The Hubble Space Telescope can see exoplanets when they pass in front of their stars. Credit: NASA

Astronomers have spotted hints of water raining in the atmosphere of a planet beyond the Solar System.

The discovery is a rare glimpse of water molecules around a distant world that is not much bigger than Earth. Named K2-18 b, the planet is 34 parsecs (110 light-years) from Earth in the constellation Leo. Notably, it lies in the ‘habitable zone’ around its star — the distance at which liquid water could exist, making extraterrestrial life possible in its hydrogen-rich atmosphere.

“That's the exciting thing about this planet,” says Björn Benneke, a planetary astronomer at the University of Montreal in Canada. He is the lead author of a paper describing the discovery that was posted on the arXiv preprint server on 10 September1.

A competing team of scientists reports their own analysis of the same planet on 11 September in Nature Astronomy2. That paper′s lead author, planetary astronomer Angelos Tsairas of the University College London (UCL), says that the finding is exciting because the planet is just twice the diameter of Earth, and because little is known about the atmospheres of such small worlds.

Astronomers have previously found water in the atmospheres of gas-giant exoplanets, but studying a distant planet’s atmosphere gets harder as the planet gets smaller. Scientists have been pushing the limits to try to scrutinize planets that are smaller than Neptune but larger than Earth — a category that turns out to be surprisingly common among the thousands of exoplanets found so far.

Flickering light

Benneke and his colleagues decided to look at K2-18 b because it falls in that range. They used the Hubble Space Telescope to watch as the planet passed in front of its star, temporarily dimming its light, on eight different occasions.

The scientists analysed how the colour of the star’s light changed as it filtered through the planet’s atmosphere. They combined this with data from the Spitzer Space Telescope, which examines more wavelengths of light. The researchers concluded that they were seeing water vapour in the planet′s atmosphere as well as signs that that vapour was condensing into liquid water.

It is the first time astronomers have seen such a water cycle — changing from gas to liquid and back again — on a small, distant world.

The UCL team that authored the second paper analysed the Hubble data from Benneke's group. The observations had been uploaded to a publicly accessible archive immediately after being collected.

The UCL researchers came up with three possible explanations for what they were seeing, any one of which is equally likely. In the first scenario, the planet has no clouds and 20–50% of its atmosphere is water. In the second and third scenarios, which involve different amounts of clouds and other molecules in the atmosphere, the planet’s atmosphere contains between 0.01% and 12.5% water.

Further questions

But the presence of water alone doesn’t mean that a planet is a good place to look for life, a point illustrated by one of Earth’s closest neighbours, Venus. It’s an Earth-sized planet in the habitable zone of its star that once had water vapour in its atmosphere — but the Sun’s rays have stripped away much of that water, leaving its surface barren.

K2-18 b might be equally unpromising. “It is highly unlikely that this world is habitable in any way that we understand based on life as we know it,” says Hannah Wakeford, a planetary astronomer at the Space Telescope Science Institute in Baltimore, Maryland.

Still, finding water in the planet's atmosphere is “extremely exciting”, says Neale Gibson, an astrophysicist at Trinity College Dublin, “and the fact that two teams find the same result is very encouraging”. Future observations, such as those that the James Webb Space Telescope will collect after its planned 2021 launch, should help pin down exactly what this distant world is like.