A new planet candidate discovered in data from NASA’s TESS mission could be an extreme lavaworld, pushed and pulled by the gravity of its own star and two other close-in planets.

Lavaworld
Artist's illustration of a lavaworld
ESO / L. Calçada

An international team of astronomers has just found hints of an extreme exoplanet that — if it exists — is bursting with so many active volcanoes that its temperature rivals that of some stars. TOI-6713.01 appears to be roughly Earth-size, just 30% larger than our own planet. But it might have more in common with Jupiter's fiery moon Io.

“It was one of those discovery moments that you think, ‘Wow, it’s amazing this can actually exist,’” says team leader Stephen Kane (University of California Riverside). “I would describe [it] as Io on steroids. At optical wavelengths, you would be able to see a glowing, red-hot planet with a molten lava surface.” So far, though, the planet candidate has only been observed indirectly, by its effects on its host star. The team’s findings are published in The Astronomical Journal.

The planet candidate TOI-6713.01 is potentially the third world belonging to the star HD 104067, located some 66 light-years away in the southern constellation Corvus, the Crow. Astronomers had already discovered one exoplanet orbiting this star using the radial velocity method, looking for the wiggle of the star toward and away from us due to its planet’s gravitational tugs. Kane's team has now identified a second planet using the same technique, reanalyzing archival data from the High Accuracy Radial velocity Planet Searcher (HARPS) at the La Silla Observatory in Chile and the High Resolution Echelle Spectrometer (HIRES) attached to the Keck 10-meter telescope on Mauna Kea, Hawaiʻi. One of the planets is a shade less massive than Uranus; the other is nearly five times more massive still.    

In the same study, the team also speculates on the existence of the third planet — TOI-6713.01 — based on a single transit seen with NASA's Transiting Exoplanet Survey Satellite (TESS).

TESS transit
The TESS mission caught a single transit event, which could be the signal of a close-in planet in the star system HD 104067. If the detection pans out, the extreme gravitational forces experienced by this planet — both from its star and two other planets — could make it a lavaworld.
S. Kane et al. / Astronomical Journal

According to their analysis, this new planet candidate, which has yet to be categorically confirmed, takes a little over two days to orbit the star. The outer two planets take around 14 and 56 days respectively. The gravitational tugs of the two outer planets might have forced TOI-6713.01 onto an eccentric orbit, which could mercilessly stretch and squeeze the planet candidate like a squash ball. Such tidal tugs could inject a huge amount of tidal energy into the planet. Jupiter does the same to its satellite Io, making the moon the most volcanically active place in the solar system.

The total tidal dissipation on Io is 1014 Watts. That’s about the same as the total energy absorbed each second by every plant on Earth during photosynthesis. According to Kane's calculations, the total tidal dissipation could be almost 10 million times higher on TOI-6713.01 than on Io. That would be enough to raise the planet candidate’s surface temperature beyond 3000 kelvin (5000°F), making it hotter than the surface of a red-dwarf star and ensuring its surface is totally molten.

While still largely hypothetical at this point, since the planet candidate hasn’t yet been confirmed, the study presents a view of the exotic physics that could be present on other worlds.

“This teaches us a lot about the extremes of how much energy can be pumped into a terrestrial planet, and the consequences of that,” Kane says. “While we know that stars contribute to the heat of a planet, the vast majority of the energy here is tidal, and that cannot be ignored.”

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