A team of scientists from the University of Cambridge, England, has determined, after analysing the mass, radius and atmospheric data of exoplanet K2-18b, that it is possible for this planet to host liquid water in habitable conditions under its hydrogen-rich atmosphere.
This planet, with an average weight between Earth and Neptune, is only 124 light-years away from Earth and... could be full of life. This discovery, published in The Astrophysical Journal Letters, expands the range of planets astronomers can include in their search for extra-terrestrial life.
K2-18b is orbiting a red dwarf star K2-18, right in the middle of the habitable zone, not so hot that liquid water ends up evaporating from the surface, and not so cold that it freezes completely.
Does the name seem familiar?
It was discovered in 2017, however the planet was in the news last year when astronomers found water vapour in its atmosphere. That was an exciting discovery as it was the first potentially habitable planet on which water vapour had been found in its atmosphere. Though, its potential habitability was not as clear at the time. Moreover, we have the clearest example on Mars. Being at the right distance from the star is not the only important characteristic in order to proclaim the habitability of a planet.
It is relatively similar to the Earth: it has a radius 2.6 times larger and a mass 8.6 times that of the Earth. Mind you, we still don’t know what its surface looks like. Researchers cannot say with certainty whether it has a rocky exterior and a thin atmosphere, like Earth, or a dense hydrogen atmosphere over a high-pressure ammonia-water ocean and a metallic core, like Neptune, conditions that are not at all suitable for life.
What’s the planet like?
With the new data it was concluded that the heart of K2-18b can be... anything from an almost pure iron ball with a hydrogen atmosphere that is not too thick to something more like Neptune, an aquatic world with a lighter atmosphere and Earth-like ocean conditions.
The researchers found that the maximum extent of the hydrogen envelope permitted by recent data is about six percent of the planet’s mass. The minimum amount of hydrogen is about one millionth by mass, similar to the mass fraction of the Earth’s atmosphere. The most suitable scenario seems to be an oceanic world, with liquid water under the atmosphere and temperatures similar to those found in the Earth’s oceans.
Is it habitable?
It’s not conclusive proof that K2-18b is habitable, but it does show that exoplanets like this are something that could happen. Therefore, we must not limit our search for habitable worlds to rocky planets of habitable zones. We must be more open-minded.
“Future observations, for example with the James Webb space telescope, will have the potential to refine our findings," wrote the authors. “We argue that planets like K2-18b may have the potential to approach habitable conditions and bio-signature searches should not necessarily be limited to smaller rocky planets".
Reference: Nikku Madhusudhan et al. The interior and atmosphere of the habitable-zone exoplanet K2-18b. The Astrophysical Journal Letters, 2020 DOI: 10.3847/2041-8213/ab7229