The Mystery of K2-18 b: An Earth-Like Planet's Atmospheric Properties

Saturday - September 23 2023, 20:17 UTC - 1 year ago

K2-18 b is a exoplanet located 120 light-years away in the Leo constellation in the habitable zone. It was identified by astronomers as a sub-Neptune with a size similar to Earth, and is believed to be a Hycean exoplanet, meaning it has the potential of having oceans of liquid water and possibly be home to life. However, its large size could mean that the ocean is too hot to be habitable.

The first insight into the atmospheric properties of this habitable-zone exoplanet came from observations with NASA’s Hubble Space Telescope, which prompted further studies that have since changed our understanding of the system.

K2-18 b orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light-years from Earth in the constellation Leo. Exoplanets such as K2-18 b, which have sizes between those of Earth and Neptune, are unlike anything in our solar system. This lack of equivalent nearby planets means that these ‘sub-Neptunes’ are poorly understood, and the nature of their atmospheres is a matter of active debate among astronomers.

The suggestion that the sub-Neptune K2-18 b could be a Hycean exoplanet is intriguing, as some astronomers believe that these worlds are promising environments to search for evidence for life on exoplanets.

While K2-18 b lies in the habitable zone, and is now known to harbor carbon-bearing molecules, this does not necessarily mean that the planet can support life. The planet’s large size — with a radius 2.6 times the radius of Earth — means that the planet’s interior likely contains a large mantle of high-pressure ice, like Neptune, but with a thinner hydrogen-rich atmosphere and an ocean surface. Hycean worlds are predicted to have oceans of water. However, it is also possible that the ocean is too hot to be habitable or be liquid.

“Although this kind of planet does not exist in our solar system, sub-Neptunes are the most common type of planet known so far in the galaxy,” explained team member Subhajit Sarkar of Cardiff University. "We have obtained the most detailed spectrum of a habitable-zone sub-Neptune to date, and this allowed us to work out the molecules that exist in its atmosphere." .