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Astronomers spot Earth-sized exoplanet probably 'carpeted' by volcanoes

The floor is lava. Literally. But at least there might be water

Astronomers have discovered a rocky Earth-sized exoplanet they believe is "likely carpeted with volcanoes" and may be capable of supporting an atmosphere and liquid water, according to a paper published in Nature on Wednesday.

The strange new world, romantically dubbed LP 791-18d, has radius and a mass similar to Earth. It orbits a red dwarf star about 86 light years away in the southern constellation Crater. Scientists previously spotted two planets around the star, and now a team of researchers led by the Trottier Institute for Research on Exoplanets based at the University of Montreal have found a third.

LP 791-18d lies very close to its parent star, and completes an orbit every 2.8 days. On that journey it sometimes passes within just 1.5 million kilometers of its neighboring planet, LP 791-18c. That's such a close pass that LP 791-18c exerts a gravitational pull on d, forcing its orbit to become stretched and elliptical. 

As LP 791-18d is also gravitationally influenced by its host star, tidal forces deform its shape. The strain causes internal friction and huge amounts of heat, powering geologically active regions and that carpet of volcanoes.

"The significant friction generated by tidal heating in the planet is responsible for heating its interior to a considerable extent, ultimately enabling the existence of a subsurface magma ocean," Caroline Piaulet, co-author of the study and a PhD student at the University of Montreal, explained in a statement. "In our Solar System, we know that Jupiter's moon Io is affected by Jupiter and its other moons in a similar way, and that world is the most volcanic we know."

"Other planets could have subsurface magma oceans, but the interesting thing about this planet in particular is that its star is small enough that we could detect the imprints of volcanic outgassing on its atmosphere," Piaulet told The Register.

Despite the exoplanet's close orbit to its sun, that star is small and dim so LP 791-18d lies in the habitable zone – where surface temperatures are mild enough to support liquid water. It's likely that only one side of the exoplanet might host liquid water, however, as it is tidally locked and one side always faces its sun.

"The day side would probably be too hot for liquid water to exist on the surface. But the amount of volcanic activity we suspect occurs all over the planet could sustain an atmosphere, which may allow water to condense on the night side," suggested Björn Benneke, a co-author of the paper and a professor at the University of Montreal, in a statement. 

The team speculated the exoplanet's atmosphere could be made up of carbon dioxide or nitrogen.

LP 791-18d was spotted by sifting through data collected by NASA's defunct Spitzer Space Telescope, the Transiting Exoplanet Survey Satellite, and other observatories on Earth.

Researchers will continue studying the LP 791-18 system, and have obtained approval to observe planet c with the James Webb Space Telescope (JWST). Piaulet said that the space scope could uncover more clues about the presence of liquid water on planet d.

"Liquid water, because it would be found at the planet's surface, is a very tricky observation to make. However, by measuring the planet's atmosphere with the JWST, we could get a clue of whether clouds cover at least part of the atmosphere, which has the potential to lower the surface temperature enough to sustain liquid water," she told The Register.

"This system provides astronomers with a precious laboratory for testing various hypotheses related to the formation and evolution of terrestrial planets," Benneke added. "The newly found planet d, an Earth-size world likely covered in volcanoes in a multiplanetary system, provides unprecedented opportunities to advance not only astronomy but many other fields of science, notably geology, planetary sciences, atmospheric sciences, and possibly astrobiology." ®

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