TOI-269 b: An eccentric sub-Neptune transiting a M2 dwarf revisited with ExTrA

TL;DR

TOI-269 b is a confirmed eccentric sub-Neptune orbiting an M2 dwarf, with well-measured properties, making it a prime candidate for atmospheric studies and offering insights into planetary migration and eccentricity.

Contribution

This study confirms and characterizes TOI-269 b with combined photometry and radial velocities, revealing its high eccentricity and potential for atmospheric characterization.

Findings

TOI-269 b has a radius of 2.77 Rearth and a mass of 8.8 Mearth.

It has a high eccentricity of 0.425, unusual for short-period planets.

The system is likely several Gyr old, indicating a dynamically mature system.

Abstract

We present the confirmation of a new sub-Neptune close to the transition between super-Earths and sub-Neptunes transiting the M2 dwarf TOI- 269 (TIC 220479565, V = 14.4 mag, J = 10.9 mag, Rstar = 0.40 Rsun, Mstar = 0.39 Msun, d = 57 pc). The exoplanet candidate has been identified in multiple TESS sectors, and validated with high-precision spectroscopy from HARPS and ground-based photometric follow-up from ExTrA and LCO-CTIO. We determined mass, radius, and bulk density of the exoplanet by jointly modeling both photometry and radial velocities with juliet. The transiting exoplanet has an orbital period of P = 3.6977104 +- 0.0000037 days, a radius of 2.77 +- 0.12 Rearth, and a mass of 8.8 +- 1.4 Mearth. Since TOI-269 b lies among the best targets of its category for atmospheric characterization, it would be interesting to probe the atmosphere of this exoplanet with transmission spectroscopy in order to compare it to other sub-Neptunes. With an eccentricity e = 0.425+0.082-0.086, TOI-269 b has one of the highest eccentricities of the exoplanets with periods less than 10 days. The star being likely a few Gyr old, this system does not appear to be dynamically young. We surmise TOI-269 b may have acquired its high eccentricity as it migrated inward through planet-planet interactions.