Current theories predict that low-mass stars should rarely host planets with masses exceeding that of Neptune (17 Earth masses).
An artist’s impression of a giant exoplanet. Image credit: Sci.News.
“In the spectral classification of stars, the lowest mass stars — the M-dwarfs — are the most common stars in the solar neighborhood and throughout the Milky Way Galaxy,” said Princeton University astronomer Guðmundur Stefánsson and colleagues.
“Gas giant planets are much rarer around M dwarfs than the more massive F, G, and K dwarfs, and planets orbiting M-dwarfs tend to be less massive than Neptune.”
“However, the properties of planets around the least massive (less than 0.25 solar masses) and coolest M dwarfs — the ultracool dwarfs — are still poorly known.”
“This is because ultracool dwarfs are faint and emit most of their radiation at infrared wavelengths, where planet-finding technologies are not as well developed as at optical wavelengths.”
“Two emblematic planetary systems around ultracool dwarfs are TRAPPIST-1 and Teegarden’s Star, which both feature compact systems of small and potentially rocky planets,” they added.
“The formation of such systems is compatible with the theory of core accretion, where the outcome of planet formation depends strongly on the assumed total mass of small solid particles (dust) within the protoplanetary disk.”
“Observations of protoplanetary disks show that the dust mass tends to be lower for low-mass stars, making it possible to form Earth-mass planets but not much more massive planets.”
“However, the uncertainties in the models and the large observed dispersion in dust masses raise the intriguing possibility that planetesimal core accretion might, at least occasionally, allow a low-mass star to form close-orbiting planets with masses exceeding 10 Earth masses.”
“Massive planet candidates have indeed been detected around a few ultracool stars, but in all cases the planets have very wide orbits.”
Using precise radial velocity observations from the Habitable-zone Planet Finder (HPF) spectrograph, Dr. Stefánsson and co-authors observed LHS 3154, an ultracool star that is 9 times less massive than the Sun.
Also known as LP 224-38, TIC 24108819 or NLTT 42028, the star is located 51.4 light-years away in the constellation of Hercules.
The astronomers detected Doppler shifts in their data due to an exoplanet of 13.2 Earth masses, which is almost the mass of Neptune.
Named LHS 3154b, the alien world has an orbital period is 3.7 days.
The researchers also performed planet formation simulations to show that the amount of dust in the protoplanetary disk that formed LHS 3154b would need to be at least 10 times greater than is typically observed in the protoplanetary disks surrounding low-mass stars.
“The detection of LHS 3154b confirms for the first time that ultracool dwarfs can form massive close-in planets,” they said.
“Both the core accretion and gravitational instability theories for planet formation struggle to account for this system.”
“In the core-accretion scenario, in particular, the dust mass of the protoplanetary disk would need to be an order of magnitude higher than typically seen in protoplanetary disk observations of ultracool stars.”
The team’s paper was published in the journal Science.
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Guðmundur Stefánsson et al. 2023. A Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models. Science 382 (6674): 1031-1035; doi: 10.1126/science.abo0233
