LP 791-18d is a tidally-locked Earth-sized planet that orbits a red dwarf star close to the inner edfe of the habitable zone. The gravitational pull of a larger neighbouring planet in the system forces LP 791-18d onto a slightly elliptical orbit. This results in the strength of the star’s gravity felt by LP 791-18d varying as the planet orbits, causing the planet surface to flex (rather like repeatedly squeezing a stress ball). The flexing produces tidal heating, and quite possibly drives strong volcanic activity on the planet surface.
In tidal lock, this planet has one side of enternal day and one of night, which can risk atmospheric collapse. But It is possible that the strong volcanic activity may support the atmosphere, and allow heat to circulate between the day and night sides. This could allow for liquid water to exist on the night side, as shown here.
JPL Press release on the discovery of LP 791-18d: https://www.jpl.nasa.gov/news/nasas-spitzer-tess-find-potentially-volcano-covered-earth-size-world
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