Exoplanet twice the size of Earth 'could be habitable'

A potentially habitable exoplanet more than double the size of Earth has just been identified.

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

A team from the University of Cambridge used the mass, radius, and atmospheric data of the exoplanet and determined it can host liquid water beneath its hydrogen-rich atmosphere.

The discovery opens the search for life up to planets significantly larger than Earth, but smaller than Neptune.

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Dr Nikku Madhusudhan, from Cambridge’s Institute of Astronomy, who led the study, said: “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.

“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.”

Last year two different teams reported the detection of water vapour in K2-18b’s hydrogen-rich atmosphere.

But the extent of the atmosphere and the conditions of the interior underneath remained unknown.

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Because of the size of K2-18b, some suggest it would be more like a smaller version of Neptune than a larger version of Earth.

It is thought a mini-Neptune would have a significant hydrogen envelope surrounding a layer of high-pressure water, with an inner core of rock and iron.

If this envelope is too thick, it would be too hot, and pressure at the surface of the water layer beneath would be far too great to support life.

According to the study published in The Astrophysical Journal Letters, despite the size of K2-18b, its hydrogen envelope is not necessarily too thick and the water layer could support life.

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Astronomers also found that levels of other chemicals such as methane and ammonia were lower than expected in the exoplanet.

But whether these levels can be attributed to biological processes remains to be seen.

The researchers found the maximum extent of the hydrogen envelope allowed by the data is around 6% of the planet’s mass, though most of the solutions require much less.

The minimum amount of hydrogen is about one-millionth by mass, similar to the mass fraction of the Earth’s atmosphere.