Discovery and characterisation of two Neptune-mass planets orbiting HD 212729 with TESS

Abstract

We report the discovery of two exoplanets orbiting around HD 212729 (TOI\,1052, TIC 317060587), a $T_{\mathrm{eff}}=6146$K star with V=9.51 observed by TESS in Sectors 1 and 13. One exoplanet, TOI-1052b, is Neptune-mass and transits the star, and an additional planet TOI-1052c is observed in radial velocities but not seen to transit. We confirm the planetary nature of TOI-1052b using precise radial velocity observations from HARPS and determined its parameters in a joint RV and photometry analysis. TOI-1052b has a radius of $2.87_{-0.24}^{+0.29}$ R${}_{\oplus }$, a mass of $16.9\pm 1.7$ M${}_{\oplus }$, and an orbital period of 9.14 days. TOI-1052c does not show any transits in the TESS data, and has a minimum mass of $34.3_{-3.7}^{+4.1}$ M${}_{\oplus }$ and an orbital period of 35.8 days, placing it just interior to the 4:1 mean motion resonance. Both planets are best fit by relatively high but only marginally significant eccentricities of $0.18_{-0.07}^{+0.09}$ for planet b and $0.24_{-0.08}^{+0.09}$ for planet c. We perform a dynamical analysis and internal structure model of the planets as well as deriving stellar parameters and chemical abundances. The mean density of TOI-1052b is $3.9_{-1.3}^{+1.7}$ g cm${}^{-3}$ consistent with an internal structure similar to Neptune. A nearby star is observed in Gaia DR3 with the same distance and proper motion as TOI-1052, at a sky projected separation of ~1500AU, making this a potential wide binary star system.

Accepted to MNRAS. 11 pages