“This is data that we acquired and in some ways we had done all the work," Benneke says. "The UCL team decided to just take our data and write their own publication of this. It's not illegal or anything, but it's bad practice."
On Tuesday, Benneke and his colleagues published a paper to the arXiv preprint server that demonstrates how the team’s analysis also detected water vapor around K2-18b. But the paper, which is currently under review for publication, goes a bit further than the UCL team’s report. Benneke and his colleagues’ models suggest there are clouds around the planet, which means that it could rain on K2-18b. Although Benneke’s team was beaten to publication by UCL, he says there is a silver lining. The fact that both teams independently arrived at the same result is a strong indicator that the water vapor is actually there.
K2-18b may be the most habitable exoplanet ever found, but it is unlikely we’ll find life or even liquid water on its surface. At this point it just meets the most criteria on astronomers’ and astrobiologists’ laundry list of what’s necessary to support life. “This is the planet that satisfies more requirements than any other we know right now,” says Tsiaras. But, he added, “this is definitely not a second Earth.”
Specifically, it has an atmosphere with water vapor, orbits its host star at a distance that could support liquid water, and has a similar temperature to Earth. But just because Tsiaras and his colleagues refer to the planet as a “super-Earth” exoplanet, a term often but not always reserved for planets up to 10 times more massive than Earth, does not mean it is Earth-like on the surface.
Abel Mendez, director of the Planetary Habitability Laboratory at the University of Puerto Rico, says K2-18b’s habitability ultimately depends on whether it turns out that the planet has a dense rocky core and a thick atmosphere like Neptune or is covered in a planet-wide ocean. Only in the latter case could the planet possibly be inhabited by microbial life.
“K2-18b is one of the potentially habitable planets in our Habitable Planets Catalog, but it’s not one of the top 21,” Mendez says. “It’s the first planet in our catalog to have an atmosphere detected, but it’s not at all a good candidate for life.”
Even the planet’s status as a “super-Earth” is up for debate. Sara Seager, an astrophysicist and planetary scientist at MIT, says she would personally call K2-18b a “mini-Neptune” or “sub-Neptune” rather than a super-Earth. Like super-Earth, this designation is metonymic—it refers to planets that have a similar size and mass to Neptune, but not necessarily the same composition. Neptune is four times the size of Earth and 17 times more massive. It has a thick atmosphere rich in hydrogen and helium, and is believed to have a dense rocky core wrapped in a water- and ammonia-rich mantle.
The biggest argument in favor of the planet being a mini-Neptune rather than a super-Earth has to do with its radius, says Drake Deming, an astronomer and exoplanet atmosphere expert at the University of Maryland. In a 2017 survey of hundreds of exoplanets detected by the Kepler Space Telescope, the astronomer Benjamin Fulton and his colleagues found that most of the planets fell into one of two groups: Those with a radius less than 1.5 times the radius of Earth, and those with a radius of two to three times that of Earth. Another study found that most exoplanets greater than 1.6 Earth radii aren’t rocky, making them more like mini-Neptunes than super-Earths.
Giovanna Tinetti, an astronomer at University College London and one of the authors on the Nature Astronomy paper, acknowledged that K2-18b has some of the characteristics of a mini-Neptune, but stood by its designation as a super-Earth because its density more closely matches that of our home planet. But Tinetti also says it’s “entirely possible” that K2-18b is covered in a planet-scale ocean, which would make it a bigger version of our own solar system’s water worlds, Titan and Europa.