A key scene of the 2005 movie Star Wars Episode III: Revenge of the Sith is the lightsaber battle between Obi-Wan and a – now turned to the dark side – Anakin Skywalker on Mustafar, a planet covered entirely in dark rocks and ash, igneous in origin as the many lava flows and active volcanoes suggest.
At the time, no real exoplanet with comparable geological features was known, but now a team of astronomers announced the discovery of a world likely covered by active volcanoes – very much like the fictional Mustafar.
The discovery was made thanks to data from NASA’s Transiting Exoplanet Survey Satellite Telescope, launched in 2021, and the now-retired Spitzer Space Telescope.
The Earth-sized planet, named LP 791-18d, orbits a small red dwarf star about 90 light years away as part of a star system with two other larger planets.
“During each orbit around the star, the planets pass very close to each other. As they do, the massive size of the outermost planet produces a gravitational tug that makes the inner planet’s orbit more elliptical, rather than perfectly circular. These deformations to the orbit create friction that heats the planet’s interior, producing volcanic activity at the surface,” writes Jules Bernstein for the University of California-Riverside. A similar effect is known in our solar system, as the volcanic activity on Io, the most active celestial body known so far, is a result of the gas-giant Jupiter’s gravitational pull constantly deforming the small moon.
Active volcanoes on other worlds could provide some key elements needed for sustaining alien life.
“A big question in astrobiology, the field that broadly studies the origins of life on Earth and beyond, is if tectonic or volcanic activity is necessary for life,” so Jessie Christiansen, study co-author and California Institute of Technology research scientist, in an interview.
The main constituents of volcanic emissions are carbon dioxide and water vapor, playing an important role in regulating a planet’s surface temperature and providing the basic chemical compounds for the existence of life.
“Why is volcanism important? It is the major source contributing to a planetary atmosphere [in Revenge of the Sith, Mustafar has indeed a breathable atmosphere], and with an atmosphere you could have surface liquid water – a requirement for sustaining life as we know it,” explains UC-Riverside astrophysicist Stephen Kane.
“In addition to potentially providing an atmosphere, these processes could churn up materials that would otherwise sink down and get trapped in the crust, including those we think are important for life, like carbon,” Christiansen concludes.
Another key feature of the planet is the fact that it does not rotate.
“LP 791-18d is tidally locked, which means the same side constantly faces its star,” said Björn Benneke, corresponding co-author of the paper and astronomy professor at the Trottier Institute for Research on Exoplanets, based at the University of Montreal.
“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,” Benneke concludes.
Just a few months ago, the discovery of Venusian volcanoes was announced, suggesting that active volcanism is far more widespread in space than previously thought. On Venus the volcanic emissions contributed to a runaway greenhouse effect, causing the planet to become a hellish landscape with an average temperature over the melting point of lead and zinc. On Earth, with over 1,300 erupting volcanoes, they play a role in controlling the climate and “fertilizing” the land by releasing elements like sodium, potassium and sulfur into the environment.
The paper “A temperate Earth-sized planet with tidal heating transiting an M6 star” is published in the journal Nature (2023).
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