TOI-1670 b and c: An Inner Sub-Neptune with an Outer Warm Jupiter Unlikely to have Originated from High-Eccentricity Migration

Abstract

We report the discovery of two transiting planets around the bright ($V=9.9$ mag) main sequence F7 star TOI-1670 by the Transiting Exoplanet Survey Satellite. TOI-1670 b is a sub-Neptune ($R_{\mathrm{b}}=2.06_{-0.15}^{+0.19}$ $R_{\oplus }$) on a 10.9-day orbit and TOI-1670 c is a warm Jupiter ($R_{\mathrm{c}}=0.987_{-0.025}^{+0.025}$ $R_{\mathrm{Jup}}$) on a 40.7-day orbit. Using radial velocity observations gathered with the Tull coudé Spectrograph on the Harlan J. Smith telescope and HARPS-N on the Telescopio Nazionale Galileo, we find a planet mass of $M_{\mathrm{c}}=0.63_{-0.08}^{+0.09}$ $M_{\mathrm{Jup}}$ for the outer warm Jupiter, implying a mean density of ${\rho }_c=0.81_{-0.11}^{+0.13}$ g cm${}^{-3}$. The inner sub-Neptune is undetected in our radial velocity data ($M_{\mathrm{b}}<0.13$ $M_{\mathrm{Jup}}$ at the 99% confidence level). Multi-planet systems like TOI-1670 hosting an outer warm Jupiter on a nearly circular orbit ($e_{\mathrm{c}}=0.09_{-0.04}^{+0.05}$) and one or more inner coplanar planets are more consistent with "gentle" formation mechanisms such as disk migration or $in$ $situ$ formation rather than high-eccentricity migration. Of the 11 known systems with a warm Jupiter and a smaller inner companion, 8 (73%) are near a low-order mean-motion resonance, which can be a signature of migration. TOI-1670 joins two other systems (27% of this subsample) with period commensurabilities greater than 3, a common feature of $in$ $situ$ formation or halted inward migration. TOI-1670 and the handful of similar systems support a diversity of formation pathways for warm Jupiters.

23 pages, 9 figures, accepted for publication in AJ

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