Astronomers have observed a distant planet that may be completely covered in a deep-water ocean, in findings that advance the search for habitable conditions beyond Earth.
The observations, by Nasa’s James Webb Space Telescope (JWST), revealed water vapor and chemical signatures of methane and carbon dioxide in the atmosphere of the exoplanet, which is twice Earth’s radius and about 70 light-years away. This chemical mix is consistent with a water world where the ocean would span the entire surface, and a hydrogen-rich atmosphere, according to researchers from the University of Cambridgealthough they do not envision a balmy, inviting seascape.
“The sea can be over 100 degrees [Celsius] or more,” said Prof Nikku Madhusudhan, who led the analysis. At high atmospheric pressure, an ocean this hot could still be liquid, “but it’s not clear that it would be habitable,” he added.
This interpretation is favored in a paper published in the journal Astronomy and Astrophysics Lettersbut is disputed by a Canadian team who made additional observations of it exoplanet, which is known as TOI-270 d. They detected the same atmospheric chemicals, but the planet would be too hot for liquid water — possibly 4,000 C — and would instead have a rocky surface with an incredibly dense atmosphere of hydrogen and water vapor.
Whichever view wins, these latest observations show the astonishing insights James Webb is providing into the nature of planets outside our solar system. The telescope captures the starlight filtered through the atmospheres of orbiting planets to provide detailed descriptions of the chemical elements present. From this, astronomers can build up a picture of conditions on a planet’s surface – and the likelihood that life will be able to survive there.
The evidence for TOI-270 d’s ocean is based on the absence of ammonia, which basic chemistry predicts should occur naturally in a hydrogen-rich atmosphere. But ammonia is highly soluble in water and would therefore be depleted in the atmosphere if there was an ocean below. “One interpretation is that it is a so-called ‘hycean’ world – with a water ocean under a hydrogen-rich atmosphere,” said Madhusudhan.
Conditions would be very different from those on Earth. TOI-270 d is tidally locked, meaning that one side is permanently facing its star and the other is bathed in eternal darkness, creating an extreme temperature contrast.
“The sea will be very warm on the day side. The night side could potentially host habitable conditions,” said Madhusudhan. But there would be a crushing atmosphere, with tens or hundreds of times the pressure on Earth’s surface, and steam rolling off the ocean. The waters are likely to reach depths of tens to hundreds of kilometers, with a high-pressure ice seabed, and beneath it a rocky core.
Prof Björn Benneke, from the University of Montreal, made additional observations of the planet and questions the “hycean world” hypothesis. “The temperature in our opinion is too hot for water to be liquid,” he said, adding that the atmosphere appears to contain significant amounts of water vapor – too much for the existence of an ocean to be plausible. At the surface, temperatures can reach 4000C, Benneke estimates, with water existing in a supercritical state, where the distinction between a liquid and a gas is blurred. “It’s almost like a thick, warm liquid,” he said.
Both teams detected carbon disulfide, which is linked to biological processes on Earth but can also be produced by other sources. However, there was no sign of it another biosignature molecule, dimethyl sulfide (DMS).
“We can’t tie down [carbon disulphide] to biological activity,” said Madhusudhan. “In a hydrogen-rich atmosphere, it is relatively easy to make it. But if we can measure the unique molecule, it is promising that we will be able to measure habitable planets in the future.
“We have to be extremely careful about how we communicate findings about this kind of object,” he added. “It’s easy for the public to jump on the idea that we’re already finding life.”
Dr Jo Barstow, an astronomer at the Open University who was not involved in the latest work, said: “Spectra of these small planets with JWST are really exciting because they are brand new environments for which we have no solar system equivalent. “
Barstow added that further observations to determine the abundance of water vapor in the atmosphere will help explain the likelihood of an ocean. “It’s really fascinating and really nice that two teams looked at the same data set and came up with the same chemical composition,” she added.
