An 11 Earth-Mass, Long-Period Sub-Neptune Orbiting a Sun-like Star

Abstract

Although several thousands of exoplanets have now been detected and characterized, observational biases have led to a paucity of long-period, low-mass exoplanets with measured masses and a corresponding lag in our understanding of such planets. In this paper we report the mass estimation and characterization of the long-period exoplanet Kepler-538b. This planet orbits a Sun-like star (V = 11.27) with M_* = 0.892 +/- (0.051, 0.035) M_sun and R_* = 0.8717 +/- (0.0064, 0.0061) R_sun. Kepler-538b is a 2.215 +/- (0.040, 0.034) R_earth sub-Neptune with a period of P = 81.73778 +/- 0.00013 d. It is the only known planet in the system. We collected radial velocity (RV) observations with HIRES on Keck I and HARPS-N on the TNG. We characterized stellar activity by a Gaussian process with a quasi-periodic kernel applied to our RV and cross correlation function full width at half maximum (FWHM) observations. By simultaneously modeling Kepler photometry, RV, and FWHM observations, we found a semi-amplitude of K = 1.68 +/- (0.39, 0.38) m s^-1 and a planet mass of M_p = 10.6 +/- (2.5, 2.4) M_earth. Kepler-538b is the smallest planet beyond P = 50 d with an RV mass measurement. The planet likely consists of a significant fraction of ices (dominated by water ice), in addition to rocks/metals, and a small amount of gas. Sophisticated modeling techniques such as those used in this paper, combined with future spectrographs with ultra high-precision and stability will be vital for yielding more mass measurements in this poorly understood exoplanet regime. This in turn will improve our understanding of the relationship between planet composition and insolation flux and how the rocky to gaseous transition depends on planetary equilibrium temperature.

arXiv commentPublished in AJ, 17 pages, 8 figures, 3 tables

References in corpus (52)

• Gaia Data Release 2. Summary of the contents and survey properties
• The Gaia mission
• MultiNest: an efficient and robust Bayesian inference tool for cosmology and particle physics
• Analytic Lightcurves for Planetary Transit Searches
• Gaia Data Release 2: The astrometric solution
• The James Webb Space Telescope
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• batman: BAsic Transit Model cAlculatioN in Python
• Efficient, uninformative sampling of limb darkening coefficients for two-parameter laws
• On the Unique Solution of Planet and Star Parameters from an Extrasolar Planet Transit Light Curve
• No planet for HD 166435
• The evaporation valley in the Kepler planets
• The California Kepler Survey VII. Precise Planet Radii Leveraging Gaia DR2 Reveal the Stellar Mass Dependence of the Planet Radius Gap
• Planets and Stellar Activity: Hide and Seek in the CoRoT-7 system
• An asteroseismic view of the radius valley: stripped cores, not born rocky
• False positive probabilties for all Kepler Objects of Interest: 1284 newly validated planets and 428 likely false positives
• Mass-Radius Relation for Rocky Planets based on PREM
• Planetary Candidates Observed by Kepler. VIII. A Fully Automated Catalog With Measured Completeness and Reliability Based on Data Release 25
• The California Planet Survey I. Four New Giant Exoplanets
• A Gaussian process framework for modelling stellar activity signals in radial velocity data
• The California-Kepler Survey. IV. Metal-rich Stars Host a Greater Diversity of Planets
• How to Constrain Your M dwarf II: the mass-luminosity-metallicity relation from 0.075 to 0.70$M_{\odot }$
• Astrometric exoplanet detection with Gaia
• Extracting Planet Mass and Eccentricity From TTV data
• PandExo: A Community Tool for Transiting Exoplanet Science with JWST & HST
• Improving Stellar and Planetary Parameters of Transiting Planet Systems: The Case of TrES-2
• Revised Radii of Kepler Stars and Planets Using Gaia Data Release 2
• Parametrizing the exoplanet eccentricity distribution with the Beta distribution
• Predictions of the WFIRST Microlensing Survey I: Bound Planet Detection Rates
• Planetary Candidates Observed by Kepler VI: Planet Sample from Q1-Q16 (47 Months)
• A Detailed Model Grid for Solid Planets from 0.1 through 100 Earth Masses
• Discovery and Characterization of Transiting SuperEarths Using an All-Sky Transit Survey and Follow-up by the James Webb Space Telescope
• Angular Momentum Loss from Cool Stars: An Empirical Expression and Connection to Stellar Activity
• Scientific Discovery with the James Webb Space Telescope
• How formation time-scales affect the period dependence of the transition between rocky super-Earths and gaseous sub-Neptunes and implications for ${\eta }_{\oplus }$
• EXPRES: A Next Generation RV Spectrograph in the Search for Earth-like Worlds
• An ultra-short period rocky super-Earth with a secondary eclipse and a Neptune-like companion around K2-141
• Exo-Transmit: An Open-Source Code for Calculating Transmission Spectra for Exoplanet Atmospheres of Varied Composition
• Gas giant planets as dynamical barriers to inward-migrating super-Earths
• Planet-planet scattering leads to tightly packed planetary systems
• Correcting the spectroscopic surface gravity using transits and asteroseismology. No significant effect on temperatures or metallicities with ARES+MOOG in LTE
• ARES+MOOG - a practical overview of an EW method to derive stellar parameters
• A Bayesian periodogram finds evidence for three planets in HD 11964
• Information Content Analysis for Selection of Optimal JWST Observing Modes for Transiting Exoplanet Atmospheres
• The Kepler-19 system: a thick-envelope super-Earth with two Neptune-mass companions characterized using Radial Velocities and Transit Timing Variations
• Homogeneous spectroscopic parameters for bright planet host stars from the northern hemisphere
• Exploring the realm of scaled Solar System analogs with HARPS
• Planet Formation: An Optimized Population-Synthesis Approach
• The Habitable Zone Planet Finder: A Proposed High Resolution NIR Spectrograph for the Hobby Eberly Telescope to Discover Low Mass Exoplanets around M Dwarfs
• Survival Function Analysis of Planet Size Distribution with GAIA Data Release 2 Updates
• The GAPS Programme with HARPS-N@TNG XII: Characterization of the planetary system around HD108874

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• pyaneti II: A multidimensional Gaussian process approach to analysing spectroscopic time-series
• The Kepler Giant Planet Search. I: A Decade of Kepler Planet-host Radial Velocities from W. M. Keck Observatory
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