Ars Technica reports on a bafflingly huge exoplanet spotted by the Webb Space Telescope. It’s the size of Neptune, but closely orbiting a star much smaller than ours – a red dwarf named LHS 3154. This means the guy is too big to have formed there in any way we understand:
The researchers used radial velocity measurements to determine the planet’s mass. This method detects Doppler shifts in the star’s light that are caused by the planet’s gravity pulling the star either closer or farther from Earth, depending on where the planet’s orbit takes it. This method can only calculate a lower limit on the planet’s mass. That’s because the planet’s orbit may be tilted relative to Earth, and so some of its gravitational pull will be off-axis.
These estimates make the newly discovered planet, LHS 3154b, at least 13 times Earth’s mass, making it slightly smaller than Neptune. (Again, that’s a lower estimate, so it might be larger.) Planets this large are rarely found around stars this small. And, when they are, they tend to be found much farther from their stars than LHS 3154b, which only requires 3.7 days to complete an orbit. So LHS 3154b is unusual enough that it seems to demand explanation.
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They obtained estimates of the amount of material in planet-forming disks around stars with a mass similar to that of LHS 3154. They then used that to simulate formation of planets through either the accretion of small rocky material or via collisions among planetesimals. These generally failed to produce planets that were large enough.
To consistently form something as big as LHS 3154b, the researchers had to change the starting conditions so that there was 10 times as much material in the planet-forming disk.
That’s an indication that our current models of planet-forming mechanisms can’t explain at least one of our observations. LHS 3154b could be an extreme outlier, and if so, we shouldn’t expect to see much like it either in our models or further observations. If you combine the uncertainties in the models and the uncertainties in the disk measurements, however, it’s possible that they could allow for something like this planet.
But the researchers also considered the idea that LHS 3154b might be telling us something about our models.