In a recent development, astronomers have found a potentially habitable exoplanet, which is more than twice the size of Earth. The planet, named K2-18b, is located 124 light-years away from Earth and measures 2.6 times the radius and 8.6 times the mass of our planet. It orbits its star within the habitable zone where temperatures could allow liquid water to exist.
The K2-18b had been in the news last year as well when two different teams reported the detection of water vapour in the planet’s hydrogen-rich atmosphere. However, the extent of the atmosphere and the conditions of the interior underneath remained unknown at that time. Now, a team from the University of Cambridge used the mass, radius, and atmospheric data of the exoplanet to determine that it’s possible for the K2-18b to host liquid water at habitable conditions beneath its hydrogen-rich atmosphere. The results are reported in The Astrophysical Journal Letters.
Why the discovery is crucial
Because of its size, the K2-18b looks more like a smaller version of Neptune than a larger version of Earth. This study opens the search for habitable conditions to exoplanets that are significantly larger than Earth, beyond Earth-like exoplanets. The atmospheric constraints obtained in this study can be refined using future observations with large facilities such as the upcoming James Webb Space Telescope.
K2-18b could support life
“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.”
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The K2-18b was expected to have a hydrogen ‘envelope’ surrounding a layer of “high-pressure water”, with an inner core of rock and iron. If the hydrogen envelope is too thick, the temperature and pressure at the surface of the water layer beneath would be far too great to support life. However, Madhusudhan and his team have shown that despite the size of K2-18b, its hydrogen envelope is not necessarily too thick and the water layer could have the right conditions to support life.
How they came to the conclusion
They used the existing observations of the atmosphere, mass, and radius of the exoplanet to determine its composition and structure. They confirmed the atmosphere to be hydrogen-rich with a significant amount of water vapour as well as confirmed the levels of other chemicals such as methane and ammonia. The team then used the atmospheric properties as boundary conditions for models of the planetary interior to obtain the range of possible interior conditions 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,” said co-author Matthew Nixon, a PhD student at the Institute of Astronomy. “While this is a question with multiple solutions, we’ve shown that you don’t need much hydrogen to explain all the observations together.”
Madhusudan and his team found that the maximum extent of the hydrogen envelope on the exoplanet is around 6 per cent of its 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,” researchers said.
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