The CARMENES search for exoplanets around M dwarfs: A low-mass planet in the temperate zone of the nearby K2-18

Abstract

K2-18 is a nearby M2.5 dwarf, located at 34 pc and hosting a transiting planet which was first discovered by the {\it K2} mission and later confirmed with {\it Spitzer Space Telescope} observations. With a radius of $\sim 2R_{\oplus }$ and an orbital period of $\sim 33$ days, the planet lies in the temperate zone of its host star and receives stellar irradiation similar to Earth. Here we perform radial velocity follow-up observations with the visual channel of CARMENES with the goal of determining the mass and density of the planet. We measure a planetary semi-amplitude of $K_b\sim 3.5$ \mpersec\ and a mass of $M_b\sim 9M_{\oplus }$, yielding a bulk density around ${\rho }_b\sim 4\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$. This indicates a low-mass planet with a composition consistent with a solid core and a volatile-rich envelope. A signal at 9 days was recently reported using radial velocity measurements taken with the HARPS spectrograph. This was interpreted as being due to a second planet. We see a weaker, time and wavelength dependent signal in the CARMENES data set and thus favor stellar activity for its origin. \ktwo\ joins the growing group of low-mass planets detected in the temperate zone of M dwarfs. The brightness of the host star in the near-infrared makes the system a good target for detailed atmospheric studies with the {\it James Webb Space Telescope}.

arXiv commentfixing author list

References in corpus (50)

• Astropy: A Community Python Package for Astronomy
• emcee: The MCMC Hammer
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• Habitable Zones Around Main-Sequence Stars: New Estimates
• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• A terrestrial planet candidate in a temperate orbit around Proxima Centauri
• Most 1.6 Earth-Radius Planets are not Rocky
• Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass
• RadVel: The Radial Velocity Modeling Toolkit
• Improving the Efficiency of Markov Chain Monte Carlo for Analyzing the Orbits of Extrasolar Planets
• Quantifying the Uncertainty in the Orbits of Extrasolar Planets
• Spectrum radial velocity analyser (SERVAL). High-precision radial velocities and two alternative spectral indicators
• Mass-Radius Relation for Rocky Planets based on PREM
• How Thermal Evolution and Mass Loss Sculpt Populations of Super-Earths and Sub-Neptunes: Application to the Kepler-11 System and Beyond
• The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets
• The HARPS-TERRA project I. Description of the algorithms, performance and new measurements on a few remarkable stars observed by HARPS
• A temperate rocky super-Earth transiting a nearby cool star
• Radial velocity planets de-aliased. A new, short period for Super-Earth 55 Cnc e
• Stellar Activity Masquerading as Planets in the Habitable Zone of the M dwarf Gliese 581
• Compositional imprints in density-distance-time: a rocky composition for close-in low-mass exoplanets from the location of the valley of evaporation
• 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 CARMENES search for exoplanets around M dwarfs: High-resolution optical and near-infrared spectroscopy of 324 survey stars
• Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program I. Comparison of activity indices
• Detecting Planets Around Very Low Mass Stars with the Radial Velocity Method
• The Habitable-Zone Planet Finder: A Stabilized Fiber-Fed NIR Spectrograph for the Hobby-Eberly Telescope
• Stellar and Planetary Properties of K2 Campaign 1 Candidates and Validation of 17 Planets, Including a Planet Receiving Earth-like Insolation
• The low-level radial velocity variability in Barnard's star (=GJ 699). Secular acceleration, indications for convective redshift, and planet mass limits
• Ghost in the time series: no planet for Alpha Cen B
• A nearby M star with three transiting super-Earths discovered by K2
• The CARMENES search for exoplanets around M dwarfs. First visual-channel radial-velocity measurements and orbital parameter updates of seven M-dwarf planetary systems
• Flat-relative optimal extraction. A quick and efficient algorithm for stabilised spectrographs
• SPIRou: the near-infrared spectropolarimeter/high-precision velocimeter for the Canada-France-Hawaii telescope
• Accounting for velocity jitter in planet search surveys
• Radial Velocity Detection of Earth-mass Planets in the Presence of Activity Noise: The Case of Alpha Centauri Bb
• Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization
• A path towards understanding the rotation-activity relation of M dwarfs with K2 mission, X-ray and UV data
• Stellar activity mimics a habitable-zone planet around Kapteyn's star
• 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
• A temperate exo-Earth around a quiet M dwarf at 3.4 parsecs
• Radial Velocity Planet Detection Biases at the Stellar Rotational Period
• Disentangling Planets and Stellar Activity for Gliese 667C
• The Impact of Stellar Rotation on the Detectability of Habitable Planets Around M Dwarfs
• The CARMENES search for exoplanets around M dwarfs - HD 147379b: A nearby Neptune in the temperate zone of an early-M dwarf
• The Ca II infrared triplet's performance as an activity indicator compared to Ca II H and K
• Transmission spectroscopy of the inflated exo-Saturn HAT-P-19b
• Comment on "Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581"
• Long-lived, long-period radial velocity variations in Aldebaran: A planetary companion and stellar activity
• Transmission spectroscopy of HAT-P-32b with the LBT: confirmation of clouds/hazes in the planetary atmosphere
• Proxima Centauri as a Benchmark for Stellar Activity Indicators in the Near Infrared
• Periodic Halpha variations in GL 581: Further evidence for an activity origin to GL 581d

