Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit

Abstract

The bright M dwarf K2-18 at 34 pc is known to host a transiting super-Earth-sized planet orbiting within the star's habitable zone; K2-18b. Given the superlative nature of this system for studying an exoplanetary atmosphere receiving similar levels of insolation as the Earth, we aim to characterize the planet's mass which is required to interpret atmospheric properties and infer the planet's bulk composition. We obtain precision radial velocity measurements with the HARPS spectrograph and couple those measurements with the K2 photometry to jointly model the observed radial velocity variation with planetary signals and a radial velocity jitter model based on Gaussian process regression. We measure the mass of K2-18b to be $8.0\pm 1.9$ M${}_{\oplus }$ with a bulk density of $3.7\pm 0.9$ g/cm${}^3$ which may correspond to a predominantly rocky planet with a significant gaseous envelope or an ocean planet with a water mass fraction $\gtrsim 50$%. We also find strong evidence for a second, warm super-Earth K2-18c at $\sim 9$ days with a semi-major axis 2.4 times smaller than the transiting K2-18b. After re-analyzing the available light curves of K2-18 we conclude that K2-18c is not detected in transit and therefore likely has an orbit that is non-coplanar with K2-18b. A suite of dynamical integrations with varying simulated orbital eccentricities of the two planets are used to further constrain each planet's eccentricity posterior from which we measure $e_b<0.43$ and $e_c<0.47$ at 99% confidence. The discovery of the inner planet K2-18c further emphasizes the prevalence of multi-planet systems around M dwarfs. The characterization of the density of K2-18b reveals that the planet likely has a thick gaseous envelope which along with its proximity to the Solar system makes the K2-18 planetary system an interesting target for the atmospheric study of an exoplanet receiving Earth-like insolation.

arXiv comment13 pages, 8 figures including 4 interactive figures best viewed in Adobe Acrobat. Submitted to Astronomy & Astrophysics. Comments welcome

References in corpus (45)

• emcee: The MCMC Hammer
• A survey of cross-validation procedures for model selection
• Analytic Lightcurves for Planetary Transit Searches
• Habitable Zones Around Main-Sequence Stars: New Estimates
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• The Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4)
• REBOUND: An open-source multi-purpose N-body code for collisional dynamics
• batman: BAsic Transit Model cAlculatioN in Python
• Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b
• Most 1.6 Earth-Radius Planets are not Rocky
• Architecture of Kepler's Multi-transiting Systems: II. New investigations with twice as many candidates
• The CORALIE survey for southern extra-solar planets VII. Two short-period Saturnian companions to HD108147 and HD168746
• The Transiting Exoplanet Survey Satellite: Simulations of planet detections and astrophysical false positives
• Improving the Efficiency of Markov Chain Monte Carlo for Analyzing the Orbits of Extrasolar Planets
• WHFast: A fast and unbiased implementation of a symplectic Wisdom-Holman integrator for long term gravitational simulations
• A temperate rocky super-Earth transiting a nearby cool star
• A new list of thorium and argon spectral lines in the visible
• Detailed Models of super-Earths: How well can we infer bulk properties?
• Transits of Earth-Like Planets
• Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b
• A simple method to estimate radial velocity variations due to stellar activity using photometry
• The rotation and Galactic kinematics of mid M dwarfs in the Solar Neighborhood
• The HARPS search for southern extra-solar planets. X. A m sin i = 11 Mearth planet around the nearby spotted M dwarf GJ 674
• The Atmospheric Signatures of Super-Earths: How to Distinguish Between Hydrogen-Rich and Hydrogen-Poor Atmospheres
• A Detailed Model Grid for Solid Planets from 0.1 through 100 Earth Masses
• EVEREST: Pixel Level Decorrelation of K2 Light curves
• Bulk Composition of GJ 1214b and other sub-Neptune exoplanets
• They are Small Worlds After All: Revised Properties of Kepler M Dwarf Stars and their Planets
• Magnetic activity in the HARPS M-dwarf sample. The rotation-activity relationship for very low-mass stars through R'HK
• Stellar and Planetary Properties of K2 Campaign 1 Candidates and Validation of 17 Planets, Including a Planet Receiving Earth-like Insolation
• The Kepler-10 planetary system revisited by HARPS-N: A hot rocky world and a solid Neptune-mass planet
• TTVFast: An efficient and accurate code for transit timing inversion problems
• Feasibility Studies for the Detection of $O_2$ in an Earth-like Exoplanet
• The HARPS search for southern extra-solar planets I. HD330075 b: a new 'hot Jupiter'
• Comparing HARPS and Kepler surveys: The alignment of multiple-planet systems
• On The Relative Sizes of Planets Within Kepler Multiple Candidate Systems
• Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization
• Uncovering the planets and stellar activity of CoRoT-7 using only radial velocities
• Differential rotation of main-sequence dwarfs and its dynamo-efficiency
• The HARPS search for southern extra-solar planets XXXV. Planetary systems and stellar activity of the M dwarfs GJ 3293, GJ 3341, and GJ 3543
• Transit timing to first order in eccentricity
• Transit confirmation and improved stellar and planet parameters for the super-Earth HD 97658 b and its host star
• Model Spectra of the First Potentially Habitable Super-Earth - Gl581d
• Pinning down the mass of Kepler-10c: the importance of sampling and model comparison
• On the radial velocity detection of additional planets in transiting, slowly rotating M-dwarf systems: the case of GJ 1132

Cited by in corpus (61)

