For the first time ever, astronomers have detected water vapor in the atmosphere of a rocky exoplanet. Intriguingly, the planet is in the so-called habitable zone of its star, making it one of the most promising places in the galaxy to look for signs of extraterrestrial life.

Located 110 light-years away, planet K2-18b was discovered by the Kepler Space Observatory in 2015. Preliminary scans of the exoplanet suggested it was a super-Earth, that is, a rocky planet larger than Earth, and that it featured an atmosphere. These observations, plus the fact that K2-18b is parked inside the habitable zone -- the sweet spot in a star system where water could persist in liquid form -- warranted further study.

Using the Hubble Space Telescope, a team from University College London performed a spectrographic survey of K2-18b's atmosphere, resulting in the detection of a distinctive water signature. Details of this important discovery are published today in Nature.

"It's the first detection of water vapor 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. "This is the first detection of its kind -- a planet orbiting within the habitable zone -- and the only [terrestrial] planet we know outside of our solar system that... has water in it," he said, adding: "It's the best candidate for habitability that we know right now."

Water vapor has been detected in exoplanets before, but only in gas giants. There's a slim possibility that K2-18b is an ice giant, similar to Neptune or Uranus, but the greater likelihood is that it's a super-Earth. Astronomers can only see this exoplanet's upper atmosphere, but it's twice the size and has eight times mass than Earth. Importantly, its density is similar to the density of Mars, which is three-times that of a typical gas giant. This leads scientists to conclude that K2-18b is probably terrestrial in nature, featuring a solid, rocky core.

K2-18b orbits a dim red dwarf, and it takes just 33 days for the planet to complete a single orbit of its host star. At such a close proximity, it might not sound like K2-18b is inside the habitable zone, but because red dwarfs emit low levels of radiation, their habitable zones are much nearer compared to systems like ours. K2-18b receives about as much radiation as Earth, and it features temperatures comparable to what we experience here, noted the researchers at the press conference.

Sadly, exoplanets in orbit around red dwarfs are considered poor candidates for habitability, owing to the propensity of this group of stars to produce tremendously powerful and frequent solar flares. As Tsiaras pointed out at the press conference, conditions on K2-18b are likely "more hostile" compared to Earth.

Hubble observations resulted in a distinct water signal, but the UCL researchers weren't certain about the amount of water packed into the atmosphere of K2-18b, or whether liquid water exists at the surface. Using a series of models, the scientists showed that the atmosphere of K2-18b could contain as little as 0.01 percent water or as much as 50 percent.

That's obviously a huge discrepancy.

The reason for this unusually wide range, explained study co-author Giovanna Tinetti in response to a Gizmodo question on the matter, is that Hubble was only able to detect the spectrographic water signal, and that "it's not easy to quantify the amount of water compared to other molecules." But the water signal is "very strong," said Tinetti at the press conference, "regardless of the quantity." Future research will focus on narrowing down this large variance, she said.

An interesting characteristic of super-Earths is that most of them are likely to be water worlds -- terrestrial planets covered entirely by a deep global ocean. When Gizmodo asked Tinetti about the potential for K2-18b to be a water world, she said it's a "good possibility," given the results of the models, but the current data can neither confirm nor rule out this scenario.

Li Zeng, a planetary scientist at Harvard University not involved with the new study, said the results came as "no surprise," as he expects many exoplanets similar in size to K2-18b to have water "as a major constituent of their bulk interiors," he told Gizmodo in an email. The new research, he said, is consistent with his own work. Earlier this year, Li and he colleagues presented evidence showing that super-Earth water worlds are likely to feature oceans that are hundreds -- and even thousands -- of kilometers deep.

In addition to water, Hubble also detected traces of hydrogen, an observation that intrigued Tom Louden, a physicist at the University of Warwick and an expert on exoplanetary atmospheres. Louden said the results of the new paper "are certainly exciting," and "quite significant in determining the evolutionary history of exoplanet atmospheres." At the same time, however, the new findings could represent a blow to the planet's potential to foster life, said Louden, who's not affiliated with the new research.

"The results suggest that the planet K2-18b has kept some, or perhaps all, of its 'primary atmosphere' of hydrogen and helium which the planet [collected] during its formation," said Louden in an email to Gizmodo. "This indicates that the radiation from the star has not been too fierce or efficient in stripping away its atmosphere, which might be good news for the many Earth-like planets we expect to exist around stars of this type, since it might mean that their atmospheres can remain stable."

At the same time, this news is potentially bad for habitability. If "most planets are born with a large atmosphere of hydrogen and helium that they can't get rid of, it might make it more difficult for complex life to develop -- we simply don't know enough at this stage," he said.

During the press conference, Ingo Waldmann discussed how the presence of primordial gases on K2-18b could impact the planet's ability to foster life.

"Hydrogen atmospheres can be habitable," he said, saying there's no reason to suggest this is not the case based on theoretical models. Our biases about habitability, he noted, are currently very "Earth-centric," which is reasonable given that Earth is the only habitable planet we know of. Accordingly, Waldmann said future research should investigate habitability in the context of hydrogen-rich atmospheres.

Clearly, K2-18b presents a mixed bag in terms of its capacity to host life. Like Earth, it has a temperate climate, a rocky core, and water. On the downside, K2-18b is in orbit around a potentially hostile red dwarf and its atmosphere is packed with primordial gasses -- not to mention its large size and mass compared to Earth, an unknown variable as far as habitability is concerned.

This exoplanet is a fantastic and tantalizing subject for future research, but one thing remains abundantly clear: The search for a truly Earth-like planet continues.

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