First traces of atmospheric water vapour detected on 'super-Earth' in habitable zone

Roughly 111 light-years away, toward the constellation of Leo, lies a dim star less than half the mass of our sun, with two planets in close orbit. Now, astronomers have revealed that one of those planets has an atmosphere containing water vapour.

The discovery, published Wednesday in the journal Nature, is the first of its kind. While other, larger, gaseous planets — called hot Jupiters — have revealed some hints about the chemical elements contained in their atmospheres, this is the first time water vapour has been detected on a potentially habitable planet, an historic first.

The planet is called K2-18 b, and orbits its parent star — K2-18 — at roughly 0.14 astronomical units (one AU is the distance from our sun to Earth). Because it is so much closer to its parent star, it orbits quite quickly: one year on K2-18 b is equal to just 33 days here on Earth.

K2-18 b is roughly eight times the mass of Earth and about twice the radius, making it a super-Earth or sub-Neptune in the somewhat loose classification of exoplanets.

But what's most important is that K2-18 b resides within the star's habitable zone, a region around a star where liquid water can exist on a planet's surface. Depending on what kind of star it is, the distance varies. In this case, K2-18 b orbits the dwarf star within what would be Mercury's orbit in our solar system. 

The search is on

K2-18 b was discovered in 2015, and due to its proximity to Earth and its dim star, it was considered a good candidate for detecting an atmosphere. The team of astrophysicists examined data from the Hubble Space Telescope collected in 2016 and 2017. While not ideal for detecting a wide range of molecular elements in distant exoplanets, Hubble is, however, capable of detecting water vapour. 

The team additionally used spectroscopic data (where light is broken up into different wavelengths that reveal various molecular elements). What they found was strong evidence that K2-18 b had water vapour in its atmosphere.

Running several models, they concluded that three conditions were equally likely to account for the water vapour detected: that it could be a type of "waterworld," with an abundance of water; that it could contain gases such as hydrogen and nitrogen, with a little water; and finally, that it could contain very little water, but with high-altitude clouds.

Those three models mean that the water that was detected could range from 0.01 per cent to 50 per cent.

It's a large range, to be sure.

"We don't really know how much water there is," said Angelos Tsiaras, lead author of the study. "This is related to the size of the atmosphere. We need wider wavelengths to cover it."

Hubble finds water vapour on distant exoplanet

While Hubble may only be able to scan narrow wavelengths in its observations, a new space telescope is set to revolutionize astronomy: the James Webb Space Telescope. That, the authors say, may blow the search for atmospheres around potentially habitable exoplanets wide open.

One small step

The detection of water vapour around a potentially habitable planet is being hailed as a first step in what will become a wider understanding of exoplanets.

"The first evidence for an atmospheric feature in a habitable-zone planet is just fantastic," said exoplanetary researcher Ryan Cloutier of the Center for Astrophysics at Harvard University. As a PhD student at the University of Toronto, he lead the research that not only determined that K2-18 b was a super-Earth, but that there was another planet in the system. "Habitable-zone planets are the holy grail for atmospheric studies."

Exoplanets that are Jupiter-sized or larger are easier to find than smaller ones, particularly those closer to the size of Earth. So, if simply finding them is so challenging, determining the molecules in their atmospheres is even more difficult. 

But finding water vapour alone in an atmosphere of a potentially habitable exoplanet doesn't mean life exists on K2-18 b. 

And water vapour doesn't necessarily mean water exists on the surface. 

The temperature of the planet is roughly –73 C to 46 C, which is similar to Earth. The range is so large because of various unknown factors, including the temperature of the star and the distance between the star and the planet and the planet's atmosphere and pressure, which is why it's unclear if water exists on the surface.

However, the James Webb Space Telescope — which is scheduled to launch in 2021 — will be capable of finding other molecules such as methane and ozone, which could add more evidence to the potential of life on an exoplanet, though it won't be definitive proof of life.

"I think it's very cool, and it's a step in the right direction. These objects are incredibly enigmatic, the so-called sub-Neptunes … and we have no idea what they are," said Sara Seager, an MIT exoplanet researcher originally from Toronto, who was not involved in the study. "We really want to understand this type of planet, and we're hoping the atmospheres will provide some clue as to what they are."

Tsiaras is optimistic about the future of exoplanet habitability research.

"It's always one small step at a time," said Tsiaras. "This time it was the first atmosphere, then it will be the first methane, then probably, why not, some detection of ozone. So one small step at a time."