British astronomers say they have found that a well-known exoplanet more than twice the size of Earth is potentially habitable.
A team from the University of Cambridge used the mass, radius and atmospheric data of K2-18b to determine that it could host liquid water at habitable conditions beneath its hydrogen-rich atmosphere.
The results are reported in The Astrophysical Journal Letters.
K2-18b is around 124 light-years away and orbits its star within the habitable zone, where temperatures could allow liquid water to exist.
It was the subject of great interest last year when two different teams reported the detection of water vapour in its hydrogen-rich atmosphere. However, the extent of the atmosphere and the conditions of the interior underneath remained unknown.
“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,” says Cambridge’s Nikku Madhusudhan, 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.”
Given its size (2.6 times the radius and 8.6 times the mass of Earth), it has been suggested that K2-18b would be more like a smaller version of Neptune than a larger version of Earth.
A “mini-Neptune” would be expected to have a significant hydrogen envelope surrounding a layer of high-pressure water, with an inner core of rock and iron. If the envelope is too thick, the temperature and pressure at the surface of the water layer beneath would be too great to support life.
Now, Madhusudhan and his team say they have shown that K2-18b’s envelope is not necessarily too thick and the water layer could have the right conditions to support 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.
The researchers confirmed the atmosphere to be 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 for such an atmosphere. Whether these levels can be attributed to biological processes remains to be seen.
The team then used the atmospheric properties as boundary conditions for models of the planetary interior. They explored a wide range of models that could explain the atmospheric properties as well as the mass and radius of the planet.
This allowed them to obtain the range of possible conditions in the interior, including the extent of the hydrogen envelope and the temperatures and pressures in the water layer.
“We wanted to know the thickness of the hydrogen envelope — how deep the hydrogen goes,” says co-author Matthew Nixon. “While this is a question with multiple solutions, we’ve shown that you don’t need much hydrogen to explain all the observations together.”
The researchers found that 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. In particular, a number of scenarios allow for an ocean world, with liquid water below the atmosphere at pressures and temperatures similar to those found in Earth’s oceans.
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