A publication in Nature

TRAPPIST and SPECULOOS observatories help discover LP791-18d, an exo-Earth potentially riddled with volcanoes

Illustration of LP 791-18 d, an Earth-sized world about 90 light-years away. The gravitational pull of a more massive planet in the system, shown as a blue disk in the background, could lead to internal heating and volcanic eruptions - as much as on Jupiter's moon Io, the most geologically active body in the solar system. Astronomers discovered and studied the planet using data from NASA's Spitzer Space Telescope and Transiting Exoplanet Survey Satellite, as well as many other observatories including those of the SPECULOOS-South and TRAPPIST-South projects led by the University of Liège.Credit: NASA Goddard Space Flight Center/Chris Smith (KRBwyle)

An international team of astronomers has discovered an Earth-sized exoplanet that could be particularly geologically active. The frequency of volcanic eruptions on LP791-18d could be as great as on Io, one of the large moons of Jupiter, known as the most volcanically active body in the solar system. This discovery, led by the iREx (University of Montreal) is published in the scientific journal Nature.


ocated 90 light-years away in the southern constellation of Crater, LP791-18d was discovered from data acquired by NASA's TESS and Spitzer space telescopes and confirmed by various observatories on Earth, including SPECULOOS-South and TRAPPIST-South, led by the University of Liège and located at ESO in the Atacama Desert in Chile.

The planet is in orbit around a red dwarf, a star of very low mass and low temperature. The scientific team that made the discovery estimates that LP791-18d is only slightly larger and more massive than Earth. It is the third planet discovered in this system. The first, LP791-98b, is internal to LP791-18d and about 20% larger. The second, LP791-98c, is external to it, and is 2.5 times larger and about nine times more massive than the Earth.

The planets LP791-98d and LP791-98c orbit very close to each other, and since LP791-98c is much more massive, it strongly disturbs the orbit of LP791-98d at each conjunction. These perturbations maintain LP791-98d in an elliptical orbit. Such an orbit generates large tidal forces on the planet from its star, which results in an effective heating of its interior by frictional effect. Calculations show that this heating could sustain a particularly violent volcanism on the surface of the planet. The same mechanism applies to Io, the third largest moon of Jupiter on which a strong volcanic activity is recorded.

"This new planet is clearly not habitable. It is indeed almost three times more irradiated by its star than Venus by the Sun. Nevertheless, the possibility that it is a kind of super-analogue of Io surrounded by an atmosphere maintained by active volcanism makes it particularly interesting, especially since this hypothesis could be confirmed or informed by future observations with the JWST telescope," says Michaël Gillon, Director of the SPECULOOS observatory and of the TRAPPIST exoplanet program. 

Scientific reference

A temperate Earth-sized planet with tidal heating transiting an M6 star, Peterson et al. 2023, Nature


Michaël GILLON

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