A group of scientists has detected for the first time water vapor in the atmosphere of an exoplanet of comparable size to Earth, which makes him "the best candidate so far to be habitable," according to a study published Wednesday in the journal "Nature Astronomy."

The planet K2-18b has a mass eight times that of the Earth and a size twice as large. It was discovered in 2015 and can be both a rocky body with a wide atmosphere and an icy planet with a high concentration of water inside.

Until now, most of the exoplanets in which it has been possible to determine the composition of their atmosphere were gas giants, due to the difficulties presented by the characterization of smaller bodies.

A University College London team led by Angelos Tsiaras has now analyzed K2-18b through data obtained by the Hubble Space Telescope in 2016 and 2017

"Finding water in a potentially habitable world (…) brings us closer to the answer to the fundamental question: is the Earth unique?" Tsiaras congratulated himself.

Scientists have used a technique known as transit spectroscopy, which analyzes the light that filters through the atmosphere when the planet passes in front of its star for traces of chemical elements.

With that system, researchers have found strong evidence of the presence of water vapor and also suggest that there could be a significant amount of hydrogen in the atmosphere.

«This is the only planet we know for now outside the solar system that has the correct temperature to contain water, an atmosphere and in which water has been detected. That makes him the best candidate so far to be habitable, ”Tsiaras said in a press conference.

"You cannot deduce that there is liquid water on the surface of the exoplanet but I think it is very possible," Giovanna Tinetti, also of the University College of London and co-author of the study, told AFP.

Although the precise composition of the gases has not been determined for now, the models developed by the authors indicate that up to half of their atmosphere could be water.

The studied exoplanet orbits around a red dwarf star, K2-18, about 110 light years away from Earth, in the constellation Leo. It is surely composed of silicates, such as Earth, Mars and Venus, and ice.

Given the high level of activity of the star, scientists believe that the planet is exposed to more radiation than Earth, so it can be a more hostile environment for life.

The researchers hope that the next generation of space telescopes, including the James Webb, will be able to study these types of small planets in more detail.

"Over the next two decades we hope to find many new super-earthworks, so this is likely to be the first discovery of many other potentially habitable planets," said Ingo Waldmann, co-author of the study.

And the adventure with the K2-18b is not over yet: it remains to determine the amount of water vapor present in the atmosphere, the presence or not of evidence of organic chemistry, oxygen, ozone …

«For now, the only planet we know that harbors life is Earth. But this does not mean that to be habitable, a planet must be by force exactly like this one, ”according to Tinetti.

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