In the dim, red light of an alien sun, scientists have found the first evidence for water in the atmosphere of a rocky planet — offering a tantalizing new target in the search for life in the universe.
The intriguing world, which goes by the impersonal designation K2-18b, lies 110 light-years away in the constellation Leo. More important: It sits in its star’s “habitable zone,” where it is bathed in the right amount of warmth to allow for liquid water on its surface.
Twice as large as our own planet and eight times as massive, K2-18b possesses powerful gravity that would make it difficult to walk upon. It orbits close to a red dwarf star, much smaller and cooler than our sun. And aside from water vapor, its atmosphere contains mostly hydrogen gas — a molecule that makes up less than 1 part per million of our own atmosphere.
It is no “second Earth,” said astronomer Angelos Tsiaras, the lead author of a study on the planet published Wednesday in the journal Nature Astronomy, but “this is the best candidate for habitability that we know right now.”
Scientists had previously detected water only in the atmospheres of “gas giants” — huge exoplanets that lack solid surfaces, much like Jupiter and Saturn in our solar system. Rocky exoplanets are smaller, making them harder to find and more difficult to study. Even a planet like K2-18b can be examined only with humanity’s most sensitive space telescope — the Hubble.
When K2-18b was discovered in 2015, Tsiaras and his colleagues thought it would be a good candidate for a form of analysis called transit spectroscopy, which involves studying the changes in a star’s light as a planet “transits,” or passes in front of it.
The planet is so close to its cool host star that it takes just 33 days to orbit. So, month after month, the researchers waited with Hubble to capture the moment of transit.
As the starlight filtered through the planet’s atmosphere, some of it would be absorbed by the gases in the planet’s air. By separating the light into its component parts, the scientists could look for signatures of particular molecules.
What they found suggests that the composition of K2-18b’s atmosphere could range from 0.1 percent water vapor (about the proportion in Earth’s upper atmosphere) to a whopping 50 percent. (At its highest, water vapor concentration in Earth’s lower atmosphere is about 4 percent.)
The Hubble isn’t sensitive to the right types of light to detect other important molecules, such as nitrogen or methane, so the researchers can only speculate about the precise composition of the atmosphere. The planet could have thick clouds, like Venus, that would heat its surface to an intolerable degree. Or its atmosphere might be so thin and insubstantial that it offers no protection from the perils of space, like the atmosphere of Mars.
Researchers must wait for more sophisticated tools, such as the long-delayed James Webb Space Telescope, to reveal more details about the distant world.
Yet co-author Giovanna Tinetti, a colleague of Tsiaras’s at University College London, said it’s possible — even likely — that there’s liquid water somewhere on its surface.
“Everything sort of suggests that the ‘water world’ could be a very good explanation of this planet,” she said, “But we’re going to need more observations. We need to know much more about the planet.”