result will encourage astronomers to make further searches, and Madhusudhan says a handful of small watery exoplanets might be within reach for Hubble. Beyond that, the hunters will have to wait for Hubble’s successor, the James Webb Space Telescope (JWST), which is due for launch in 2021. “JWST is going to be spectacular,” Madhusudhan says, and will find “dozens” of such planets.

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