An article published in “The Astrophysical Journal” reports a study that proposes a new type of habitable planet that was named Hycean. Nikku Madhusudhan, Anjali AA Piette, and Savvas Constantinou of the British University of Cambridge’s Institute of Astronomy started from the study of the exoplanet K2-18b to argue that some planets with conditions between those of super-Earths and mini-Neptunes can be covered in oceans and have hydrogen-rich atmospheres offering the possibility of developing carbon-based life forms. These Hycean planets could potentially be habitable even orbiting outside what is commonly considered a star system habitable zone.
In over twenty years of search for exoplanets, some have been found that show enough similarities to Earth to be considered potentially habitable. This type of research is logical because at the moment the Earth is the only planet on which the presence of life forms is known but many studies are examining the possibility that other types of planets or moons with an underground ocean such as Jupiter’s moon Europa and Saturn’s moon Enceladus host life forms.
Mini-Neptunes seem very common but according to some studies of the past years, the pressure and temperature under their very thick and hydrogen-rich atmospheres would be too high to allow the development of life forms. However, an article published in “The Astrophysical Journal Letters” in February 2020 reported a study conducted by a team of researchers led by Dr. Nikku Madhusudhan on, K2-18b, a mini-Neptune that under certain conditions could support life.
In short, among the many mini-Neptunes discovered, it’s possible that there are combinations of characteristics that are interesting for astrobiologists, and this could also apply to exoplanets classified as super-Earths. If rocky planets can have different conditions like Venus, Earth, and Mars why expect all super-Earths and all mini-Neptunes to be the same?
Dr. Nikku Madhusudhan continued to study these possibilities together with Anjali A. A. Piette, and Savvas Constantinou joined them. The three researchers took into consideration planets with a hydrogen-rich atmosphere and covered by an ocean: the English words hydrogen and ocean combined formed the term Hycean which was proposed to indicate this new class of potentially habitable planets.
Hycean planets can be significantly larger than Earth: up to 2.6 times larger with masses up to 10 times greater. Their atmosphere can reach temperatures of up to nearly 200° Celsius but the conditions in their oceans can remain favorable for life, similar to those in Earth’s oceans.
An interesting conclusion is that the habitable zone for Hycean planets could be larger than that of the rocky planets. This is another element that could greatly expand astrobiological research by offering many candidates for the examination.
Planets close to their star are tidally locked and always show the same face to their star. The consequence is that that face tends to be too hot to allow the development of life forms similar to those on Earth but the night face is far colder. The researchers called this subclass dark Hycean.
Planets far from their star receive little energy from it and for this reason, researchers named this subclass cold Hycean. These planets’ mass could be sufficient to generate enough internal heat to have an ocean of liquid water. High atmospheric pressure would help maintain surface heat. With low pressure, the surface would be frozen but it would still be possible to have an underground ocean, just like on Europa and Enceladus.
The researchers also proposed some possibilities for finding biological signatures in Hycean candidates. Compounds such as methyl chloride and dimethyl sulfide may be abundant in Hycean planets where life forms have developed. When the James Webb Space Telescope is launched, planets such as K2-18b can be adequately examined to verify the Hycean planet theory. If confirmed, it would open up very interesting new study scenarios.