Cited by in corpus (46)

• 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
• Habitability and Biosignatures of Hycean Worlds
• Planetary system around the nearby M dwarf GJ 357 including a transiting, hot, Earth-sized planet optimal for atmospheric characterization
• Conclusive evidence for a population of water-worlds around M-dwarfs remains elusive
• The CARMENES search for exoplanets around M dwarfs. Guaranteed time observations Data Release 1 (2016-2020)
• The CARMENES search for exoplanets around M dwarfs -- Photospheric parameters of target stars from high-resolution spectroscopy. II. Simultaneous multiwavelength range modeling of activity insensitive lines
• 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
• Aurora: A Generalised Retrieval Framework for Exoplanetary Transmission Spectra
• RV-detected planets around M dwarfs: Challenges for core accretion models
• Stellar surface inhomogeneities as a potential source of the atmospheric signal detected in the K2-18 b transmission spectrum
• The CARMENES search for exoplanets around M dwarfs. Measuring precise radial velocities in the near infrared: the example of the super-Earth CD Cet b
• The CARMENES search for exoplanets around M dwarfs. A Neptune-mass planet traversing the habitable zone around HD 180617
• The CARMENES search for exoplanets around M dwarfs: Nine new double-line spectroscopic binary stars
• Exomoons in the habitable zones of M dwarfs
• The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b
• ARES IV: Probing the atmospheres of the two warm small planets HD 106315 c and HD 3167 c with the HST/WFC3 camera
• The CARMENES search for exoplanets around M dwarfs: The warm super-Earths in twin orbits around the mid-type M dwarfs Ross 1020 (GJ 3779) and LP 819-052 (GJ 1265)
• The CARMENES search for exoplanets around M dwarfs: Stable radial-velocity variations at the rotation period of AD~Leonis -- A test case study of current limitations to treating stellar activity
• Gliese 49: Activity evolution and detection of a super-Earth
• Cleaning our Hazy Lens: Exploring Trends in Transmission Spectra of Warm Exoplanets
• Dynamical evolution of two-planet systems and its connection with white dwarf atmospheric pollution
• The CARMENES search for exoplanets around M dwarfs - The enigmatic planetary system GJ 4276: One eccentric planet or two planets in a 2:1 resonance?
• Insufficient evidence for DMS and DMDS in the atmosphere of K2-18 b. From a joint analysis of JWST NIRISS, NIRSpec, and MIRI observations
• Magnitude-squared coherence: A powerful tool for disentangling Doppler planet discoveries from stellar activity
• Revisiting Radial Velocity Measurements of the K2-18 System with the Line-by-Line Framework
• The CARMENES search for exoplanets around M dwarfs. A super-Earth planet orbiting HD 79211 (GJ 338 B)
• Detecting Biosignatures in the Atmospheres of Gas Dwarf Planets with the James Webb Space Telescope
• The CARMENES search for exoplanets around M dwarfs Detection of a mini-Neptune around LSPM J2116+0234 and refinement of orbital parameters of a super-Earth around GJ 686 (BD+18 3421)
• Exoplanet atmosphere evolution: emulation with neural networks
• Evolution of steam worlds: energetic aspects
• Detailed stellar activity analysis and modelling of GJ 832: Reassessment of the putative habitable zone planet GJ 832c
• Potential Interior Structures and Habitability of Super-Earth Exoplanets LHS 1140 b, K2-18 b, TOI-1452 b and TOI-1468 c
• A photospheric and chromospheric activity analysis of the quiescent retrograde-planet host $\nu$ Octantis A
• Can Carbon Fractionation Provide Evidence for Aerial Biospheres in the Atmospheres of Temperate Sub-Neptunes?
• Three-dimensional Transport-induced Chemistry on Temperate sub-Neptune K2-18b, Part I: the Effects of Atmospheric Dynamics
• Is the high-energy environment of K2-18b special?
• Precise radial velocities of giant stars -- XVII. Distinguishing planets from intrinsically induced radial velocity signals in evolved stars
• The CARMENES search for exoplanets around M dwarfs : Understanding the wavelength dependence of radial velocity measurements
• Exploring the habitability and interior composition of exoplanets lying within the extended habitable zone
• The atmosphere of K2-18 b: The role of hazes, clouds and photoelectrons
• CARMENES input catalog of M dwarfs VI. A time-resolved Ca II H&K catalog from archival data
• Extreme precision photometry from the ground with beam-shaping diffusers for K2, TESS and beyond
• Multi-Wavelength High-Resolution Spectroscopy for Exoplanet Detection: Motivation, Instrumentation and First Results
• Three-dimensional transport-induced chemistry on temperate sub-Neptune K2-18b, Part II: the combined effects of atmospheric dynamics and chemical reactions