TOI-969: a late-K dwarf with a hot mini-Neptune in the desert and an eccentric cold Jupiter

Abstract

The current architecture of a given multi-planetary system is a key fingerprint of its past formation and dynamical evolution history. Long-term follow-up observations are key to complete their picture. In this paper we focus on the confirmation and characterization of the components of the TOI-969 planetary system, where TESS detected a Neptune-size planet candidate in a very close-in orbit around a late K-dwarf star. We use a set of precise radial velocity observations from HARPS, PFS and CORALIE instruments covering more than two years in combination with the TESS photometric light curve and other ground-based follow-up observations to confirm and characterize the components of this planetary system. We find that TOI-969 b is a transiting close-in ($P_b\sim 1.82$ days) mini-Neptune planet ($m_b=9.1_{-1.0}^{+1.1}$ M${}_{\oplus }$, $R_b=2.765_{-0.097}^{+0.088}$ R${}_{\oplus }$), thus placing it on the {lower boundary} of the hot-Neptune desert ($T_{\mathrm{eq},\mathrm{b}}=941\pm 31$ K). The analysis of its internal structure shows that TOI-969 b is a volatile-rich planet, suggesting it underwent an inward migration. The radial velocity model also favors the presence of a second massive body in the system, TOI-969 c, with a long period of $P_c=1700_{-280}^{+290}$ days and a minimum mass of $m_c\sin i_c=11.3_{-0.9}^{+1.1}$ M${}_{\mathrm{Jup}}$, and with a highly-eccentric orbit of $e_c=0.628_{-0.036}^{+0.043}$. The TOI-969 planetary system is one of the few around K-dwarfs known to have this extended configuration going from a very close-in planet to a wide-separation gaseous giant. TOI-969 b has a transmission spectroscopy metric of 93, and it orbits a moderately bright ($G=11.3$ mag) star, thus becoming an excellent target for atmospheric studies. The architecture of this planetary system can also provide valuable information about migration and formation of planetary systems.

Accepted for publication in A&A. 25 pages, 15 figures, 12 tables