TOI-1268b: the youngest, hot, Saturn-mass transiting exoplanet

Abstract

We report the discovery of TOI-1268b, a transiting Saturn-mass planet from the TESS space mission. With an age of less than one Gyr, derived from various age indicators, TOI-1268b is the youngest Saturn-mass planet known to date and contributes to the small sample of well characterised young planets. It has an orbital period of $P=8.1577080\pm 0.0000044$ days, and transits an early K dwarf star with a mass of $M_{\star }$ = $0.96\pm 0.04$ $M_{\odot }$, a radius of $R_{\star }$ = $0.92\pm 0.06$ $R_{\odot }$, an effective temperature of $T_{\mathrm{eff}}=5300\pm 100$ K, and a metallicity of $0.36\pm 0.06$ dex. By combining TESS photometry with high-resolution spectra acquired with the Tull spectrograph at McDonald observatory, and the high-resolution spectrographs at Tautenburg and Ondrejov observatories, we measured a planetary mass of $M_{\mathrm{p}}=96.4\pm 8.3M_{\oplus }$ and a radius of $R_{\mathrm{p}}=9.1\pm 0.6R_{\oplus }$. TOI-1268 is an ideal system to study the role of star-planet tidal interactions for non-inflated Saturn-mass planets. We used system parameters derived in this paper to constrain the planet tidal quality factor to the range of $10^{4.5-5.3}$. When compared with the sample of other non-inflated Saturn-mass planets, TOI-1268b is one of the best candidates for transmission spectroscopy studies.

21 pages, 17 figures, accepted for publication in Astronomy & Astrophysics; see independent work by Dong et al. for Rossiter-McLaughlin measurement of TOI-1268b