The exoplanet K2-18b, about 124 light-years from Earth, is a kind of mini-Neptune, as astronomers discovered this past year. It is seven to ten times heavier than Earth and its radius is 2.7 times larger. K2-18b orbits its host star, a red dwarf, once every 33 days. Thus, it is located in its star’s habitable zone.
For astronomers, however, it has one other special noteworthy feature: hydrogen, helium, and water vapor have been detected in its atmosphere. In the media, K2-18b has even been described as “Earth 2.0,” which it very definitively is not. The researchers who studied it in 2019 described it at the time as “very likely more harmful to life” than Earth.
Hydrogen in the atmosphere is not generally considered especially friendly to life: it is very reactive; any available oxygen molecules would be immediately converted to water, thus ruling out any sort of breathing as we know it. But does that mean we should write off K2-18b as a candidate for extraterrestrial life? That would be a premature conclusion according to scientists. As an experiment, they tested how life might function in a hydrogen-rich atmosphere in a laboratory.