This Volcanic Hellworld of an Exoplanet Might Be Habitable, Actually

The new exoplanet sounds like a horrible place, but it may be just what life needs to gain a foothold around a red dwarf star.

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image of a reddish planet with a cracked surface dotted with small bright explosions, against a blac...

This newly-discovered exoplanet is an Earth-sized version of Jupiter’s notoriously volcanic moon Io, with one important difference: it just might be habitable.

The tidal pull of an immense neighbor keeps exoplanet LP 791-18d’s interior hot and churning, turning its surface into a volcanic hellscape. According to a recent study, however, the planet’s violent volcanism may actually make it a possible, albeit very weird, home for alien life. University of Montreal astronomer Merrin Peterson and colleagues published their findings in a recent paper in the journal Nature.

New Exoplanet Just Dropped

Exoplanet LP 791-18d orbits a red dwarf star about 90 light years away. It seems to be a rocky world about the size and mass of Earth, and it’s even the third planet out from its star. That’s where the similarities with our world end, though. If life exists at all on LP 791-18d, it lives on a strange, dark world wracked by frequent convulsions, lit only by distant starlight and the fires of erupting volcanoes.

This artist’s image shows what exoplanet LP 791-18d might look like. It’s hard to imagine there might be life down there, but it’s technically possible.


Like most of the rocky planets astronomers have discovered around red dwarfs, LP 791-18d orbits very close to its small, dim star: the planet completes a lap around the star once every 2.75 days. At that distance, it’s tidally locked, with one side exposed to constant, scorching-hot daylight and the other in perpetual, chilly darkness. And then there are the volcanoes.

Every time LP 791-18d orbits its star, it also makes a close pass by its much larger neighbor, LP 791-18c, a planet about seven times more massive than Earth. The much larger planet’s gravity pulls at LP 791-18d, stretching its orbit into a longer, narrower ellipse — and also stretching the planet itself, pulling at its entire body the way our Moon pulls at Earth’s oceans. LP 791-18c’s tidal pull creates friction in the rocky surface and interior of the smaller planet, and the heat of that tidal friction melts rock and fuels violent geological activity. Picture a worldwide volcanic apocalypse that never actually ends.

It’s the same force that’s at work on Jupiter’s moon Io, and astronomers expect that LP-971-18d is every bit as volcanically active as Io despite being much larger.

A Homeworld for Volcano Aliens?

LP 791-18d sounds like the least hospitable place you can imagine, despite being right at the innermost edge of the star’s habitable zone, but all that volcanism might actually make the planet livable. We think of erupting volcanoes as a destructive force, but they also pump out huge amounts of gas, including carbon dioxide and sulfur dioxide. The combined gas from all of LP 791-18d’s constant eruptions might be enough to keep the star swathed in a thick atmosphere despite the nearby star’s constant efforts to strip it away.

If LP 791-18d has an atmosphere, it would carry some of the heat from the dayside over to the cold, dark night side. A volcano-supplied atmosphere could also make it rain.

“The day side would probably be too hot for liquid water to exist at the surface,” says University of Montreal astronomer Bjorn Benneke, a coauthor of the recent study, “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.”

Meanwhile, the volcanoes could also pull chemical ingredients for life — things like carbon, oxygen, and even water — up from deep in the planet’s interior and make them available on the surface, potentially giving the chemistry of life a chance to kickstart itself amid all the constant fiery upheaval.

Of course, this picture of life evolving amid the fires of a volcanic hellworld is mostly informed speculation at this point; astronomers can safely draw conclusions about volcanoes on LP 791-18d based on how its orbit interacts with its much larger neighbor LP 791-18c. And if there are volcanoes, there may be an atmosphere, but so far, astronomers haven’t observed one — although that could change in the next couple of years.

A team of astronomers has already scheduled time to study 18c with the James Webb Space Telescope, which may be able to see changes in starlight filtering through the Neptune-sized planet’s atmosphere, if it has one. Similar observations in the future might also shed light on whether there’s an atmosphere around 18d, and if so, what it’s made of. That could help astronomers confirm some of these ideas and maybe even draw some conclusions about whether the planet really could be habitable.

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