Water discovered for first time in atmosphere of habitable exoplanet

Waterworld: up to 50% of the atmosphere of K2-18b may be composed of water. ESA/UCL
An international team of astronomers has discovered a new solar system with a planet that could be habitable, a Spanish astrophysicist who led the research said Thursday.
Three new planets were discovered orbiting GJ 357, a red dwarf — a small and cooling star — 31 light years away, relatively close in space terms, said Rafael Luque of Spain\’s Institute of Astrophysics in the Canary Islands.
The discovery was also reported by NASA, whose TESS planet-hunting satellite made it possible.
The planet known as GJ 357d — the furthest away from the star — was particularly intriguing as researchers estimate it could be habitable. The other two are deemed too hot.
Signs of habitability in any planet include a rocky terrain, a size similar to Earth and a distance from their sun — the temperate "Goldilocks" zone neither too close nor too far — that allows the right temperature for liquid water, a key requirement for life.
Given its distance from its star, similar to that of Mars to our Sun, researchers estimate the planet has temperatures of -53 degrees Celsius (-63.4 Fahrenheit), Luque told AFP.
"That seems a little cold at first," he said.
But "if this planet had an atmosphere (unlike Mars), it could retain the heat it receives from its star, and water could be liquid."
Researchers also estimate GJ 357d could be roughly the same size as Earth or up to twice the size.
It is not the first potentially habitable planet to have been discovered close to us.
In 2016, the discovery of Proxima b at a mere four light years from the Solar System made waves.
But there is a hitch.
Proxima b and GJ 357d were discovered via so-called radial velocity, which involves looking for signs of a wobble in a star from the gravitational tug of an orbiting planet.
But Luque says the method is not precise enough to ascertain whether it actually is habitable.
As things stand, in order to measure its size, density and composition, the planet has to pass directly between its star and an observer, the so-called "transit" method, he says.
That has not been possible for Proxima b and other nearer potentially habitable planets, Luque says.
In the coming months, Luque and his team will be working to try and catch GJ 357d in "transit" to try and confirm it as a habitable planet.
"The probability that a planet passes in front of a star from our line of vision on Earth is pretty small," he adds.
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