- Exoplanet K2-18b could be able to host liquid water, meaning it could be habitable.
- Though water vapour was detected in its atmosphere last year, astronomers weren’t sure if the conditions on the surface were right for liquid water.
- Astronomers also found that levels of methane and ammonia were lower than expected.
An exoplanet more than double the size of Earth could potentially be habitable, astronomers have found. This opens the search for life up to planets significantly larger than Earth, but smaller than Neptune.
A team from the University of Cambridge used the mass, radius, and atmospheric data of the exoplanet K2-18b and determined it is possible for the planet to host liquid water at habitable conditions beneath its hydrogen-rich atmosphere.
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.
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Last year, two different teams reported the detection of water vapour in the hydrogen-rich atmosphere of the exoplanet – a planet outside our Solar System. But the extent of the atmosphere and the conditions of the interior underneath remained unknown.
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.”
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 new 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. Astronomers used the existing observations of the atmosphere, to confirm the atmosphere is hydrogen-rich with a significant amount of water vapour.
They also found that levels of other chemicals such as methane and ammonia were lower than expected. But whether these levels can be attributed to biological processes remains to be seen.
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, 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.
What does it mean if an exoplanet is ‘habitable’?
All forms of life that we know of depend on one critical component: liquid water. So, in the search for life, astronomers focus on planets where liquid water could exist, which they call ‘habitable’.
Every star has a ‘habitable zone’, also called the ‘Goldilocks zone’, where it is not too hot and not too cold. A planet in the habitable zone gets the right amount of energy from the star to support liquid water. Any closer in to the star and water would boil, and any further out and it would freeze.
However, this doesn’t guarantee that liquid water would exist on a planet in the habitable zone. The planet’s atmosphere could be too thick, raising the temperature even higher. And even if liquid water does exist on the planet, habitable doesn’t mean inhabited.