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exoplanet with water space

Earth-dwarfing exoplanets could be capable of supporting alien life

Image credit: UNIVERSITY OF CAMBRIDGE

An exoplanet over twice the size of Earth which holds liquid water and has potentially habitable conditions has made scientists reconsider the types of celestial bodies that may be capable of supporting alien life.

K2-18b, which is 124 light years away, is 2.6 times the radius and 8.6 times the mass of Earth. It orbits its star within the habitable zone, where temperatures could allow liquid water to exist.

Its atmosphere is expected to consist of primarily hydrogen, but the planet made headlines last year when evidence of water vapour in the atmosphere was discovered.

Previously, little else was known about K2-18b and it was thought that if the hydrogen atmosphere is thick enough it would make conditions on the surface below too dense for life to exist.

Now, a team from the University of Cambridge believe they have demonstrated that the thickness of the atmosphere is not too great to eliminate the possibility of life.

They used the existing observations of the atmosphere, as well as the mass and radius, to determine the composition and structure of both the atmosphere and interior using detailed numerical models and statistical methods to explain the data.

While they confirmed the atmosphere to be hydrogen rich, levels of other chemicals such as methane and ammonia were lower than expected.

They took this data and created models of the planet’s interior that could explain the conditions seen in the atmosphere.

“Water vapour has been detected in the atmospheres of a number of exoplanets but, even if the planet is in the habitable zone, that doesn’t necessarily mean there are habitable conditions on the surface,” said Dr Nikku Madhusudhan from Cambridge’s Institute of Astronomy, who led the new research.

“To establish the prospects for habitability, it is important to obtain a unified understanding of the interior and atmospheric conditions on the planet - in particular, whether liquid water can exist beneath the atmosphere.”

The researchers found that the maximum extent of the hydrogen envelope allowed by the data is around 6 per cent of the planet’s mass and the minimum is about one-millionth by mass, similar to the mass fraction of the Earth's atmosphere.

“We wanted to know the thickness of the hydrogen envelope - how deep the hydrogen goes,” said co-author Matthew Nixon, a PhD student. “While this is a question with multiple solutions, we’ve shown that you don’t need much hydrogen to explain all the observations together.”

This study opens the search for habitable conditions and bio-signatures outside the solar system to exoplanets that are significantly larger than Earth, beyond Earth-like exoplanets.

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