TOI-5205b: A Short-period Jovian Planet Transiting a Mid-M Dwarf

Abstract

We present the discovery of TOI-5205b, a transiting Jovian planet orbiting a solar metallicity M4V star, which was discovered using Transiting Exoplanet Survey Satellite photometry and then confirmed using a combination of precise radial velocities, ground-based photometry, spectra, and speckle imaging. TOI-5205b has one of the highest mass ratios for M-dwarf planets, with a mass ratio of almost 0.3%, as it orbits a host star that is just 0.392 ± 0.015 M ⊙. Its planetary radius is 1.03 ± 0.03 R J, while the mass is 1.08 ± 0.06 M J. Additionally, the large size of the planet orbiting a small star results in a transit depth of ∼7%, making it one of the deepest transits of a confirmed exoplanet orbiting a main-sequence star. The large transit depth makes TOI-5205b a compelling target to probe its atmospheric properties, as a means of tracing the potential formation pathways. While there have been radial-velocity-only discoveries of giant planets around mid-M dwarfs, this is the first transiting Jupiter with a mass measurement discovered around such a low-mass host star. The high mass of TOI-5205b stretches conventional theories of planet formation and disk scaling relations that cannot easily recreate the conditions required to form such planets.

TOI-3235 b: A Transiting Giant Planet around an M4 Dwarf Star

“…Most of these planets (HATS-6 b, Hartman et al 2015;HATS-74 b and HATS-75 b, Jordán et al 2022;Kepler-45 b, Johnson et al 2012; TOI-530 b, Gan et al 2022; TOI-3714 b, Canas et al 2022;WASP-80 b, Triaud et al 2013) are hosted by early M dwarfs with M å 0.5 M ☉ . The sole other exception, aside from TOI-3235 b itself, is TOI-5205 b, a Jupiter-sized planet transiting a 0.392 M ☉ star (Kanodia et al 2023). Save for TOI-530 b, which has a somewhat longer period of 6.39 days, these planets also cluster together in period-radius space (Figure 4, center panel), forming a group of midrange close-in Jupiters with periods between 1.63 and 3.33 days.…”

“…Since the host stars have the same mass, we can extrapolate from their analysis; the main difference is that TOI-3235 b is rather less massive than TOI-5205 b. As regards planetary heavy-element mass, using the relations of Thorngren et al (2016) we find a heavy-element mass of M Z ∼ 45 M ⊕ corresponding to 75% of that of TOI-5205 b; therefore, assuming a solid core and scaling the results of Kanodia et al (2023), the required disk mass for TOI-3235 b becomes ∼2%-23% the mass of the host star for 100%-10% formation efficiency, respectively. While lower than the disk mass required to explain TOI-5205 b, given typical disk masses of the order of ∼0.1%-5% (Pascucci et al 2016), the formation of TOI-3235 b still requires either an extremely high formation efficiency or a very massive disk.…”

“…Both these stars are therefore in the transition region between partially and fully convective M dwarfs and as such are likely to undergo periodic changeovers from partially to fully convective and vice versa that alter their radius and luminosity (e.g., van Saders & Pinsonneault 2012; Baraffe & Chabrier 2018). As noted by Silverstein et al (2022) and Kanodia et al (2023), these oscillations may impact the planetary orbital parameters and equilibrium temperature. It is possible that the similar planets of these similar stars may share similar formation and/ or evolution histories.…”

“…It is possible that the similar planets of these similar stars may share similar formation and/ or evolution histories. Kanodia et al (2023) studied the disk mass necessary to form TOI-5205 b. Since the host stars have the same mass, we can extrapolate from their analysis; the main difference is that TOI-3235 b is rather less massive than TOI-5205 b.…”

“…However, as noted by Schlecker et al (2022), gravitational instability is expected to form very massive planets of ≈10M J on large orbits, while the planets found to date are mainly of Jupiter mass, and many have short orbital periods. Likewise, most of these planets orbit early M dwarfs, for which the Bern model can, though rarely, produce gas giants; the first, and until now only, exception was TOI-5205 b (Kanodia et al 2023), which orbits an M4 star. It is also worth noting that the Bern models normally assume a smooth initial gas surface density distribution in the protoplanetary disk; a nonsmooth density distribution could modify the migration history and potentially facilitate the formation of these planets.…”