An unusually low density ultra-short period super-Earth and three mini-Neptunes around the old star TOI-561
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Based on HARPS-N radial velocities (RVs) and TESS photometry, we present a full characterisation of the planetary system orbiting the late G dwarf 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.
Lacedelli , G , Malavolta , L , Borsato , L , Piotto , G , Nardiello , D , Mortier , A , Stalport , M , Cameron , A C , Poretti , E , Buchhave , L A , López-Morales , M , Nascimbeni , V , Wilson , T G , Udry , S , Latham , D W , Bonomo , A S , Damasso , M , Dumusque , X , Jenkins , J M , Lovis , C , Rice , K , Sasselov , D , Winn , J N , Andreuzzi , G , Cosentino , R , Charbonneau , D , Di Fabrizio , L , Martinez Fiorenzano , A F , Ghedina , A , Harutyunyan , A , Lienhard , F , Micela , G , Molinari , E , Pagano , I , Pepe , F , Phillips , D F , Pinamonti , M , Ricker , G , Scandariato , G , Sozzetti , A & Watson , C A 2021 , ' An unusually low density ultra-short period super-Earth and three mini-Neptunes around the old star TOI-561 ' , Monthly Notices of the Royal Astronomical Society , vol. 501 , no. 3 , pp. 4148–4166 . https://doi.org/10.1093/mnras/staa3728
Monthly Notices of the Royal Astronomical Society
Copyright © 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the author created accepted manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1093/mnras/staa3728.