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Exploring the Habitable Zone of Other Stars
The discovery of water vapor in the atmosphere of the exoplanet K2–18b, which orbits a red dwarf (M-class) star 110 light-years from Earth labeled K2-18, made it an ideal object of study for scientists seeking to understand the planets that lie within the ‘habitable zone’ of other star systems.
The award of the 2019 Nobel Prize in Physics to Professor James Peebles and Professor Didier Queloz for the discovery of the first exoplanet orbiting a sun-like star back in 1995, sent a clear message to the scientific community that the field of ‘exoplanet-hunting’ had reached maturity.
It is perhaps unsurprising that one of the most discussed aspects of exoplanet analysis is identifying planets that can support life. When we consider the criteria for the evolution and proliferation of life, one molecule is the most important: water.
The very definition of what makes a ‘habitable zone’ around a star is the distance at which a planet would theoretically be able to host liquid water. A planet must be close enough to its star that water does not freeze, and far enough away that it does not boil away. This criteria of not too hot, not too cold, but just right, has led many astronomers to call it ‘the Goldilocks zone.’
With all this considered, the context of the discovery of water vapor in the atmosphere of the exoplanet K2–18b, a world which orbits within the habitable zone of a red dwarf — or M-class — star 110 light-years from Earth, is clear. Also clear is the reason researchers have chosen this particular exoplanet to learn more about habitable zones, their occupants and the chances that their atmospheres could harbor life.
There are other good reasons to investigate K2–18: red dwarfs are the most common stellar objects in the Milky Way; they have low temperatures; and, low-mass planets are common around such stars. This means they are easy to spot and investigate.
Astronomers refer to this as the ‘small star opportunity’ and it has led to the discovery of many exoplanets aside from K2–18b, most notably the Trappist system of seven Earth-like planets.
Super-Earths and Mini-Neptunes
K2–18b was first discovered by astronomers examining data beamed back to Earth from NASA’s Kepler Space Telescope in 2015. It is part of a class of planets known as ‘super-Earths’ - planets with a mass greater than that of Earth, but less than the ice-giant Neptune.
K2–18b has a mass that is approximately nine times that of our home planet, and a radius just over twice its size. Despite this, K2–18b is still the smallest planet outside the solar system upon which we have detected water.
The detection of water vapor in the atmosphere of K2–18b was made last year by a team of astronomers using data collected by the Wide Field Camera 3, which is aboard the Hubble Space Telescope. The discovery was documented in the journal Nature Astronomy.
While the discovery of water vapor within the atmosphere of a super-Earth sparked speculation about the possibility of K2–18b being habitable, many astronomers greeted the news with restraint. A planet must meet other criteria before it can be considered as a possible home for even simple life.
It is also important to note that discovering water vapor doesn’t tell us a great deal about K2–18b. The name ‘super-Earth’ may summon images of a rocky world like our own, but it is more than possible that K2–18b is an icy world like Neptune : ‘super-Earths could be called ‘mini-Neptunes.’
In terms of habitability, life is more likely on a super-Earth than a mini-Neptune. It is vital for researchers to discover more about K2–18b’s atmosphere and to even investigate beneath its surface, to begin to guess whether it could harbor life. A study recently published in the journal The Astrophysical Journal Letters aims to do just this.
Diving Deeper into K2-18b’s Atmosphere and Interior
The team of astronomers that pioneered this deeper investigation into K2–18b did so using the exoplanet’s bulk properties and atmospheric spectrum. As such, they were able to place important constraints upon the planet’s atmosphere, concluding that it is rich in hydrogen molecules with a water content consistent with previous investigations. The astronomers were unable to detect clouds or water vapor ‘haze’ during their investigation.
The constraints placed on K2–18b allowed them to develop three possible models for the planet’s interior.
Firstly, they considered a rocky planet with a core of mostly iron, with the planet surrounded by a hydrogen and helium envelope. This would fit the criteria of a ‘super-Earth.’
Secondly, they modeled K2–18b as a ‘mini-Neptune’, with a significant part of its composition made-up of water-ice.
Thirdly, they considered that K2–18b could be a ‘water-world’, which has minimal atmosphere. The astronomers deemed this last model unlikely due to the difficultly of a planetesimal comprised mostly of water, and without a rocky core, accumulating enough gas and dust to become a fully-fledged planet.
Interestingly, the team found a depletion of methane and ammonia in the atmosphere of K2–18b. This chemical disequilibrium could indicate the presence of processes such as photochemistry and other biochemical processes, and provides evidence for life, although it must be stressed there are other non-life related phenomena that could have given rise to such an imbalance.
K2–18b is a prime target for further investigation by the James Webb Space Telescope when it launches in 2021. As the search for biosignatures becomes a real possibility with improved telescopes, it would seem that astronomers are far from finished with K2–18b.
References and Further Reading
Tsiaras. A, Waldmann. I.P, Tinetti. G, et al, ‘Water vapor in the atmosphere of the habitable zone eight-Earth-mass planet K2–18 b,’ Nature Astronomy, (2019), DOI: 10.1038/s41550–019–0878–9
Madhusudhan. N, Nixon. M.C, Welbanks. L, et al, ‘The Interior and atmosphere of habitable-zone exoplanet K2–18b,’ The Astrophysical Journal Letters, (2020), DOI: 10.3847/2041–8213/ab7229