Exoplanet characterisation in the longest known resonant chain: the K2-138 system seen by HARPS

Abstract

The detection of low-mass transiting exoplanets in multiple systems brings new constraints to planetary formation and evolution processes and challenges the current planet formation theories. Nevertheless, only a mere fraction of the small planets detected by Kepler and K2 have precise mass measurements, which are mandatory to constrain their composition. We aim to characterise the planets that orbit the relatively bright star K2-138. This system is dynamically particular as it presents the longest chain known to date of planets close to the 3:2 resonance. We obtained 215 HARPS spectra from which we derived the radial-velocity variations of K2-138. Via a joint Bayesian analysis of both the K2 photometry and HARPS radial-velocities (RVs), we constrained the parameters of the six planets in orbit. The masses of the four inner planets, from b to e, are 3.1, 6.3, 7.9, and 13.0 ${\mathrm{M}}_{\oplus }$ with a precision of 34%, 20%, 18%, and 15%, respectively. The bulk densities are 4.9, 2.8, 3.2, and 1.8 g cm${}^{-3}$, ranging from Earth to Neptune-like values. For planets f and g, we report upper limits. Finally, we predict transit timing variations of the order two to six minutes from the masses derived. Given its peculiar dynamics, K2-138 is an ideal target for transit timing variation (TTV) measurements from space with the upcoming CHaracterizing ExOPlanet Satellite (CHEOPS) to study this highly-packed system and compare TTV and RV masses.

References in corpus (26)

• The Dartmouth Stellar Evolution Database
• The K2 Mission: Characterization and Early results
• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• Mass-Radius Relationships for Solid Exoplanets
• Most 1.6 Earth-Radius Planets are not Rocky
• A Technique for Extracting Highly Precise Photometry for the Two-Wheeled Kepler Mission
• Measuring the rotation period distribution of field M-dwarfs with Kepler
• Homogeneous studies of transiting extrasolar planets. I. Light curve analyses
• Spitzer Secondary Eclipses of the Dense, Modestly-irradiated, Giant Exoplanet HAT-P-20b Using Pixel-Level Decorrelation
• Three-dimensional simulations of multiple protoplanets embedded in a protostellar disc
• EVEREST: Pixel Level Decorrelation of K2 Light curves
• ARES v2 - new features and improved performance
• PASTIS: Bayesian extrasolar planet validation. I. General framework, models, and performance
• Observable Consequences of Planet Formation Models in Systems with Close-in Terrestrial Planets
• The Kepler-10 planetary system revisited by HARPS-N: A hot rocky world and a solid Neptune-mass planet
• Distances and parallax bias in Gaia DR2
• APASS Landolt-Sloan BVgri photometry of RAVE stars. I. Data, effective temperatures and reddenings
• A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system
• Correcting the spectroscopic surface gravity using transits and asteroseismology. No significant effect on temperatures or metallicities with ARES+MOOG in LTE
• New planetary and EB candidates from Campaigns 1-6 of the K2 mission
• Constraints on Super-Earths Interiors from Stellar Abundances
• The K2-138 System: A Near-Resonant Chain of Five Sub-Neptune Planets Discovered by Citizen Scientists
• Revisiting the transits of CoRoT-7b at a lower activity level
• The TROY project: Searching for co-orbital bodies to known planets. I. Project goals and first results from archival radial velocity
• Precise masses for the transiting planetary system HD 106315 with HARPS
• Co-orbital exoplanets from close period candidates: The TOI-178 case

Cited by in corpus (29)

• Six transiting planets and a chain of Laplace resonances in TOI-178
• The CARMENES search for exoplanets around M dwarfs. Two planets on the opposite sides of the radius gap transiting the nearby M dwarf LTT 3780
• A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067
• A framework for the architecture of exoplanetary systems. I. Four classes of planetary system architecture
• The dynamics of the TRAPPIST-1 system in the context of its formation
• HD 23472: A multi-planetary system with three super-Earths and two potential super-Mercuries
• Shallower radius valley around low-mass hosts: Evidence for icy planets, collisions or high-energy radiation scatter
• Masses for the seven planets in K2-32 and K2-233. Four diverse planets in resonant chain and the first young rocky worlds
• TOI-216: Resonant Constraints on Planet Migration
• Unveiling the internal structure and formation history of the three planets transiting HIP 29442 (TOI-469) with CHEOPS
• Resonant Chains of Exoplanets: Libration Centers for Three-body Angles
• Resonant sub-Neptunes are puffier
• Confirming the 3:2 Resonance Chain of K2-138
• Water content trends in K2-138 and other low-mass multiplanetary systems
• Dynamical Evolution of Closely Packed Multiple Planetary Systems Subject to Atmospheric Mass-Loss
• TOI-220 $b$: a warm sub-Neptune discovered by TESS
• K2-138 g: Spitzer Spots a Sixth Planet for the Citizen Science System
• Refining the properties of the TOI-178 system with CHEOPS and TESS
• Higher Water Loss on Earth-like Exoplanets in Eccentric Orbits
• Photo-dynamical characterisation of the TOI-178 resonant chain
• Tidal interactions shape period ratios in planetary systems with three-body resonant chains
• Detection capability of ground-based meter-sized telescopes for shallow exoplanet transits
• Exploiting periodic orbits as dynamical clues for Kepler and K2 systems
• Resonant chains in triple-planetary systems
• The orbital evolution of resonant chains of exoplanets incorporating circularisation produced by tidal interaction with the central star with application to the HD 158259 and EPIC 245950175 systems
• The Dynamical History of the Kepler-221 Planet System
• Particle dynamics in TOI-178 planetary system
• The Occurrence-weighted Median Planets Discovered by Transit Surveys Orbiting Solar-type Stars and Their Implications for Planet Formation and Evolution
• How to measure tidal dissipation in long resonant chains