Abstract
ABSTRACT Based on HARPS-N radial velocities (RVs) and TESS photometry, we present a full characterization of the planetary system orbiting the late G dwarf TOI-561. After the identification of three transiting candidates by TESS, we discovered two additional external planets from RV analysis. RVs cannot confirm the outer TESS transiting candidate, which would also make the system dynamically unstable. We demonstrate that the two transits initially associated with this candidate are instead due to single transits of the two planets discovered using RVs. The four planets orbiting TOI-561 include an ultra-short period (USP) super-Earth (TOI-561 b) with period Pb = 0.45 d, mass Mb = 1.59 ± 0.36 M⊕ and radius Rb = 1.42 ± 0.07 R⊕, and three mini-Neptunes: TOI-561 c, with Pc = 10.78 d, Mc = 5.40 ± 0.98 M⊕, Rc = 2.88 ± 0.09 R⊕; TOI-561 d, with Pd = 25.6 d, Md = 11.9 ± 1.3 M⊕, Rd = 2.53 ± 0.13 R⊕; and TOI-561 e, with Pe = 77.2 d, Me = 16.0 ± 2.3 M⊕, Re = 2.67 ± 0.11 R⊕. Having a density of 3.0 ± 0.8 g cm−3, TOI-561 b is the lowest density USP planet known to date. Our N-body simulations confirm the stability of the system and predict a strong, anti-correlated, long-term transit time variation signal between planets d and e. The unusual density of the inner super-Earth and the dynamical interactions between the outer planets make TOI-561 an interesting follow-up target.
Highlights
The Transiting Exoplanet Survey Satellite (TESS, Ricker et al 2014) is a NASA all-sky survey designed to search for transiting planets around bright and nearby stars, and targeting stars that could reveal planets with radii smaller than Neptune
Based on HARPS-N radial velocities (RVs) and TESS photometry, we present a full characterisation of the planetary system orbiting the late G dwarf TOI-561
If two or more observations are gathered during the same night and they span a large fraction of the orbital phase, the RV semi-amplitude of the ultra-short period (USP) planet can be precisely measured by just applying nightly offsets to remove all the other signals (e.g. Hatzes et al 2010; Howard et al 2013; Pepe et al 2013; Frustagli et al 2020 for a recent example)
Summary
The Transiting Exoplanet Survey Satellite (TESS, Ricker et al 2014) is a NASA all-sky survey designed to search for transiting planets around bright and nearby stars, and targeting stars that could reveal planets with radii smaller than Neptune. Since the beginning of its observations in 2018, TESS has discovered more than 66 exoplanets, including about a dozen multi-planet systems (e.g. Dragomir et al 2019; Dumusque et al 2019; Günther et al 2019). Multi-planet systems, orbiting the same star and having formed from the same protoplanetary disc, offer a unique opportunity for comparative planetology. They allow for investigations of the formation and evolution processes, i.e. through studies of relative planet sizes and orbital separations, orbital inclinations relative to the star’s rotation
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