No one agrees what it means for a planet to be “habitable”

A little less than a month ago, scientists announced that water vapor had been spotted in the atmosphere of K2-18b, an exoplanet 110 light-years away. Crucially, the planet was in its star’s “habitable zone” (the region around a star that’s temperate enough for liquid surface water, sometimes referred to as the “Goldilocks zone”). But the use of that phrase is pretty controversial. Though humans certainly couldn’t live on K2-18b, there is little agreement among experts about whether some form of extreme microbial life could be found there. It might have been in the “habitable zone,” but no one could agree whether K2-18b was truly “habitable” or not.