For most of the exoplanets we’ve discovered, we know very few details. We know very little about the star it orbits and perhaps a partial list of other planets in the same system. And we usually know either how big it is or how heavy it is. There is not much to go on.
But we can infer a lot when we start to incorporate these details. Such is the case for a newly discovered exoplanet orbiting a young star about 90 light-years from Earth. The planet itself has a very similar radius and mass to Earth, indicating that it also has a rocky composition. Based on what we know about the star, it likely contains liquid water. And based on the forces exerted by the nearby planets, it likely has very active geology, likely to include volcanoes.
The outer system in the star LP 791-18 was first detected by the Transiting Exoplanet Survey Satellite (TESS). TESS has detected two planets orbiting LP 791-18, which is one of the youngest — and therefore darkest — 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 its orbit, which means it is close enough to the star to be extremely hot. Farther out, with a five-day orbit, is LP 791-18c, a sub-Neptune more than twice the size of Earth.
Their discovery led to some of the last observations by the Spitzer Space Telescope before it shut down, saving more than five days of observations in all—just enough to capture two transits of LP 791-18c, the exoplanet, as it passed through line-of-sight between its host star and Earth. But this data also captured two additional transits, indicating the possibility of another planet orbiting between the two known planets.
That was enough to launch a multi-year, multi-telescope effort that confirmed LP 791-18d’s existence, which takes just under three days to complete an orbit. But the effort went considerably further by measuring differences in the timing of the planetary transit time in front of LP 791-18. These differences in the timing of transits are caused by the relative positions of the three planets, which determine whether they produce gravitational tugs that either slow or speed up their orbits.
By measuring enough transit timing changes, the researchers inferred how strong those gravitational tugs were and used that to get mass estimates for two exoplanets.
For the outermost planet, LP 791-18c, the mass is estimated to be about seven times that of Earth. Based on a radius 2.4 times larger than that of Earth, if a planet has an Earth-like composition, we would expect it to be about 25 times the mass of Earth, which would indicate that it contains a lot of lighter material. The research team concluded that it either has a large hydrogen/helium atmosphere, or that roughly half of the planet is made up of icy material.
And the second, outermost planet, newly discovered LP 791-18d, has an Earth-like radius (officially 1.03 times that of Earth, with error bars including Earth’s radii). Its mass is somewhat less, at 0.9 times that of Earth, but this is still consistent with largely lithological composition.
Like and unlike
However, the planet differs from Earth in several key ways. Because it is so close to its host star, it is likely to be a little hotter. If the planet absorbs as much light as Earth, researchers estimate that its average temperature would be more than 120 degrees Celsius. Celsius). However, the exact temperature will depend greatly on the levels of greenhouse gases in the atmosphere.
However, the planet is close enough to be tidally locked on its host star, which means that one side of LP 791-18d is always lit and the other is always dark. Depending on how 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 very massive sub-Neptune orbiting fairly close together which prevents it from adopting a circular orbit. The resulting elliptical orbit means that the tidal forces exerted by the star vary depending on where it is in its orbit. The forces exerted by sub-Neptune will also vary. As a result, the planet is likely to experience something similar to Jupiter’s moon Io, which is constantly being flexed by the massive planet and other nearby moons, creating internal friction that heats the moon.
The result is volcanoes. Many, many of them. Io is probably the most volcanically active body in the solar system. And based on its rock composition, there is every reason to believe that LP 791-18d will also be unusually active. The researchers estimate that tidal heating alone would cause twice as much heat flux as we currently see at Earth’s surface.
Aside from being very cool if you find volcanoes cool (and really, I can’t understand why not), this has consequences for the potential atmosphere of LP 791-18d. It is thought that planets that approach dwarf stars are likely to have been blown away by stellar explosions early in their history, which may have been powerful enough to heat up any atmospheres they might have. But volcanic activity should regularly release so many gases that it can constantly restore the atmosphere. This makes LP 791-18d an excellent candidate if we are interested in studying exoplanet atmospheres.
The Webb telescope is already scheduled to search for the atmosphere on the innermost planet of the LP 791-18 system, and NASA press release The new discovery acknowledges that scientists now believe the middle part deserves similar attention.
Nature, 2019. DOI: 10.1038 / s41586-023-05934-8 (about DOIs).
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