Published on Wednesday in the journal Nature, a new paper reveals a new, roughly Earth-sized temperate world around a nearby small star that suffers from much the same gravitational tug-of-war from a Neptune-like sister planet that Jupiter’s moons do. As a result it’s expected to have strong volcanic activity on its surface.
However, the star itself—a cool red dwarf star in the southern hemisphere constellation of Crater—is only slightly bigger than Jupiter.
“The discovery of this exoplanet is an extraordinary find,“ said Professor Björn Benneke from Canada’s Université de Montréal’s Department of Physics. “The similarity in the properties of LP 791-18 d and Earth as well as the prospect of detectable geological activity and volcanism on it make it a key object to better understand how terrestrial worlds form and evolve.”
LP 791-18 d was found in some of the data from NASA’s now-dead Spitzer Space Telescope before it was decommissioned in 2019, as well as in data from NASA’s Transiting Exoplanet Survey Satellite (TESS).
What Makes It Earth-Like?
The planet orbits within its star’s habitable zone—close enough for liquid water to exist on its surface—and is suspected to be geologically and able to maintain an atmosphere. The temperature on LP-791-18 d is only slightly higher than on Earth, largely because its host star is so small and cool.
However, the newly found planet might be considered Earth-like, but that comparison only goes so far. LP 791-18 d orbits its star in just 2.8 Earth-days and is tidally locked, with a permanent day-side and night-side. Only on the latter could water condense on its surface, suspect the researchers.
“The main discovery here is that we found a planet the size of Earth, much smaller than the planets b and c,” said co-author Ian Crossfield, associate professor of physics and astronomy at the University of Kansas. “We don’t really know if it has an atmosphere, or if it has water, or if it has life, or if it could support life. We don’t know any of that.”
How Do They Know It’s Volcanic?
Researchers suspect there are volcanoes and possibly even an ocean of magma, but there’s no firm proof. The clues come from the fact that LP 791-18 d orbits its star in between two other planets:
LP 791-18 b: 20% bigger than Earth that orbits in a little less than one Earth-day.
LP 791-18 c: 2.5 times Earth’s size that orbits in five Earth-days.
In the data, researchers could see the new planet passing very close to LP 791-18 c. Since the other planet is so massive, relatively speaking, its gravity tugs the tiny planet and forces it into an elliptical orbit, which squashes and deforms it enough to create internal friction that heats the planet’s interior. Cue that heat trying to escape as volcanic activity at its surface.
“The significant friction generated by tidal heating in the planet is responsible for heating its interior to a considerable extent, ultimately enabling the existence of a subsurface magma ocean,” said Caroline Piaulet, a Ph.D student at Université de Montréal, who was involved in the discovery. “In our solar system, we know that Jupiter’s moon Io is affected by Jupiter and its other moons in a similar way, and that world is the most volcanic we know.”
For now, actual proof is lacking. “We don’t know that there are any volcanoes here,” said Crossfield. “All we know is that this is a small planet that’s experiencing a straight-up periodic stretching due to its orbit around its star and near the other planets.”
Why Is This Discovery So Important?
LP 791-18 d “provides unprecedented opportunities to advance not only astronomy but many other fields of science, notably geology, planetary sciences, atmospheric sciences, and possibly astrobiology,” said Benneke.
The researchers suspect that an atmosphere like that of the Earth, Venus, or Saturn’s moon Titan could exist on LP 791-18 d and hope that it will soon be the target of the James Webb Space Telescope (JWST). Given the closeness of this star system, JWST should be able to tease-out whether this volcanic world has an atmosphere or not.
That could be crucial for those hunting for life outside the solar system. “Only a small proportion of the exoplanets discovered so far are thought to be able to support life,” said Karen Collins, astronomer at the Center for Astrophysics. “Our discovery of LP 791-18 d gives us more hope that we might one day find signs of life on another planet.”