Astronomers took a second look at an exoplanet and discovered it probably is a super-Earth. They also found out the planet probably has a neighbor.

The two planets orbit a star called K2-18, which is a red dwarf star (dimmer and smaller than our sun) lying about 111 light-years from Earth. The star forms part of the constellation Leo. In 2015, another research team discovered the planet K2-18b and noted that it lies with the habitable zone. Since K2-18b is likely rocky, this means the planet could have liquid water on its surface, which is one of many conditions for supporting life.

"Being able to measure the mass and density of K2-18b was tremendous, but to discover a new exoplanet was lucky and equally exciting," lead author Ryan Cloutier, an astronomy and astrophysics Ph.D. student at the University of Toronto and the University of Montreal’s Institute for Research on Exoplanets, said in a statement.

K2-18b's newly found super-Earth status comes after a slew of other recent, rocky planet-related discoveries. Last month, another research team announced that the Earth-mass Ross 128b, which orbits a red dwarf star 11 light-years from Earth, is perhaps the most likely candidate for hosting alien life. It's the closest rocky planet found in a habitable zone found since Proxima Centauri b in 2016, which is located only four light-years away. And in February, yet another science team announced the planet TRAPPIST-1 hosts seven rocky planets, including some that may be in the habitable zone.

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The K2-18b researchers used data from the prolific planet-finding High Accuracy Radial Velocity Planet Searcher (HARPS) at the La Silla Observatory in Chile. HARPS is designed to measure radial velocities of stars, which are influenced by the presence of orbiting planets.

Previously, researchers were unsure if K2-18b was a super-Earth — meaning it is slightly larger than our planet, but mainly rocky — or a mini-Neptune composed mostly of gas. HARPS measurements provided information on the planet's mass, while the planet's radius was determined by other instruments that measure how much light it blocks from its parent 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," Cloutier said. The researchers' data showed K2-18b is either a rocky planet with a small gas atmosphere (similar to Earth) or a water planet that has a lot of ice on its surface.

Figuring out which is actually true will require more observations, perhaps from NASA's high-powered James Webb Space Telescope, which is set to launch in 2019. But the research team acknowledged that many other teams will request use of the new telescope, so they will need to justify their target carefully.

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The second planet popped up when Cloutier noticed a different signal in the data than from K2-18b, which orbits its star every 33 days. The second signal happened every nine days. After ruling out the possibility of noise, the astronomers announced a second planet called K2-18c. The planet is also a super-Earth, but likely orbits too close to its parent star to have liquid water on its surface.

Other researchers on the team are affiliated with the Geneva Observatory in Switzerland, France’s University of Grenoble, and the University of Porto in Portugal. The research will be published in the journal Astronomy and Astrophysics and is currently available on the website Arxiv.

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