Four of seven Earth-sized exoplanets may have large quantities of water

Four of seven mysterious worlds orbiting a nearby star might bear large quantities of water, scientists have revealed, offering a tantalising boost to the possibility of finding life beyond our solar system.

Just 39 light-years away from Earth, the solar system of seven Earth-sized planets orbiting the small, cool star known as Trappist-1 was discovered earlier this year, with the announcement met with excitement by experts.

While scientists said all of the planets could harbour some water, given the right atmosphere, three of the four outermost planets lie within the star’s so-called “habitable zone” – the distance at which temperatures are suitable for liquid water to exist – making them the most likely candidates.

But it remained unclear whether any of the planets were indeed wet. Now scientists say they are a step closer to finding out.

“What we found tells us that probably the [three] inner planets are not good planets to search for life,” said Vincent Bourrier, first author of the study from the University of Geneva.

Writing in the Astronomical Journal, Bourrier and an international team of researchers reveal how they sought to calculate whether the planets could bear water using the Hubble space telescope to probe the levels of ultraviolet radiation emitted by Trappist-1.

“Knowing how much energy is emitted by a star in the ultraviolet is very important because it can strongly affect the atmospheres of the planets,” said Bourrier, adding that the lower energies of ultraviolet radiation can break water molecules into hydrogen and oxygen, while higher energies can be absorbed by the gases, allowing them to escape into space.

The team fed data on the star’s UV radiation and the distances of the planets into computer models which also took into account other factors, including how the star’s radiation and its habitable zone have shifted over millennia.

The results reveal that over 8bn years all but the outermost planet could have lost the equivalent of 20 times the amount of water contained in the Earth’s oceans. But, they add, if water loss drops once a planet enters the habitable zone, the outer four planets could have lost less than three times the water in Earth’s oceans over 8bn years, suggesting it could still be present in substantial amounts. What’s more, they found that the outer planets could have “regained” significant amounts of water through volcanic events that released water from the planets’ interiors.

But, said Bourrier, the calculations assume the planets start with an infinite amount of water, adding that further work on the density of the planets is needed to figure out how much water they actually formed with, and hence the amount of water remaining.

There is more. “When the hydrogen escapes it can form a very extended, comet-like tail around the planet,” said Bourrier. “If you detect such a cloud escaping from the Trappist-1 planets, it could inform us on the presence of water that is being [lost] into the atmosphere.”

To probe the possibility, the team looked at the UV radiation emitted from Trappist-1 as the second planet out passed in front of the star – a dip in such radiation would have suggested the planet had a hydrogen “halo” absorbing energy. Unfortunately the observations were inconclusive.

“[The star’s] energy is changing over time, so that makes it difficult to search for this hydrogen cloud,” he said, adding that the team is planning further observations looking at both UV and infrared wavelengths to hunt for signs of water in the planets’ atmospheres.

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Ignas Snellen, an astrophysicist at the Leiden Observatory in the Netherlands who was not involved in the study, described the Trappist-1 system as exciting. “We are all very eager to learn more about the possible climates on these planets, and in particular whether they could host liquid water,” he said.

But while Snellen welcomed the research, he said that further observations were crucial.

“Personally, I think that you can make all the calculations you want, but in the end we need measurements,” he said.

Snellen added that the mystery could soon be solved, pointing out that the James Webb Space Telescope is set to launch next year and is expected to shed light on the atmospheres of the planets.

“If one or more of these planets turn out to contain water it will be an enormous breakthrough,” said Snellen. “We can really start to think seriously about whether these worlds may harbour life.”