Hubble telescope observes rainy water on a distant world

Astronomers have spotted traces of water that was raining in the atmosphere of a planet beyond the solar system.

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

"That's what's exciting on this planet," says Björn Benneke, planetary astronomer at the University of Montreal in Canada. He is the main author of an article describing the discovery that was published on the pre-print server arXiv on September 10th.¹.

A team of competing scientists presents its own analysis of the same planet on September 11 Nature Astronomy². The principal author of this document, the planet astronomer Angelos Tsairas of University College London (UCL), says that this discovery is exciting because the planet is only twice the diameter of the Earth and we know little about the atmospheres of such small worlds.

Astronomers have already found water in the atmospheres of giant exoplanets, but it is increasingly difficult to study the atmosphere of a distant planet as it is being reduced. Scientists have pushed the boundaries to try to scan planets smaller than Neptune but larger than Earth – a category that is surprisingly common among the thousands of exoplanets discovered so far.

Flickering light

Benneke and his colleagues decided to look at K2-18 b because it falls within this range. They used the Hubble Space Telescope to observe the passage of the planet in front of its star, temporarily lowering its light, on eight different occasions.

Scientists have analyzed the evolution of the color of the light of the star that filtered through the atmosphere of the planet. They combined this with data from Spitzer Space Telescope, which examines more wavelengths of light. The researchers concluded that they saw water vapor in the atmosphere of the planet as well as signs of condensation of this vapor in liquid water.

This is the first time that astronomers see such a cycle of water – going from gas to liquid and again – in a small world and far away.

The UCL team that drafted the second article analyzed Hubble data from the Benneke group. The observations had been uploaded to publicly accessible archives immediately after their collection.

UCL researchers have proposed three possible explanations for what they saw, each of them being equally probable. In the first scenario, the planet has no clouds and 20 to 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 atmosphere of the planet contains between 0.01% and 12.5% ​​water.

Any other questions

But the presence of water alone does not mean that a planet is a good place to look for life, a point illustrated by one of the closest neighbors of the Earth, Venus. It is a planet the size of the Earth located in the habitable zone of its star that once had water vapor in its atmosphere – but the rays of the Sun have removed much of this water, leaving its surface sterile .

K2-18 b could be just as unpromising. "It is highly unlikely that this world will be liveable in any way, based on life as we know it," says Hannah Wakeford, a world astronomer at the Space Telescope Science Institute in Baltimore, Maryland.

Nevertheless, finding water in the atmosphere of the planet 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 launch in 2021, should help pinpoint exactly what this distant world looks like.