Separated twins or just siblings? A multi-planet system around an M dwarf including a cool sub-Neptune

TL;DR

This paper reports the discovery and validation of a multi-planet system around an M dwarf, including the longest-period M dwarf exoplanet found by TESS, with potential for atmospheric characterization.

Contribution

First validation of a multi-planet system around an early M dwarf with a long-period sub-Neptune, including the longest-period M dwarf exoplanet detected by TESS.

Findings

Two sub-Neptunes orbiting TOI-904 with measured periods of 10.9 and 84 days.

The outer planet, TOI-904 c, is the coldest and longest-period M dwarf exoplanet found by TESS.

The system's planets are promising targets for atmospheric follow-up and composition studies.

Abstract

We report the discovery of two TESS sub-Neptunes orbiting the early M dwarf TOI-904 (TIC 261257684). Both exoplanets, TOI-904 b and c, were initially observed in TESS Sector 12 with twin sizes of ${2.426}_{-0.157}^{+0.163}$ and ${2.167}_{-0.118}^{+0.130}$ R$_\oplus$, respectively. Through observations in five additional sectors in the TESS primary mission and the first and second extended missions, the orbital periods of the planets were measured to be 10.887$\pm$0.001 and 83.999$\pm$0.001 days, respectively. Reconnaissance radial velocity measurements (taken with EULER/CORALIE and SMARTS/CHIRON) and high-resolution speckle imaging with adaptive optics (obtained from SOAR/HRCAM and Gemini South/ZORRO) show no evidence of an eclipsing binary or a nearby companion, which, together with the low false-positive probabilities calculated with the statistical validation software TRICERATOPS, establishes the planetary nature of these candidates. The outer planet, TOI-904 c, is the longest-period M dwarf exoplanet found by TESS, with an estimated equilibrium temperature of 217 K. As the three other validated planets with comparable host stars and orbital periods were observed by Kepler around much dimmer stars (J mag > 12), TOI-904 c, orbiting a brighter star (J mag = 9.6), is the coldest M dwarf planet easily accessible for atmospheric follow-up. Future mass measurements and transmission spectroscopy of the similar-sized planets in this system could determine whether they are also similar in density and composition, suggesting a common formation pathway, or whether they have distinct origins.