TOI-1468: A system of two transiting planets, a super-Earth and a mini-Neptune, on opposite sides of the radius valley

Abstract

We report the discovery and characterization of two small transiting planets orbiting the bright M3.0V star TOI-1468 (LSPM J0106+1913), whose transit signals were detected in the photometric time series in three sectors of the TESS mission. We confirm the e planetary nature of both of them using precise radial velocity measurements from the CARMENES and MAROON-X spectrographs, and supplement them with ground-based transit photometry. A joint analysis of all these data reveals that the shorter-period planet, TOI-1468 b ($P_{\mathrm{b}}$ = 1.88 d), has a planetary mass of $M_{\mathrm{b}}=3.21\pm 0.24$ $M_{\oplus }$ and a radius of $R_{\mathrm{b}}=1.280_{-0.039}^{+0.038}{R}_{\oplus }$, resulting in a density of $\ddot{I}{}_{\mathrm{b}}=8.39_{-0.92}^{+1.05}$ g cm${}^{-3}$, which is consistent with a mostly rocky composition. For the outer planet, TOI-1468 c ($P_{\mathrm{c}}=15.53$ d), we derive a mass of $M_{\mathrm{c}}=6.64_{-0.68}^{+0.67}$ $M_{\oplus }$, a radius of $R_{\mathrm{c}}=2.06\pm 0.04R_{\oplus }$, and a bulk density of $\ddot{I}{}_c=2.00_{-0.19}^{+0.21}$ g cm${}^{-3}$, which corresponds to a rocky core composition with a H/He gas envelope. These planets are located on opposite sides of the radius valley, making our system an interesting discovery as there are only a handful of other systems with the same properties. This discovery can further help determine a more precise location of the radius valley for small planets around M dwarfs and, therefore, shed more light on planet formation and evolution scenarios.

26 pages, 15 figures