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The Hubble telescope spies water raining on a distant world

  The Hubble Space Telescope after the capture of the giant observatory of the space shuttle Atlantis.

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

Astronomers have discovered 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 not much larger than the Earth. The planet is called K2-1

8b and is 34 parsecs (110 light years) from Earth in the constellation Leo. It is remarkable that it lies in the "inhabited zone" around its star – the distance to which liquid water can exist, which makes extraterrestrial life possible in its hydrogen-rich atmosphere.

"That's exciting about this planet," says Björn Benneke, a planetary astronomer at the University of Montreal, Canada. He is the lead author of a paper describing the discovery that was published on the arXiv suppression server on September 10 1 .

A competing research group reports on its own analysis of the same planet on September 11 in Nature Astronomy 2 . This essay's lead author, planet astronomer Angelos Tsairas of University College London (UCL), says the discovery is exciting because the planet is only twice the diameter of the earth, and because little is known about the atmosphere in such small worlds.

Astronomers have previously found water in the atmospheres of gas giant exoplanets, but studying a distant planet's atmosphere becomes harder as the planet becomes smaller. Scientists have pushed the boundaries to try to study planets smaller than Neptune but larger than Earth – a category that proves to be surprisingly common among the thousands of exoplanets found to date.

Flickering lights

Benneke and his colleagues decided to look at K2-18 b because it falls within 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 researchers analyzed how the color 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 investigates more wavelengths for light. The researchers concluded that they saw water vapor in the planet's atmosphere and signs that the steam was condensing into liquid water.

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

The UCL team that wrote the second paper analyzed the Hubble data from Benneke's group. The observations had been uploaded to a publicly available archive immediately after they were collected.

The UCL researchers came up with three possible explanations for what they saw, some of which are 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, involving 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 does not mean that a planet is a good place to look for life, a point illustrated by one of the Earth's closest neighbors, Venus. It is an earth size in the habitable zone of its star that once had water vapor in its atmosphere – but the sun's rays have removed much of that water and left its surface barren.

K2-18 b may be equally unreacted. "It is highly unlikely that this world is habitable in any way we understand based on the life we ​​know it," said Hannah Wakeford, a planetary astronomer at the Space Telescope Science Institute in Baltimore, Maryland.

Still 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 results is very encouraging." Future observations, such as those that the James Webb Space Telescope will collect after the planned launch in 2021, should help determine exactly what this distant world is.

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