Company for the Ultra-high Density, Ultra-short Period Sub-Earth GJ 367 b: Discovery of Two Additional Low-mass Planets at 11.5 and 34 Days

Creators: Goffo, Elisa^{1, 2}; Gandolfi, Davide¹; Egger, Jo Ann³; Mustill, Alexander J.⁴; Albrecht, Simon H.⁵; Hirano, Teruyuki⁶; Kochukhov, Oleg⁷; Astudillo-Defru, Nicola⁸; Barragan, Oscar⁹; Serrano, Luisa M.¹; Hatzes, Artie P.²; Alibert, Yann³; Guenther, Eike²; Dai, Fei¹⁰; Lam, Kristine W. F.¹¹; Csizmadia, Szilárd¹¹; Smith, Alexis M. S.¹¹; Fossati, Luca¹²; Luque, Rafael¹³; Rodler, Florian¹⁴; Winther, Mark L.⁵; Rørsted, Jakob L.⁵; Alarcon, Javier¹⁴; Bonfils, Xavier¹⁵; Cochran, William D.¹⁶; Deeg, Hans J.^{17, 18}; Jenkins, Jon M.¹⁹; Korth, Judith⁴; Livingston, John H.^{6, 20}; Meech, Annabella⁹; Murgas, Felipe^{17, 18}; Orell-Miquel, Jaume^{17, 18}; Osborne, Hannah L. M.²¹; Palle, Enric^{17, 18}; Persson, Carina M.²²; Redfield, Seth²³; Ricker, George R.²⁴; Seager, Sara²⁵; Vanderspek, Roland²⁵; Van Eylen, Vincent²¹; Winn, Joshua N.²⁶

1. University of Turin
2. Thüringer Landessternwarte Tautenburg
3. University of Bern
4. Lund University
5. Aarhus University
6. National Astronomical Observatory of Japan
7. Uppsala University
8. Catholic University of the Most Holy Conception
9. University of Oxford
10. California Institute of Technology
11. German Aerospace Center
12. Space Research Institute
13. University of Chicago
14. European Southern Observatory
15. Institut de Planétologie et d'Astrophysique de Grenoble
16. The University of Texas at Austin
17. Instituto de Astrofísica de Canarias
18. University of La Laguna
19. Ames Research Center
20. The Graduate University for Advanced Studies, SOKENDAI
21. University College London
22. Chalmers University of Technology
23. Wesleyan University
24. MIT Kavli Institute for Astrophysics and Space Research & MIT Physics Department, USA
25. Massachusetts Institute of Technology
26. Princeton University

Abstract

GJ 367 is a bright (V ≈ 10.2) M1 V star that has been recently found to host a transiting ultra-short period sub-Earth on a 7.7 hr orbit. With the aim of improving the planetary mass and radius and unveiling the inner architecture of the system, we performed an intensive radial velocity follow-up campaign with the HARPS spectrograph—collecting 371 high-precision measurements over a baseline of nearly 3 yr—and combined our Doppler measurements with new TESS observations from sectors 35 and 36. We found that GJ 367 b has a mass of M_b = 0.633 ± 0.050 M_⊕ and a radius of R_b = 0.699 ± 0.024 R_⊕, corresponding to precisions of 8% and 3.4%, respectively. This implies a planetary bulk density of ρ_b = 10.2 ± 1.3 g cm⁻³, i.e., 85% higher than Earth's density. We revealed the presence of two additional non-transiting low-mass companions with orbital periods of ∼11.5 and 34 days and minimum masses of Mcsin⁡ic = 4.13 ± 0.36 M_⊕ and Mdsin⁡id = 6.03 ± 0.49 M_⊕, respectively, which lie close to the 3:1 mean motion commensurability. GJ 367 b joins the small class of high-density planets, namely the class of super-Mercuries, being the densest ultra-short period small planet known to date. Thanks to our precise mass and radius estimates, we explored the potential internal composition and structure of GJ 367 b, and found that it is expected to have an iron core with a mass fraction of 0.91_(−0.23)^(+0.07). How this iron core is formed and how such a high density is reached is still not clear, and we discuss the possible pathways of formation of such a small ultra-dense planet.

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Acknowledgement

We thank the anonymous referee for a thoughtful review of our paper and very positive feedback. This work was supported by the KESPRINT collaboration, an international consortium devoted to the characterization and research of exoplanets discovered with space-based missions (www.kesprint.science). Based on observations made with the ESO-3.6 m telescope at La Silla Observatory under programs 1102.C-0923 and 106.21TJ.001. We are extremely grateful to the ESO staff members for their unique and superb support during the observations. We acknowledge the use of public TESS data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. TESS data presented in this paper were obtained from the Milkulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute. The specific observations analyzed can be accessed via doi:10.17909/rkwv-t847. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. E.G. acknowledges the generous support from Deutsche Forschungsgemeinschaft (DFG) of the grant HA3279/14-1. D.G. gratefully acknowledges financial support from the Cassa di Risparmio di Torino (CRT) foundation under Grant No. 2018.2323 "Gaseous or rocky? Unveiling the nature of small worlds". Y.A. and J.A.E. acknowledge the support of the Swiss National Fund under grant 200020_192038. C.M.P. gratefully acknowledges the support of the Swedish National Space Agency (DNR 65/19). R.L. acknowledges funding from University of La Laguna through the Margarita Salas Fellowship from the Spanish Ministry of Universities ref. UNI/551/2021-May 26, and under the EU Next Generation funds. K.W.F.L. was supported by Deutsche Forschungsgemeinschaft grants RA714/14-1 within the DFG Schwerpunkt SPP 1992, Exploring the Diversity of Extrasolar Planets. S.A. acknowledges the support from the Danish Council for Independent Research through grant No. 2032-00230B. O.B. received funding from the ERC under the European Union's Horizon 2020 research and innovation program (grant agreement No. 865624). H.J.D. acknowledges support from the Spanish Research Agency of the Ministry of Science and Innovation (AEI-MICINN) under grant 'Contribution of the IAC to the PLATO Space Mission' with reference PID2019-107061GB-C66. A.J.M. acknowledges support from the Swedish National Space Agency (career grant 120/19C). O.K. acknowledges support by the Swedish Research Council (grant agreement No. 2019-03548), the Swedish National Space Agency, and the Royal Swedish Academy of Sciences. J.K. gratefully acknowledges the support of the Swedish National Space Agency (SNSA; DNR 2020-00104) and of the Swedish Research Council (VR: Etableringsbidrag 2017-04945).

Additional Information

Based on observations made with the ESO-3.6 m telescope at La Silla Observatory under programs 1102.C-0923 and 106.21TJ.001.

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