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HomescienceThe Neptune-sized exoplanet is too large relative to its host star

The Neptune-sized exoplanet is too large relative to its host star

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You win a little and you lose a little. Earlier this week, observations by the Webb Space Telescope provided new data supporting what we thought we understood about the planet’s formation. On Thursday, news came that astronomers had spotted a large planet orbiting close to a young star, a star so small that it doesn’t have enough material around it to form a planet that size.

This does not mean that the planet is “impossible.” But this means we may not fully understand some aspects of planet formation.

Big mismatch

LHS 3154 is, by all reasonable standards, a small, faint star. Images taken by the team behind the new work indicate that the red dwarf’s mass is only 11% of the mass of the Sun. Temperature estimates are around 2,850 K, which is well below the Sun’s temperature of 5,800 K and barely warm enough to keep it out of the category of ultra-cold dwarfs. (Yes, super-cool dwarves are enough of a thing to merit Their Wikipedia entry.)

We’ve found a lot of planets around red dwarfs like this. But for the most part, it was small and rocky. This is likely because young stars simply formed inside disks without much material. So it was a bit of a surprise when data indicated a very large planet around LHS 3154. It was discovered using a telescope at the McDonald Observatory in Texas.

The researchers used radial velocity measurements to determine the planet’s mass. This method detects Doppler shifts in starlight 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 the minimum mass of the planet. This is because the planet’s orbit would be tilted relative to Earth, so some of the planet’s gravity would be off-axis.

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These estimates put the newly discovered planet, LHS 3154b, at least 13 times the mass of Earth, making it slightly smaller than Neptune. (Again, this is an underestimate, so it could be larger.) Planets this size are rarely found around stars this small. When they are, they tend to be much farther from their stars than LHS 3154b, which requires only 3.7 days to complete its orbit. So LHS 3154b is unusual enough that it seems to require an explanation.

Go big

In cases where large planets exist around small stars, they appear to form through disc fragmentation early in the star formation process – this is the same process that forms binary stars, but with a lower amount of mass that produces a planet instead. The researchers found that LHS 3154b is much larger than the planets formed in simulations of this process, and much closer to its host star.

Therefore, the researchers investigated whether other patterns of planet formation could produce something like LHS 3154b. 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 this to simulate planet formation either through accretion of small rocky material or through collisions between planetesimals. These generally failed to produce sufficiently large planets.

To consistently form something as large as LHS 3154b, researchers had to alter the starting conditions so that there was 10 times the amount of material in the planet’s forming disk.

This is an indication that our current models of planet formation mechanisms cannot explain at least one of our observations. LHS 3154b could be quite exceptional, and if so, we shouldn’t expect to see much like it in either our models or additional observations. If you combined the uncertainties in the models and the uncertainties in the disc measurements, it could potentially allow for the existence of something like this planet.

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But the researchers also took into account the idea that LHS 3154b might tell us something about our models. One option that might explain things is that most of the material in planetary-forming disks is in the form of centimeter-scale pebbles, which would be impossible to detect at the wavelengths we used to study these disks.

The other reason is that there is a timing mismatch between the time when we tend to get good images of planet-forming disks and the time when planet formation actually occurs. The amount of material in the planet’s forming disk is expected to decrease over time, as the newly formed star heats up and expels material. If LHS 3154b’s rocky core formed early enough, it could continue to capture material even after the disk has begun thinning to the types of material densities used in these simulations.

The easiest way to see which of these possibilities will help explain LHS 3154b is more observations of red dwarfs. These should tell us whether there are other similar planets and perhaps help explain how much material is in the planet-forming disks.

Science, 2023. DOI: 10.1126/science.abo0233 (About digital IDs).

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