The GAPS Programme at TNG XL: A puffy and warm Neptune-sized planet and an outer Neptune-mass candidate orbiting the solar-type star TOI-1422

Abstract

We investigate the exoplanet candidate TOI-1422b, which was discovered by the TESS space telescope around the high proper-motion G2V star TOI-1422 ($V=10.6$ mag), 155pc away, with the primary goal of confirming its planetary nature and characterising its properties. We monitored TOI-1422 with the HARPS-N spectrograph for 1.5 years to precisely quantify its radial velocity variation. The radial velocity measurements are analyzed jointly with TESS photometry and we also check for blended companions through high-spatial resolution images using the AstraLux instrument. We estimate that the parent star has a radius and a mass of $R^{\ast }=1.019_{-0.013}^{+0.014}{R}_{\odot }$, $M^{\ast }=0.981_{-0.065}^{+0.062}{M}_{\odot }$, respectively. Our analysis confirms the planetary nature of TOI-1422b and also suggests the presence of a Neptune-mass planet on a more distant orbit, the candidate TOI-1422c, which is not detected in TESS light curves. The inner planet, TOI-1422b, orbits on a period $P_{\mathrm{b}}=12.9972\pm 0.0006$ days and has an equilibrium temperature $T_{\mathrm{eq},\mathrm{b}}=867\pm 17$ K. With a radius of $R_{\mathrm{b}}=3.96_{-0.11}^{+0.13}{R}_{\oplus }$, a mass of $M_{\mathrm{b}}=9.0_{-2.0}^{+2.3}{M}_{\oplus }$ and, consequently, a density of $\ddot{I}{}_{\mathrm{b}}=0.795_{-0.235}^{+0.290}$ g cm${}^{-3}$, it can be considered a warm Neptune-size planet. Compared to other exoplanets of similar mass range, TOI-1422b is among the most inflated ones and we expect this planet to have an extensive gaseous envelope that surrounds a core with a mass fraction around $10\%-25\%$ of the total mass of the planet. The outer non-transiting planet candidate, TOI-1422c, has an orbital period of $P_{\mathrm{c}}=29.29_{-0.20}^{+0.21}$ days, a minimum mass, $M_{\mathrm{c}}\sin i$, of $11.1_{-2.3}^{+2.6}{M}_{\oplus }$, an equilibrium temperature of $T_{\mathrm{eq},\mathrm{c}}=661\pm 13$ K and, therefore, if confirmed, it could be considered as another warm Neptune.

22 pages, 20 figures, 6 tables. Accepted for publication in Astronomy & Astrophysics on July 7, 2022. Abstract abridged