For most of the exoplanets we've discovered, we know very few details. We know a bit about the star they orbit and maybe a partial list of other planets in the same system. And we typically know either how large they are or how heavy they are. It's not a lot to go on.
But we can infer a lot when we start combining those details. That's the case for a newly discovered exoplanet orbiting a small star about 90 light-years from Earth. The planet itself has a radius and mass very similar to the Earth's, suggesting it also has a rocky composition. Based on what we know of the star, it can potentially contain liquid water. And, based on the forces exerted by nearby planets, it's likely to have very active geology, potentially including volcanoes.
An extra, extra-solar planet
The exosolar system at the star LP 791-18 was first discovered by the Transiting Exoplanet Survey Satellite (TESS). TESS had spotted two planets orbiting LP 791-18, which is one of the smallest—and thus dimmest—stars known to host planets. The innermost planet, LP 791-18b, is about 20 percent larger than Earth and takes less than a day to complete an orbit, meaning it's close enough to the star to be very hot. Farther out, with a five-day orbit, is LP 791-18c, a sub-Neptune that's more than double Earth's size.
Their discovery led to some of the last observations done by the Spitzer Space Telescope before it was shut down, providing over five days of observations in total—just enough to capture two transits of LP 791-18c, the outermost planet, as it passed across the line of sight between its host star and Earth. But this data also captured two additional transits, suggesting that another planet might be present, orbiting between the two known planets.
That was enough to kick off a multi-year, multi-telescope effort that confirmed the existence of LP 791-18d, which takes a bit under three days to complete an orbit. But the effort went considerably beyond that by measuring variations in the timing of when planets transited in front of LP 791-18. These transit timing variations are caused by the relative positions of the three planets, which determine whether they produce gravitational tugs that either slow down or speed up their respective orbits.
By measuring enough of the transit timing variations, the researchers inferred how strong those gravitational tugs were and used that to get mass estimates for the outer two planets.
For the outermost planet, LP 791-18c, the estimated mass is roughly seven times Earth's. Based on its radius is 2.4 times larger than Earth's, if the planet had an Earth-like composition, then we'd expect it to be about 25 times Earth's mass, so this indicates that it has a lot of lighter materials. The research team concludes that it either has a substantial hydrogen/helium atmosphere, or that roughly half the planet is composed of icy materials.
And the second-outermost planet, the newly discovered LP 791-18d, has an Earth-like radius (officially 1.03 times Earth's, with error bars that include Earth's radius). Its mass is somewhat lower, at 0.9 times Earth's, but that's still consistent with a largely rocky composition.
Like and unlike
The planet is unlike Earth in several key ways, however. Due to its close proximity to the host star, it's likely to be quite a bit hotter. If the planet absorbs as much light as Earth does, the researchers estimate that its average temperature would be over 120° C. Even if it reflects as much light as Venus, it would still average 30° C (Earth's, for comparison, is 15° C). The exact temperature, however, would depend heavily on the levels of greenhouse gases in its atmosphere.
The planet, however, is close enough to be tidally locked to its host star, meaning that one side of LP 791-18d is perpetually lit and the other perpetually dark. Depending on how well the atmosphere distributes the heat of the star-facing side, this could allow liquid water to exist on the far side of the planet.
The other big difference is that the planet has a pretty massive sub-Neptune orbiting fairly nearby that keeps it from adopting a circular orbit. The resulting elliptical orbit means that the tidal forces exerted by the star vary based on where it is in its orbit. The forces exerted by the sub-Neptune will also vary. As a result, the planet is likely to experience something similar to Jupiter's moon Io, which is constantly flexed by the massive planet and other nearby moons, creating internal friction that heats the moon.
The result is volcanoes. Lots and lots of them. Io is likely the most volcanically active body in the Solar System. And, based on its rocky composition, there's every reason to think that LP 791-18d will also be unusually active. The researchers estimate that tidal heating alone would cause double the heat flux currently seen at Earth's surface.
Aside from being very cool if you find volcanoes cool (and really, I can't see why you wouldn't), this has consequences for the likely atmosphere of LP 791-18d. It's thought that planets that close to dwarf stars would likely be blasted by stellar outbursts early in their history, which might be strong enough to heat off any atmosphere they have. But volcanic activity should be regularly emit lots of gases that can continually restore the atmosphere. So this makes LP 791-18d an excellent candidate if we're interested in studying exoplanet atmospheres.
The Webb telescope is already scheduled to look for an atmosphere on the innermost planet of the LP 791-18 system, and NASA's press release on the new find acknowledges that scientists now think that the middle one deserves similar attention.
Nature, 2019. DOI: 10.1038/s41586-023-05934-8 (About DOIs).
