Mass determination of the 1:3:5 near-resonant planets transiting GJ 9827 (K2-135)^★

¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
e-mail: jparranz@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206, La Laguna, Tenerife, Spain
³ Dipartimento di Fisica, Universitá di Torino, Via P. Giuria 1, 10125, Torino, Italy
⁴ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁵ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeto, NJ 08544, USA
⁶ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁷ Leiden Observatory, Leiden University, 2333 CA Leiden, The Netherlands
⁸ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁹ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
¹⁰ Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹¹ Astronomy Department and Van Vleck Observatory, Wesleyan University, Middletown, CT 06459, USA
¹² Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
¹³ Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany
¹⁴ Department of Astronomy and McDonald Observatory, University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
¹⁵ National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
¹⁶ Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, NINS, Asakuchi, Okayama 719-0232, Japan
¹⁷ Rheinisches Institut für Umweltforschung an der Universität zu Köln, Aachener Strasse 209, 50931 Köln, Germany
¹⁸ Departamento de Astronomia do IAG/USP, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP, Brazil
¹⁹ Astrobiology Center, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁰ National Astronomical Observatory of Japan, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²¹ Center for Astronomy and Astrophysics, TU Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
²² Institut de Ciències de l’Espai (IEEC-CSIC), C/ Can Magrans s/n, Campus UAB, 08193 Bellaterra, Spain
²³ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
²⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117, Heidelberg, Germany

Received: 22 February 2018
Accepted: 14 July 2018

Abstract

Context. Multiplanet systems are excellent laboratories to test planet formation models as all planets are formed under the same initial conditions. In this context, systems transiting bright stars can play a key role, since planetary masses, radii, and bulk densities can be measured.

Aims. GJ 9827 (K2-135) has recently been found to host a tightly packed system consisting of three transiting small planets whose orbital periods of 1.2, 3.6, and 6.2 days are near the 1:3:5 ratio. GJ 9827 hosts the nearest planetary system (~30 pc) detected by NASA’s Kepler or K2 space mission. Its brightness (V = 10.35 mag) makes the star an ideal target for detailed studies of the properties of its planets.

Methods. Combining the K2 photometry with high-precision radial-velocity measurements gathered with the FIES, HARPS, and HARPS-N spectrographs we revised the system parameters and derive the masses of the three planets.

Results. We find that GJ 9827 b has a mass of M_b = 3.69_−0.46^+0.48 M_⊕ and a radius of R_b = 1.58_−0.13^+0.14 R_⊕, yielding a mean density of ρ_b = 5.11_−1.27^+1.74 g cm⁻³. GJ 9827 c has a mass of M_c = 1.45_−0.57^+0.58 M_⊕, radius of R_c = 1.24_−0.11^+0.11 R_⊕, and a mean density of ρ_c = 4.13_−1.77^+2.31 g cm⁻³. For GJ 9827 d, we derive M_d = 1.45_−0.57^+0.58 M_⊕, R_d = 1.24_−0.11^+0.11 R_⊕, and ρ_d = 1.51_−0.53^+0.71 g cm⁻³.

Conclusions. GJ 9827 is one of the few known transiting planetary systems for which the masses of all planets have been determined with a precision better than 30%. This system is particularly interesting because all three planets are close to the limit between super-Earths and sub-Neptunes. The planetary bulk compositions are compatible with a scenario where all three planets formed with similar core and atmosphere compositions, and we speculate that while GJ 9827 b and GJ 9827 c lost their atmospheric envelopes, GJ 9827 d maintained its primordial atmosphere, owing to the much lower stellarirradiation. This makes GJ 9827 one of the very few systems where the dynamical evolution and the atmosphericescape can be studied in detail for all planets, helping us to understand how compact systems form and evolve.