• Water Vapor and Clouds on the Habitable-Zone Sub-Neptune Exoplanet K2-18b
• Water vapour in the atmosphere of the habitable-zone eight Earth-mass planet K2-18 b
• Planetary system around the nearby M dwarf GJ 357 including a transiting, hot, Earth-sized planet optimal for atmospheric characterization
• The interior and atmosphere of the habitable-zone exoplanet K2-18b
• The Transiting Exoplanet Community Early Release Science Program for JWST
• CARMENES input catalogue of M dwarfs IV. New rotation periods from photometric time series
• A second terrestrial planet orbiting the nearby M dwarf LHS 1140
• The CARMENES search for exoplanets around M dwarfs. Guaranteed time observations Data Release 1 (2016-2020)
• The CARMENES search for exoplanets around M dwarfs: A low-mass planet in the temperate zone of the nearby K2-18
• Discovery of a Third Transiting Planet in the Kepler-47 Circumbinary System
• Characterization of the L 98-59 multi-planetary system with HARPS: two confirmed terrestrial planets and a mass upper limit on the third
• Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES
• Radial velocity follow-up of GJ1132 with HARPS. A precise mass for planet 'b' and the discovery of a second planet
• How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres
• Quantifying the Bayesian Evidence for a Planet in Radial Velocity Data
• How Deep Is the Ocean? Exploring the phase structure of water-rich sub-Neptunes
• Mass-Radius relationship for M dwarf exoplanets: Comparing nonparametric and parametric methods
• Stellar surface inhomogeneities as a potential source of the atmospheric signal detected in the K2-18 b transmission spectrum
• A hot terrestrial planet orbiting the bright M dwarf L 168-9 unveiled by TESS
• Hot, rocky and warm, puffy super-Earths orbiting TOI-402 (HD 15337)
• A Second Planet Transiting LTT 1445A and a Determination of the Masses of Both Worlds
• Exoplanet Imitators: A test of stellar activity behavior in radial velocity signals
• Exomoons in the habitable zones of M dwarfs
• The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b
• A 3D picture of moist-convection inhibition in hydrogen-rich atmospheres: Implications for K2-18 b
• Decoding the radial velocity variations of HD41248 with ESPRESSO
• Eyes on K2-3: A system of three likely sub-Neptunes characterized with HARPS-N and HARPS
• Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b
• TOI-269 b: An eccentric sub-Neptune transiting a M2 dwarf revisited with ExTrA
• Quantifying the Observational Effort Required for the Radial Velocity Characterization of TESS Planets
• K2-146: Discovery of Planet c, Precise Masses from Transit Timing, and Observed Precession
• The SOPHIE search for northern extrasolar planets XIII. Two planets around M-dwarfs Gl617A and Gl96
• ExoPlaSim: Extending the Planet Simulator for Exoplanets
• Planetary system around LTT 1445A unveiled by ESPRESSO: Multiple planets in a triple M-dwarf system
• A Gaussian Process Regression Reveals No Evidence for Planets Orbiting Kapteyn's Star
• Dynamical evolution of two-planet systems and its connection with white dwarf atmospheric pollution
• Characterising Atmospheres of Cloudy Temperate Mini-Neptunes with JWST
• Insufficient evidence for DMS and DMDS in the atmosphere of K2-18 b. From a joint analysis of JWST NIRISS, NIRSpec, and MIRI observations
• A quarter century of spectroscopic monitoring of the nearby M dwarf Gl 514. A super-Earth on an eccentric orbit moving in and out of the habitable zone
• Revisiting Radial Velocity Measurements of the K2-18 System with the Line-by-Line Framework
• So close, so different: characterization of the K2-36 planetary system with HARPS-N
• Atmospheric dynamics of temperate sub-Neptunes. Part I: dry dynamics
• Predictions of planet detections with near infrared radial velocities in the up-coming SPIRou Legacy Survey-Planet Search
• EXPRES. II. Searching for Planets Around Active Stars: A Case Study of HD 101501
• Dynamical Mass Estimates of the $\beta$ Pictoris Planetary System Through Gaussian Process Stellar Activity Modelling
• Diversity in the haziness and chemistry of temperate sub-Neptunes
• TOI-4336 A b: A temperate sub-Neptune ripe for atmospheric characterization in a nearby triple M-dwarf system
• Scheduling strategies for the ESPRESSO follow-up of TESS targets
• Tidal evolution of exoplanetary systems hosting Potentially Habitable Exoplanets. The cases of LHS-1140 b-c and K2-18 b-c
• Is the hot, dense sub-Neptune TOI-824b an exposed Neptune mantle? Spitzer detection of the hot day side and reanalysis of the interior composition
• ExoplANNET: A deep learning algorithm to detect and identify planetary signals in radial velocity data
• A transiting super-Earth close to the inner edge of the habitable zone of an M0 dwarf star
• Planetary albedo is limited by the above-cloud atmosphere: Implications for sub-Neptune climate
• Homogeneous planet masses I: Reanalysis of archival HARPS radial velocities
• Is the high-energy environment of K2-18b special?
• Three-dimensional Transport-induced Chemistry on Temperate sub-Neptune K2-18b, Part I: the Effects of Atmospheric Dynamics
• Disentangling Atmospheric Compositions of K2-18 b with Next Generation Facilities
• The atmosphere of K2-18 b: The role of hazes, clouds and photoelectrons
• Three-dimensional transport-induced chemistry on temperate sub-Neptune K2-18b, Part II: the combined effects of atmospheric dynamics and chemical reactions
• The TESS-Keck Survey. XI. Mass Measurements for Four Transiting sub-Neptunes orbiting K dwarf TOI-1246
• Friends and Foes: Conditional Occurrence Rates of Exoplanet Companions and their Impact on Radial Velocity Follow-up Surveys