A New Brown Dwarf Orbiting an M Star and an Investigation of the Eccentricity Distribution of Transiting Long-period Brown Dwarfs

The orbital eccentricities of brown dwarfs encode valuable information on their formation and evolution history, providing insights into whether they resemble giant planets or stellar binaries. Here, we report the discovery of TOI-5575 b, a long-period, massive brown dwarf orbiting a low-mass M5V star (0.21 ± 0.02 M _⊙ ) delivered by the TESS mission. The companion has a mass and radius of 72.4 ± 4.1 M _J and 0.84 ± 0.07 R _J on a 32 day moderately eccentric orbit ( e = 0.187 ± 0.002), making it the third-highest mass ratio transiting brown dwarf system known to date. Building on this discovery, we investigate the eccentricity distributions of a sample of transiting long-period (10 days ≤ P ≲ 1000 days, ∼0.1–1.5 au) giant planets, brown dwarfs, and low-mass stars. We find that brown dwarfs exhibit an eccentricity behavior nearly identical to that of giant planets: a preference for circular orbits with a long tail toward high eccentricities. Such a trend contrasts sharply with direct imaging findings, where cold (5–100 au) brown dwarfs and giant planets display distinct eccentricity distributions. Our results suggest that transiting long-period brown dwarfs and giant planets probably (1) form in different routes at exterior orbits but undergo analogous dynamical evolution processes and migrate inward or (2) both contain two subgroups, one with widely spread eccentricities while the other has circular orbits, that jointly sculpt the eccentricity distributions. The low-mass star systems appear to be a distinctive population, showing a peak eccentricity at about 0.3, akin to more massive stellar binaries.

Abstract licence: CC BY

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