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Water vapor found on an ‘Earth-sized’ exoplanet 110 light-years from home

Scientists say the planet, called K2-18b, is “the best candidate for habitability” beyond our solar system.

ByKatherine J. WuNOVA NextNOVA Next
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An artist's impression showing K2-18b, the only known exoplanet that contains both water and temperatures that could support life. Image Credit: ESA/Hubble, M. Kornmesser

This week, astronomers announced new hope for extraterrestrial habitability: an exoplanet some 110 light-years away from Earth that harbors water in its atmosphere.

The discovery, published in two independent papers published in Nature Astronomy and on the pre-print server arXiv.org, marks the first time researchers have detected an Earth-like planet orbiting a distant star that supports liquid water—a crucial ingredient for life as humans understand it.

But don’t pack your bags just yet. While it’s offered up some tantalizing tidbits, the planet, called K2-18b, likely harbors many foreign features, and is 2.5 times larger and eight times more massive than Earth. Such intermediate sized planets don’t always look like Earth, and may actually more closely resemble Neptune—the icy planet that’s one size up in our own solar system.

“K2-18b is not ‘Earth 2.0,’” Angelos Tsiaras, an astronomer at University College London and author of the Nature Astronomy paper, said in a statement. “However, [this] brings us closer to answering the fundamental question: Is the Earth unique?”

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NASA’s Kepler space telescope first spotted K2-18b in 2015. The planet closely orbits a dim red dwarf star that’s about half the size of our solar system’s sun. 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.

This isn’t the first time water’s been detected in the atmosphere of an exoplanet, but previous candidates have had extremely hostile environments—like a Saturn-like behemoth with scorching surface temperatures, reports National Geographic’s Michael Greshko.

So far, K2-18b seems considerably more hospitable—and that’s big. In the arXiv.org paper, a team led by University of Montreal astronomer Björn Benneke used the Hubble Space Telescope to pinpoint evidence of a watery atmosphere, noting how it absorbed certain wavelengths of light.

After Benneke’s group posted their data to a public archive, an independent team led by Tsiaras re-analyzed it with their own software, arriving at similar conclusions. Their study, which appears in Nature Astronomy, also suggests that K2-18b’s atmosphere contains hydrogen and helium.

As Tsiaras said during a press conference, researchers consider K2-18b “the best candidate for habitability” outside our solar system.

Both studies, however, raise many new questions. For instance, it’s unclear just how much K2-18b’s atmosphere holds. In their paper, Tsiaras’ team laid out three models, each of which contained a very different estimate, spanning between 0.01 and 50 percent of the total atmosphere.

When Benneke’s team modeled K2-18b’s climate, they discovered that the planet has clouds, as well as conditions might allow for the formation of liquid water droplets. Taken together, these two pieces of data hint that water could condense out of the planet’s clouds to create rain. If that’s true, K2-18b would be the first known exoplanet with clouds of water vapor, Greshko reports.

“It’s quite likely that this planet has liquid rain on it,” Benneke told Lisa Grossman at Science News. “This is actually one of the most exciting findings from this data.”

However, Sara Seager, an astronomer at MIT who was not involved in either study, cautioned to Grossman that the presence of rain remains speculation until further observations are made, perhaps with the more NASA’s more powerful James Webb Space Telescope.

Depending on what future observations yield, K2-18b could turn out to be a planet with a dense, rocky core swaddled by a thick atmosphere, like Neptune—not exactly the most life-friendly of places. There’s still a chance it could be bathed in a huge ocean, though, which would be more suitable for life, Abel Mendez, director of the Planetary Habitability Laboratory at the University of Puerto Rico, who was not involved in either study, told Daniel Oberhaus at WIRED.

Either way, this exoplanet remains a first. K2-18b may not be an exact replica of Earth, but in several ways, it comes much closer than previously known water-vapor-containing exoplanets, Nikole Lewis, an astronomer at Cornell University who was not involved in either study, told Lee Billings at Scientific American. Getting to know K2-18b a bit better, she said, “will be important for understanding the potential habitability of smaller ‘Earth-sized’ planets.”

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