Imagine a planet where one side is always facing the sun, baking in perpetual daylight, while the other side is shrouded in eternal darkness. Imagine a planet where volcanoes erupt constantly, spewing lava and ash across its surface. Imagine a planet where water may exist, but only on the frigid night side, where it condenses from the thin atmosphere. This is not a scene from a science fiction movie, but a possible reality for LP 791-18 d, an Earth-sized exoplanet discovered by astronomers using data from NASA’s TESS and Spitzer missions.
LP 791-18 d is located about 90 light-years away from Earth in the Crater constellation, where it orbits a small red dwarf star. It is one of three known planets in the system, along with LP 791-18 b and c, which are both larger and more massive than Earth. The planets have a complex gravitational dance that affects their orbits and shapes their geology.
LP 791-18 d is tidally locked to its star, meaning it rotates at the same rate as it orbits, keeping the same side always facing the star. This creates extreme temperature differences between the day and night sides of the planet, which could range from hundreds to thousands of degrees Celsius. The day side would be too hot for liquid water to exist on the surface, but the night side could be cold enough for water to freeze.
But LP 791-18 d is not a static world. It is constantly being stretched and squeezed by the gravitational pull of its more massive neighbor, LP 791-18 c, which passes very close to it every orbit. This causes LP 791-18 d to have an elliptical orbit that changes slightly with each cycle. The elliptical orbit also means that LP 791-18 d gets closer and farther from its star during each revolution, which adds to the temperature variations.
The gravitational tides and orbital variations create internal friction and heating inside LP 791-18 d, which could drive volcanic activity on its surface. The planet could have frequent volcanic eruptions similar to those on Jupiter’s moon Io, which is the most volcanically active body in our solar system. The volcanoes could spew lava and gases that could form or sustain an atmosphere around the planet.
An atmosphere could have important implications for the habitability of LP 791-18 d. It could moderate the temperature extremes between day and night, and allow water to condense on the night side. It could also shield the planet from harmful radiation from its star and interstellar space.
The atmosphere could also reveal clues about the composition and history of LP 791-18 d. By studying the light that passes through or reflects off the atmosphere, astronomers could determine what elements and molecules are present, and how they change over time. This could help them understand how the planet formed and evolved, and whether it has any potential for life.
LP 791-18 d is an intriguing example of how diverse and surprising exoplanets can be. It challenges our assumptions about what Earth-sized planets can look like and how they can behave. It also inspires us to explore further and deeper into the cosmos, where we may find more worlds that defy our expectations.