An artist's impression of exoplanet K2-18bESA / Hubble / M. Kornmesser

Of the 4,044 confirmed exoplanets found orbiting faraway stars, some are rocky like Earth, and others are gaseous like Jupiter. Some have thick atmospheres, others have none at all. Some planets hurtle around their stars in a matter of hours, while others can take decades to complete one orbit.

And then there’s K2-18b. It is about twice the size of Earth, but nearly nine times more massive. It orbits within its star’s habitable zone. And scientists just detected a hint of water in its atmosphere.

Astronomers have found water vapor in exoplanets before. But this distant world is the first known exoplanet that both shows signs of water and orbits in a region where temperatures are right for water, if there’s enough of it, to flow in liquid form.

According to new research published today, astronomers have stretched the famous Hubble space telescope’s limits and detected the molecular signature of water vapor from more than 100 light-years away.

K2-18b orbits close enough to its star that it receives about the same warmth as the Earth does from the sun, according to an international team led by Björn Benneke, an astronomer at the University of Montreal. Their models, based on the potential temperature conditions on this planet, suggest that the water vapor in the planet’s atmosphere could condense into liquid water.

In other words, it might rain. “The temperature is just right—or potentially right—for liquid water to exist,” Benneke says.

That doesn’t mean, however, that liquid water does exist on K2-18b. The Hubble data reveals only that water vapor exists in its atmosphere, and cannot tell astronomers how much there is.

The detection is another milestone in a field that is so packed with exoplanets that the most exciting work in exoplanet research now focuses on studying their atmospheres.

And just like the earliest days of exoplanet discoveries, it’s a competitive field; Benneke published his team’s findings the night before a separate team was scheduled to announce the same detection.

Both papers analyzed Hubble observations that Benneke had designed and requested from the space telescope, but that were made public and accessible to all astronomers. The second team published their findings in the journal Nature Astronomy today, but Benneke, whose paper has been submitted to The Astronomical Journal but not yet accepted, decided to beat them to it by posting his team’s paper on arXiv, an online repository for preprints. The second team, led by Angelos Tsiaras, an astronomer at the University College London, reports the same detection, but stops short of suggesting there’s liquid water suspended in the exoplanet’s atmosphere.

It is tempting to take these papers and their buzz phrases—exoplanet! water vapor! habitable zone!—and jump right to the idea that K2-18b could harbor extraterrestrial life. But it would take a lot more than a whiff of water vapor to suggest that another world meets the conditions conducive to life.

Despite some similarities in the temperatures of their orbits, K2-18b is not like Earth. Its atmosphere is thick and puffy with hydrogen, which suggests that the planet probably doesn’t have a rocky surface. “If you have a lot of hydrogen in your atmosphere, the pressure keeps increasing as you go deeper and deeper,” says Laura Kreidberg, an astronomer at the Harvard-Smithsonian Center for Astrophysics who studies exoplanet atmospheres, and who was not involved in this research. “You get to a liquid hydrogen layer before you reach a rocky surface.”

These conditions mean that K2-18b is more similar to Neptune, the kind of planet where life is unlikely. The detection of water vapor on an exoplanet in its star’s habitable zone is an exciting start, but astronomers want to find chemical signatures that are less ubiquitous in the universe. “Oxygen would be the real exciting thing to detect—because that is actually produced by life,” says David Charbonneau, an astronomer at the Harvard-Smithsonian Center for Astrophysics who was not involved in these studies.

It is difficult to observe the chemical composition of exoplanets, even with powerful telescopes like Hubble. (The next generation of telescopes that can do this kind of work is still a few years away.) Hubble observes the light coming from distant stars, not the planets around them. To find the planets, astronomers must dig into the starlight that has passed through the planet’s atmosphere on its way to Earth. “Some of the stellar light filters through the exoplanet’s atmosphere, and when it does that, some of that light will be absorbed by the chemistry that is in that atmosphere,” says Ingo Waldmann, an astronomer at the University College London and one of the authors of the Nature Astronomy study. “If we manage to get these photons that went through the atmosphere of that planet, then we can characterize the chemistry of the atmosphere of that planet.”

If an advanced alien civilization had its own Hubble and pointed it right at Earth as we crossed in front of the sun, they’d be able to do the same, finding the kinds of gases that would give away our presence.

K2-18b is the smallest planet so far where astronomers have managed to excavate this hard-to-detect information. This is an exciting prospect for exoplanet researchers. Planets like K2-18b—somewhere between Earth and Neptune in their properties—are the most common kind of exoplanets that telescopes have found so far. And the stars they orbit, known as red dwarfs, are the most common stars in the galaxy. These planetary systems make for great targets for research, but some exoplanet astronomers still dream of finding a world that is truly Earth-like. “That’s a planet orbiting a sun-like star, not a dwarf star like all these others, that has water vapor in its atmosphere that can indicate a liquid ocean and thin atmosphere, and that has gases that don’t belong that we might be able to attribute to life,” says Sara Seager, an MIT astronomer who was not involved in the new discovery.

Until then, astronomers will continue to dig through space-telescope data for tiny signals, and maybe even race one another to tell the world when they find them. (The findings may not always be convincing to other exoplanet searchers; some told me they found the detection of water vapor convincing, while Charbonneau says he thinks it is just on the edge of statistically significant.) The competition, if a little stressful for the astronomers themselves, presents a nice outcome for the public. Researchers provided independent analyses of the same thing, that gold standard of discovery. “If two groups analyzed the same data and got contradictory results, then we’d really be in trouble,” Charbonneau says.

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