Planet K2-18b is like Earth, but not

Astronomers have detected water vapour in the atmosphere of a rocky planet orbiting in the habitable zone of a star (the region that is neither too close nor too far from the star for it to be possible for planets to have liquid, surface water) 110 light-years away.

“This is the first detection of this kind,” says Angelos Tsiaras, an astronomer at University College London (UCL), UK, and lead author of a paper published in the journal Nature Astronomy. 

“This is the only planet we know outside the Solar System that has the correct temperature to support water, has an atmosphere, and has water in it,” he says, “making this planet the best candidate for habitability that we know right now.”

That said it’s not Earth’s twin. 

“We’re talking about a planet twice the size of the Earth and eight times the mass,” Tsiaras says. 

Not only does that mean that the surface gravity is nearly twice that of Earth, but even though its star is cool, dim, and red, it probably receives significantly more ultraviolet radiation than we do on Earth. 

“It’s not quite your vacation destination,” says Ingo Waldmann, also of UCL. “We would all get cancer relatively quickly.” 

Not that this means the planet can’t support life. “Life there may have evolved differently,” Waldmann says.

The planet, known as K2-18b, was discovered in 2015 when the Kepler Space Telescope observed its silhouette crossing between us and its star, once every 33 days – putting it just the right distance from its dim sun to receive about the same amount of solar energy as Earth.

T
discovery of water came from using the Hubble Space Telescope to obtain detailed spectroscopic analysis of the star’s light when the planet crosses in front of it. “The atmosphere leaves a characteristic fingerprint, and we measure the differences,” Tsiaras says.

How much water is present, however, is uncertain, because simply detecting it stretched the Hubble’s abilities to their limit. All that the scientists are sure of is that water is present, though it could be anywhere from 0.01% of the total atmosphere to 50%.

Giovanna Tinetti, another researcher from UCL, admits that this is an extremely large range. But, she says, “with Hubble, the only thing we can see is that the signature is there”. 

Computer models of the planet’s atmospheric signature also suggest that it contains significant amounts of hydrogen. But how much it has of either gas remains a mystery.

One possibility, Waldmann says, is that the atmosphere is hydrogen and a lot of water, “but nothing else.” Another is that it’s mostly hydrogen and some other as yet undetectable gas, with “a little bit of water.” 

“Probably the answer is somewhere in between those,” he says.

Finding the answer, Tsiaras suggests, will probably need to await the launch of the next generation of space telescopes, or the completion of ground-based instruments much larger than the best ones currently available.

“Between [NASA’s] James Webb Space Telescope, [ESA’s] ARIEL space telescope, and other measurement in the future, we’ll probably be able to find this out,” Tinetti says.

Related reading: Exoplanet with the right conditions of life?