Astronomers have observed a distant planet that may be completely covered by a deep-water ocean, findings that advance the search for habitable conditions beyond Earth.
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 has a radius twice that of Earth and is located about 70 light-years away. According to researchers from the University of Cambridge, this chemical mixture is consistent with a water world and a hydrogen-rich atmosphere where the ocean would cover the entire surface, but they do not envision a calm and inviting seascape.
“The ocean may be over 100 degrees [Celsius] or more,” said Prof Nikku Madhusudhan, who led the analysis. “At high atmospheric pressure, such a hot ocean could still be liquid, but it is unclear whether it would be habitable,” he added.
This interpretation is favored in a paper published in the journal Astronomy and Astrophysics Letters, but is disputed by a Canadian team that made additional observations of the same exoplanet known as TOI-270 d. They detected the same atmospheric chemicals, but suggested that the planet would be too hot for liquid water (possibly 4,000 degrees Celsius) and would instead have a rocky surface covered by an incredibly dense atmosphere of hydrogen and water vapor.
Whichever view wins, these latest observations reveal striking insights that James Webb offers into the nature of planets beyond our solar system. The telescope captures starlight filtered through the atmospheres of orbiting planets to give a detailed breakdown of the chemical elements present. From this, astronomers can build a picture of the conditions on a planet’s surface and the likelihood of life surviving there.
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 so if there was an ocean below it would be depleted in the atmosphere. “One interpretation is that this is a so-called ‘hycean’ world with a water ocean under a hydrogen-rich atmosphere,” Madhusudhan said.
Conditions would be very different from those on Earth. TOI-270 d is tidally locked; This means that one side is constantly facing its star, while the other side remains in eternal darkness, creating extreme temperature contrast.
“The ocean would be extremely hot during the day. “The nightside could potentially host habitable conditions,” Madhusudhan said. However, there would be tens or hundreds of times more pressure at the Earth’s surface and an overwhelming atmosphere with steam emerging from the ocean. The waters would be surrounded by a high-pressure icy seafloor and a rocky core beneath it, creating tens to hundreds of times more pressure. It is likely to reach depths of hundreds of kilometers.
Prof Björn Benneke from the University of Montreal carried out additional observations on the planet and questioned the “hycean world” hypothesis. “Our view is that the temperature is too hot for water to be liquid,” he said, adding that the atmosphere contains significant amounts of water vapor; This is too much for the existence of an ocean to be plausible. Benneke estimates that temperatures at the surface could reach 4000°C as water is in a supercritical state and the distinction between liquid and gas blurs. “It almost looks like a thick, hot liquid,” he said.
Both teams detected carbon disulfide, which is linked to biological processes on Earth but can also be produced by other sources. But there was no sign of another biosignature molecule, dimethyl sulfide (DMS).
“We can’t bind [carbon disulphide] to biological activity,” Madhusudhan said. “This is relatively easy to do in a hydrogen-rich atmosphere. But if we can measure this unique molecule, it would hopefully be possible to measure habitable planets in the future.
“We need to be extremely careful about how we communicate findings on such objects,” he added. “It’s easy for the public to get the idea that we’re already coming to life.”
Dr Jo Barstow, an astronomer at the Open University who was not involved in the latest study, said: “The spectra of these minor planets with JWST are really exciting because these are brand new environments for which we don’t have a solar system equivalent.”
Barstow added that further observations to determine the abundance of water vapor in the atmosphere would help clarify the possibility of an ocean. “It’s really fascinating and really nice that two teams look at the same data set and come up with the same chemical structure,” he added.