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

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 220 479 565, V = 14.4 mag, J = 10.9 mag, R-star = 0.40 R-circle dot, M-star = 0.39 M-circle dot, 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 R-circle plus, and a mass of 8.8 +/- 1.4 M-circle plus. 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(-0.086)(+0.082) e=0.425-0.086+0.082 , 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.

Automated summary

A new sub-Neptune planet TOI-269 close to the transition between super-Earths and sub-Neptunes has been confirmed, with high eccentricity and potential interactions leading to its current orbit. The system, with a few Gyr old star, does not display signs of being dynamically young. Further characterization of TOI-269 b's atmosphere through transmission spectroscopy could provide valuable insights into comparing it to other sub-Neptunes.