Rocky super-Earth named most habitable known world beyond solar system

Pablo Tucker
September 12, 2019

Astronomers have detected water vapour in the atmosphere of a planet known as K2-18 b, which is twice the size and eight times the mass of Earth.

Angelos Tsiaras of University College London's Centre for Space Exochemistry Data (CSED), the leader of one of the research teams, stated, "This is the only planet right now with the correct temperature [for Earth-like life] and water outside the solar system. However, it brings us closer to answering the fundamental question: Is the Earth unique?".

Still, scientists warned that K2-18b isn't "Earth 2.0" - its atmosphere and surface are different than ours.

Also on Seeing double? Its orbital path is much tighter than the one Earth follows around our sun (out there, a year would be just 33 Earth days)-but this intimate distance means the planet is still within the red dwarf star's habitable zone, the life-friendly region where temperatures are conducive to liquid water. Liquid water would only be possible if the planet is "terrestrial" in nature, NASA said.

"It's the first detection of water vapour in a planet that's not a gas giant, UCL astronomer Angaelos Tsiaras, a co-author of the new study, proclaimed at a press teleconference held yesterday". The density measurements of K2-18b suggest the atmosphere is mostly hydrogen, surrounding a rocky and icy core.

"It's super exciting to have a first glimpse into the atmosphere of a planet this small", exoplanet researcher Laura Kreidberg told Eos.

"If planets like Earth are very common", says Tsiaris, "we can say life is very common".

For the first time, scientists have found water vapor on a so-called "habitable zone exoplanent".

ARIEL is expected to launch in 2028, and aims to observe 1,000 planets in detail so it could help us gain a more accurate picture of what planet K2-18b is like. The Montreal-based team's interpretation of its data suggests the hydrogen atmosphere forms a thick, gaseous envelope around the planet.

"The Kepler Space Telescope first discovered K2-18 b, which is just 110 light-years away from us, in 2015". But they caution it's not another Earth. 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. But while the temperature range and presence of water are good signs, K2-18 b isn't necessarily habitable. Despite this abundance, probing the conditions and atmospheric properties on any of these habitable-zone planets is extremely hard and has remained elusive to date ... However, there are still some serious questions about K2-18b, including whether or not its particularly intense red dwarf star has battered the planet with too much radiation to allow life to take root.

The scientists, who spent more than a year on their work before publishing it in the journal Nature Astronomy, said they do not think K2-18b is the only planet of its kind. Our biases about habitability, he noted, are now very "Earth-centric", which is reasonable given that Earth is the only habitable planet we know of.

Where the two groups agree is that K2-18b is not another Earth. The data are not precise enough to narrow down this range or to detect other molecules the atmosphere might have, the researchers found.

"Our discovery makes K2-18b one of the most interesting targets for future study".

Only K2-18b revealed the molecular signature of water, which is a vital ingredient for life on Earth.

All this makes a pretty compelling case for pointing the next generation of wider wavelength instruments - such as the James Webb Space Telescope and the Atmospheric Remote-sensing Infrared Exoplanet Large - in the planet's direction.

That's largely because the researchers are taking a transmission spectrum of the planet as it passes in front of its host star - that is, they're looking at the star's light as it passes through a thin sliver of the planet's atmosphere.

As of now, the researchers have concluded that there is some quantity of water and likely hydrogen in the planet's atmosphere.

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