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Pair of super-Earths found orbiting star K2-18

"If you can get the mass and radius, you can measure the bulk density of the planet and that can tell you what the bulk of the planet is made of," researcher Ryan Cloutier said.

By
Brooks Hays
An artist's rendering shows the view from exoplanet K2-18b, looking out at its host star K2-18 and its planetary neighbor K2-18c. Photo by Alex Boersma/University of Montreal
An artist's rendering shows the view from exoplanet K2-18b, looking out at its host star K2-18 and its planetary neighbor K2-18c. Photo by Alex Boersma/University of Montreal

Dec. 6 (UPI) -- Astronomers have discovered not one but two super-Earths surrounding the star K2-18, a red-dwarf located 111 light-years from Earth.

The large rocky exoplanets were discovered using data collected by the European Southern Observatory's HARPS instrument.

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Astronomers were aware of one of the exoplanets, K2-18b. It was first identified in 2015. The team of researchers was attempting to find out more about its size and composition. As their observations revealed, the alien world is solid like Earth, but several times larger.

Their analysis revealed another surprise: K2-18b has a neighbor.

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"Being able to measure the mass and density of K2-18b was tremendous, but to discover a new exoplanet was lucky and equally exciting," Ryan Cloutier, a doctoral student at the University of Toronto, Scarborough, said in a news release.

To measure the mass of K2-18b, Cloutier and his research partners used radial velocities of its host star measured by HARPS. Radial velocities can reveal slight wobbles caused by the pull of an exoplanet's gravity on its host star.

"If you can get the mass and radius, you can measure the bulk density of the planet and that can tell you what the bulk of the planet is made of," said Cloutier.

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Their findings -- detailed this week in the journal Astrophysics and Astronomy -- proved the exoplanet is a scaled-up Earth, not a scaled-down Neptune. That is, it's made of mostly solid, not gas.

Researchers believe K2-18b is either mostly rock with a slight gaseous atmosphere or mostly water surrounded by a thick, icy shell.

"With the current data, we can't distinguish between those two possibilities," Cloutier said. "But with the James Webb Space Telescope we can probe the atmosphere and see whether it has an extensive atmosphere or it's a planet covered in water."

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The list of targets for the forthcoming telescope is long, but Cloutier and his colleagues think K2-18b is a deserving candidate.

"K2-18b is now one of the best targets for atmospheric study, it's going to the near top of the list," he said.

As for K2-18b's neighbor, K2-18c, data suggests the exoplanet is similar in size and composition but too close to its host star to be in the habitable zone. If it ever had water on the surface or in the atmosphere, it has likely been burned away.

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