K2-18b: Super-Earth in 'Habitable Zone' Has Water Vapor in Its Atmosphere, Scientists Say

For the first time, scientists have discovered water vapor in the atmosphere of a planet beyond our solar system. The 'super-Earth' K2-18b sits within the habitable zone of its host star, meaning it is not too hot or too cold for liquid water to exist. This raises the possibility that it could support alien life.

K2-18b is an exoplanet that is 110 light-years from Earth. It was found in 2015 with NASA's Kepler space telescope and is of interest because of its position within its solar system. It orbits a cool dwarf star once every 33 days, and, according to research yet to be published, may receive about the same amount of radiation as Earth does from the sun. While previous research has hinted water may be present on the planet, strong evidence to support this has been lacking.

Now, a team of astronomers from the U.K. has announced the successful detection of water vapor in the exoplanet's atmosphere. Findings are published in Nature Astronomy.

At present, we only have one example of life in the universe—Earth. All living things on our planet need water to survive—so when searching for other planets that may also host life, water is seen as a good potential indicator.

Using data from the Hubble Space Telescope, the team was able to analyze the starlight that filters through K2-18b's atmosphere, allowing them to work out the molecular signatures. This revealed strong evidence of water vapor—potentially accounting for up to 50 percent of the atmosphere. Their evidence also suggests the presence of hydrogen.

"Finding water in a potentially habitable world other than Earth is incredibly exciting," lead author Angelos Tsiaras, from the U.K.'s University College London, said in a statement. "K2-18b is not 'Earth 2.0' as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental question: Is the Earth unique?"

The planet has a mass about eight times that of Earth and is roughly twice the size. Its composition is unknown—researchers believe it could be a rocky planet, or a water world with a layer of ice.

Whether or not life could exist on K2-18b is unknown, but the discovery of water vapor makes it a promising target for future observations—especially for the James Webb Telescope (JWST), which is currently scheduled for launch in 2021 and will be able to detect 'biosignatures'—and is possibly an indication that life is present. The European Space Agency's ARIEL, scheduled for launch in 2028, will also provide a better insight into the composition of exoplanets by analyzing their chemical makeup and thermal structures.

"Our discovery makes K2-18b one of the most interesting targets for future study," Giovanna Tinetti, study co-author and principal investigator for ARIEL, said in a statement. "Over 4,000 exoplanets have been detected but we don't know much about their composition and nature. By observing a large sample of planets, we hope to reveal secrets about their chemistry, formation and evolution."

Andrew Coates, professor of physics at University College London, who was not involved in the research, commented on the study: "This is an exciting discovery of water vapor in the atmosphere of an exoplanet in the habitable zone."

"Water has been suggested in this type of planet in the past, but this is the first real evidence. It makes this particular super-Earth planet a promising target for future missions which will look at exoplanet atmospheres, including JWST and ARIEL," Coates told Newsweek.

Carole Haswell, from School of Physical Sciences at the U.K.'s Open University, also commented. "This is an important milestone in humanity's exploration of worlds outside our own solar system because this is the first time we have detected water in the atmosphere of a lowish mass planet which is possibly rocky like the Earth," she told Newsweek. "Water is a particularly significant molecule to find because it plays a key role in sustaining life on Earth."

She continued: "While it probably does have liquid water in clouds or possibly on its surface, it is certainly not an Earth-twin. Nonetheless, this is a major discover, blazing a trail for our scientific adventure exploring the planets of our Milky Way Galaxy."

This story has been updated to include comments from Carole Haswell.

K2-18 b
Artist impression of K2-18 b. Scientists have discovered water vapor in the planet's atmosphere for the first time. ESA/Hubble, M. Kornmesser