Gan, Tianjun, Cadieux, Charles, Ida, Shigeru, Wang, Sharon Xuesong, Mao, Shude, Lin, Zitao, Stassun, Keivan Guadalupe, Burgasser, Adam J., Howell, Steve B., & Clark, Catherine A. (2025). “A new brown dwarf orbiting an M star and an investigation of the eccentricity distribution of transiting long-period brown dwarfs.” Astrophysical Journal Letters, 988(2), L78. https://doi.org/10.3847/2041-8213/adef55
The shapes of brown dwarf orbits—how circular or stretched they are—can reveal important clues about how these objects form and evolve, and whether they behave more like giant planets or small stars. In this study, we report the discovery of TOI-5575 b, a massive brown dwarf orbiting a small, cool M5V star (about 21% the mass of the Sun) detected by the TESS mission. TOI-5575 b has a mass of 72.4 times that of Jupiter and a radius of 0.84 Jupiter radii, traveling around its star every 32 days on a moderately elongated orbit. This system represents one of the highest mass-ratio transiting brown dwarf systems known.
We then analyzed the orbit shapes (eccentricities) of a sample of transiting long-period giant planets, brown dwarfs, and low-mass stars. We found that brown dwarfs in these types of orbits behave much like giant planets: most have nearly circular orbits, with a smaller number in more elongated orbits. This is very different from results seen with objects detected by direct imaging at much larger distances, where brown dwarfs and giant planets show clearly different orbital behaviors.
Our findings suggest that transiting long-period brown dwarfs and giant planets either form differently in outer orbits but undergo similar dynamical evolution as they move inward, or that both populations contain two subgroups—one with mostly circular orbits and another with widely spread orbital shapes—that together shape their eccentricity patterns. Low-mass stars, by contrast, appear as a distinct population, peaking at moderately elongated orbits similar to more massive stellar binaries.
Figure 1. Left panel: the TESS and ASP light curves folded in phase with the transit ephemeris from the joint analysis along with the best-fit transit models. Right panel: the full SPIRou RV time series and the phase-folded RVs. The error bars presented here are the quadrature sum of the original RV uncertainties and the instrument jitter. The black solid line is the best-fit Keplerian model. The residuals are shown below each subplot.