Astronomers have found a planet a mere 87 light years away that is almost exactly the same size as Earth, orbiting its star at a distance that is neither roasting nor frozen.
Sounds perfect for Earth 2.0, right? Not so fast. The exoplanet known as LP 791-18d has been tugged so far out of an even circular orbit by another planet that its insides are probably a roiling hot mess that erupt from its surface in the form of raging volcanoes.
Even if it isn't exactly homely, the discovery could help astronomers better understand how habitable conditions emerge on Earth-like planets.
"LP 791-18d is tidally locked, which means the same side constantly faces its star," says astronomer Björn Benneke of the University of Montreal in Canada.
"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."
LP 791-18 is a tiny, cool red dwarf star just 14 percent of the mass and 17 percent of the radius of the Sun. In 2019, astronomers discovered it was home to two exoplanets; a super-Earth named LP 791-18b, 1.46 times the mass of Earth and on an orbit of 0.94 days, and a mini-Neptune named LP 791-18c, thought to be around 6 times the mass of Earth and on an orbit of 4.99 days.
Because the star is one of the coolest known to host exoplanets, a team led by astronomer Merrin Peterson of the University of Montreal used the infrared Spitzer Space Telescope to take 127 hours' worth of near continuous observations of the star. There, they saw faint dips in starlight called transits that were caused neither by LP 791-18b, nor Lp 791-18c as they orbited between us and the star.
This suggested the presence of a third, previously unknown world. Following up with observations from ground-based telescopes, the researchers confirmed the presence of LP 791-18d, an exoplanet clocking in at 1.03 times the radius and 0.9 times the mass of Earth, on an orbit of 2.753 days around the star.
That's a lot closer than Earth is to the Sun, but LP 791-18 is also a lot cooler, meaning the newly discovered exoplanet still sits in the star's habitable zone, the region of space at a distance from the star that theoretically allows for liquid water on the surface. It's not so hot that the water boils off, nor so cold that it freezes.
However, that proximity presents a different problem. The exoplanet's rotation "locks" to the same period as its orbit; in effect, the same side of the world always faces the star, the way one side of the Moon always faces Earth. In the case of LP 791-18d, this 'tidal locking' means one side is always in scorching hot daylight, the other in perpetual night.
The team captured a total of 72 transits – 43 transits from LP 791-18d, and 29 transits from mini-Neptune LP 791-18c. This allowed them to measure something called transit timing variations, which occur when gravitational interactions between the exoplanets cause small variations in the timings of their transits.
In turn, the researchers could calculate the mini-Neptune's mass with greater accuracy, determining it was the equivalent of 7.1 Earths. And they also learnt that the two exoplanets pass so close to each other on their orbital paths that the mini-Neptune pulls the smaller world into a distinctly elliptical orbit. This means that, as LP 791-18d moves closer to and farther from the star, the changing gravity stretches and compresses the planet, heating it from within.
This internal heating could manifest as volcanic activity – which may be visible as a thick atmosphere. Measuring it with an observatory like the James Webb Space Telescope could yield some insights into how planets like Earth or Venus – both volcanically active, but having taken very different evolutionary paths – end up the way they do.
"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," says astrophysicist Jessie Christiansen of the California Institute of Technology.
"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."
The discovery demonstrates how complex a thing habitability can be, and the importance of studying each planetary system holistically. It's no longer enough to find an Earth-sized world in its star's temperate zone. The impacts of the other worlds in the system also need to be considered carefully.
And it's important to keep finding and characterizing these worlds, too, whether they're habitable or not. They show us what a broad range of outcomes are possible for exoplanets that are ostensibly Earth-like, and assess the likelihood of habitable conditions in the broader galaxy.
"This discovery is just a first step," says astronomer Karen Collins of the Harvard-Smithsonian Center for Astrophysics.
"With the potential to continue studying this planet with the James Webb Space Telescope, we will be able to fine-tune our observations and learn more about the planet's likely volcanically fueled atmosphere. Future discoveries will help us understand how the ingredients of life might have come to be on worlds other than our own."
The research has been published in Nature.