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

Abstract

Context. Multi-planet systems are excellent laboratories to test planet formation models, since 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 accurately 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 (d = 30.32 ± 1.62 pc) detected by Kepler and K2. 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 revise the system parameters and derive the masses of the three planets.Results. We find that GJ 9827 b has a mass of M b = 3.74 +0.50 −0.48 M ⊕ and a radius of R b = 1.62 +0.17 −0.16 R ⊕ , yielding a mean density of ρ b = 4.81 +1.97 −1.33 g cm −3 . GJ 9827 c has a mass of M c = 1.47 +0.59 −0.58 M ⊕ , radius of R c = 1.27 +0.13 −0.13 R ⊕ , and a mean density of ρ c = 3.87 +2.38 −1.71 g cm −3 . For GJ 9827 d we derive M d = 2.38 +0.71 −0.69 M ⊕ , R d = 2.09 +0.22 −0.21 R ⊕ , and ρ d = 1.42 +0.75 −0.52 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 mini-Neptunes. We also find that the planetary bulk compositions are compatible with a scenario where all three planets formed with similar core/atmosphere compositions, and we speculate that while GJ 9827 b and GJ 9827 c lost their atmospheric envelopes, GJ 9827 d maintained its atmosphere, owing to the much lower stellar irradiation. This makes GJ 9827 one of the very few systems where the dynamical evolution and the atmospheric escape can be studied in detail for all planets, helping us to understand how compact systems form and evolve.

The Transiting Multi-planet System HD15337: Two Nearly Equal-mass Planets Straddling the Radius Gap

“…As in other systems hosting two close-in sub-Neptunemass planets (e.g., HD 3167 Gandolfi et al 2017), the radii of HD 15337 b and c lie on opposite sides of the radius gap (Fulton et al 2017;Van Eylen et al 2018), with the closer-in planet having a higher bulk density, similar to other close-in systems with measured planetary masses (e.g., HD 3167, K2-109, GJ 9827; Gandolfi et al 2017;Guenther et al 2017;Prieto-Arranz et al 2018). This gap is believed to be caused by atmospheric escape (Owen & Wu 2017;Jin & Mordasini 2018), which is stronger for closer-in planets.…”

“…For the phase and amplitude of the activity signal we adopted uniform priors. While this simple model might not fully reproduce the periodic and quasi-periodic variations induced by evolving active regions carried around by stellar rotation, it has proven to be effective in accounting for the stellar signal of active and moderately active stars (e.g., Pepe et al 2013;Gandolfi et al 2017;Barragán et al 2018;Prieto-Arranz et al 2018). Any variation not properly modeled by the coherent sinecurve, and/or any instrumental noise not included in the nominal RV uncertainties, were accounted for by fitting two RV jitter terms for the two HARPS set-ups.…”

“…TESS also enables the discovery of multi-planet systems for which both mass and radius can be precisely measured. Because such planets orbit the same star, differences in mean density and atmospheric structure among planets belonging to the same system can be ascribed mainly to differences in planetary mass and orbital separation (see, e.g., Guenther et al 2017;Prieto-Arranz et al 2018). This greatly simplifies modeling of their past evolution history, thus constraining how these planets formed (Alibert et al 2005;Alibert & Benz 2017).…”

Masses and radii for the three super-Earths orbiting GJ 9827, and implications for the composition of small exoplanets

“…We also use the K2 lightcurve to constrain the stellar activity (Section 5). We then carry out a radial velocity analysis using the same radial velocity data as used by Teske et al (2018), Niraula et al (2017) and Prieto-Arranz et al (2018), but with an additional 41 new radial velocities from the HARPS-N spectrograph (Cosentino et al 2012(Cosentino et al , 2014. As we will discuss in Section 7, we were able to constrain the masses of GJ 9827 b and d to better than "10%" and about "20%", but were not able to place a strong constraint on the mass of GJ 9827 c. We also discuss what these results imply about the typical composition of planets below the radius gap (Fulton et al 2017), a particular science goal of the HARPS-N Collaboration.…”

HD 219666 b: a hot-Neptune from TESS Sector 1

“…The work presented here is part of the ongoing RV follow-up effort carried out by two teams, namely the KESPRINT consortium (see, e.g. Johnson et al 2016;Van Eylen et al 2016;Dai et al 2017;Gandolfi et al 2017;Barragán et al 2018;Prieto-Arranz et al 2018) and the NCORES consortium (see, e.g. Armstrong et al 2015;Lillo-Box et al 2016;Barros et al 2017;Lam et al 2018;Santerne et al 2018).…”