I first wrote about K2-18, a red dwarf star some 11 light-years from Earth, and its two companion planets in December 2017. At that time, the outermost of the two planets, called K2-18b or EPIC 201912552 b and discovered in 2015, was the subject of a study to determine its mass in an attempt to better understand the planet’s possible atmospheric properties and bulk composition. This was of particular interest to scientists as K2-18b lay within its parent star’s habitable zone – where liquid water might exist on the planet’s surface.
That study ultimately revealed K2-18b has a mass of around 8 times that of Earth, putting it in the “super-Earth” category of rocky worlds, with a diameter roughly 2.3 times greater than Earth’s (see: Space Sunday: Exoplanets Update). Since then, K2-18b has continued to be the subject of study – and it has now become the first exoplanet thus far discovered confirmed to have water vapour, mostly likely liquid water clouds, within its atmosphere.
The news came via two independent studies that have been carried out using the data gathered by the Hubble Space Telescope (HST). The first study, written by the team who originally gathered the data, appeared on September 10th, 2019 on arXiv.org, but has not been peer-reviewed. The second study – which has been peer-reviewed – appeared in the September 11th edition of Nature Astronomy.
The team responsible for gathering the data – led by Björn Benneke, a professor at the Institute for Research on Exoplanets, Université de Montréal – did so after applying to use Hubble to observe K2-18b shortly after its discovery. They were ultimately granted telescope time in in 2016 and 2017, using Hubble to gather data in the light from the red dwarf star, and how that light changed under the influence of any atmosphere surrounding K2-18b as it transited in front of the star. Spectrographic analysis of the data confirmed the planet has a fairly dense atmosphere rich in hydrogen and helium – and which also contains the molecular signature of water.
After gathering the data, Benneke’s team wanted time to carry out further observations to both confirm what they had found and make additional discoveries. In the meantime, their findings were available for others to study – which is exactly what a team led by Dr. Angelos Tsiaras based at the University College London (UCL), UK did.
Using independent means of analysing the data, both teams reached the same overall conclusions concerning the major finds within K2-18b’s atmosphere – although they come to different conclusions as to the planet’s likely form. The UCL specify K2-18b as a rocky planet with a dense atmosphere, between 0.01% and 50% of which is water vapour. By comparison, the amount of water vapour in our atmosphere is put at between 0.1% and 4% – so, K2-18b could have anything from a comparable amount of water vapour in its atmosphere to Earth through to being a completely flooded world.
By contrast, Benneke’s team believe the planet is more of a “mini-Neptune”: a planet with a small, solid core surrounded with a thick atmosphere that is predominantly hydrogen / helium in nature, with only trace amounts of water vapour – albeit enough to create liquid water clouds, and possibly even rain. However, the idea that the planet is a mini-Neptune is somewhat at odds with other findings about the planet – such as the December 2017 study.
There is also some tension between the two teams. While Benneke acknowledges his team’s research was open to others to use, he is somewhat aggrieved the UCL team did not bother to contact him or his team concerning their work or their intentions. However, he also sees the results of the UCL’s work as positive in respect to understanding the nature of K2-18b.
The presence of liquid water in the planet’s atmosphere doesn’t automatically mean it is home to life. There are some significant issues around this. For one thing, while the plant is within the habitable zone, the precise surface temperature has yet to be determined, and could range from -73ºC to +47ºC (-100ºF and +116ºF), meaning it could be colder or hotter than the coldest / hottest places on Earth.
There’s also the fact that the planet is so close to its parent, orbiting once every 33 days, that it is likely tidally-locked with its star. This means one side of the planet will be in perpetual sunlight, and the other in perpetual darkness – something that could well give rise to extreme weather conditions. Finally, there’s the fact that K2-18 is a red dwarf star. These, as I’ve noted before, can be exceptionally violent, and flares and coronal mass ejections from the star are likely to both expose the planet to high levels of radiation and could strip away its atmosphere over time, although it is possible K2-18b’s atmosphere might be dense enough to help it withstand at least some of this stripping away.
Finding water on a potentially habitable world other than Earth is incredibly exciting. K2-18b is not ‘Earth 2.0’ as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental question: Is the Earth unique?
– Dr. Angelos Tsiaras (UCL Centre for Space Exochemistry Data)
co-author of the UCL study on K2-18b
The next phases in studying K2-18b will likely come in the mid-to-late 2020s. Benneke and his team are already planning to continue their work using NASA’s James Webb telescope, due to be launched in 2021, while Giovanna Tinetti, a member of the UCL team studying K2-18b also happens to be the Principal Investigator for Europe’s Atmospheric Remote-sensing Infra-red Exoplanet Large-survey (ARIEL). She has already indicated the planet will be target for study by that mission when it launches in 2028.