Two Transiting Low Density Sub-Saturns from K2

Abstract

We report the discovery and confirmation of two sub-Saturn planets orbiting a bright (V = 11.3), metal-rich ([Fe/H] = 0.42 $\pm$ 0.04 dex) G3 dwarf in the K2 Campaign 2 field. The planets are 5.68 $\pm$ 0.56 Earth-radii and 7.82 $\pm$ 0.72 Earth-radii and have orbital periods of 20.8851 $\pm$ 0.0003 d and 42.3633$\pm$0.0006 d, near to the 2:1 mean-motion resonance. We obtained 32 radial velocities (RVs) with Keck/HIRES and detected the reflex motion due to EPIC-203771098b and c. These planets have masses of 21.0 $\pm$ 5.4 Earth-masses and 27.0 $\pm$ 6.9 Earth-masses, respectively. With low densities of 0.63 $\pm$ 0.25 g/cc and 0.31 $\pm$ 0.12 g/cc, respectively, the planets require thick envelopes of H/He to explain their large sizes and low masses. Interior structure models predict that the planets have fairly massive cores of 17.6 $\pm$ 4.3 Earth-masses and 16.1 $\pm$ 4.2 Earth-masses, respectively. They may have formed exterior to their present locations, accreted their H/He envelopes at large orbital distances, and migrated in as a resonant pair. The proximity to resonance, large transit depths, and host star brightness offer rich opportunities for TTV follow-up. Finally, the low surface gravities of the EPIC-203771098 planets make them favorable targets for transmission spectroscopy by HST, Spitzer, and JWST.

12 pages, 6 figures, 3 tables, published in ApJ. v2: text updated to ApJ version, replaced EPIC-2037 with K2-24

References in corpus (12)

• The Dartmouth Stellar Evolution Database
• The K2 Mission: Characterization and Early results
• EXOFAST: A fast exoplanetary fitting suite in IDL
• The NASA-UC-UH Eta-Earth Program: IV. A Low-mass Planet Orbiting an M Dwarf 3.6 PC from Earth
• Make Super-Earths, Not Jupiters: Accreting Nebular Gas onto Solid Cores at 0.1 AU and Beyond
• Thermal Emission and Albedo Spectra of Super Earths with Flat Transmission Spectra
• The Atmospheric Signatures of Super-Earths: How to Distinguish Between Hydrogen-Rich and Hydrogen-Poor Atmospheres
• The Exoplanet Orbit Database II: Updates to exoplanets.org
• A nearby M star with three transiting super-Earths discovered by K2
• Giant Impact: An Efficient Mechanism for the Devolatilization of Super-Earths
• Detection of Stars within $\sim$0.8 arcseconds of Kepler Objects of Interest
• Sensitivity bias in the mass-radius distribution from Transit Timing Variations and Radial Velocity measurements

Cited by in corpus (32)

• 197 Candidates and 104 Validated Planets in K2's First Five Fields
• Eleven Multi-planet Systems from K2 Campaigns 1 & 2 and the Masses of Two Hot Super-Earths
• Four Sub-Saturns with Dissimilar Densities: Windows into Planetary Cores and Envelopes
• The Planetary Mass-Radius Relation and its Dependence on Orbital Period as Measured by Transit Timing Variations and Radial Velocities
• zeus: A Python implementation of Ensemble Slice Sampling for efficient Bayesian parameter inference
• Detection of Hundreds of New Planet Candidates and Eclipsing Binaries in K2 Campaigns 0-8
• An Overabundance of Low-density Neptune-like Planets
• Doppler Monitoring of five K2 Transiting Planetary Systems
• Utilizing Small Telescopes Operated by Citizen Scientists for Transiting Exoplanet Follow-up
• TOI-257b (HD 19916b): A Warm sub-Saturn Orbiting an Evolved F-type Star
• Close-in sub-Neptunes reveal the past rotation history of their host stars: atmospheric evolution of planets in the HD3167 and K2-32 planetary systems
• Dynamics and Formation of the Near-Resonant K2-24 System: Insights from Transit-Timing Variations and Radial Velocities
• A multiplicity study of transiting exoplanet host stars. I. High-contrast imaging with VLT/SPHERE
• High-resolution Imaging of Transiting Extrasolar Planetary systems (HITEP). II. Lucky Imaging results from 2015 and 2016
• K2-19b and c are in a 3:2 Commensurability but out of Resonance: A Challenge to Planet Assembly by Convergent Migration
• Kepler-1656b: a Dense Sub-Saturn With an Extreme Eccentricity
• Two warm, low-density sub-Jovian planets orbiting bright stars in K2 campaigns 13 and 14
• Transit-Timing Variation Signature of Planet Migration: The Case of K2-24
• VaTEST I: Validation of Sub-Saturn Exoplanet TOI-181b in Narrow Orbit from its Host Star
• Characterizing K2 Candidate Planetary Systems Orbiting Low-Mass Stars III: A High Mass & Low Envelope Fraction for the Warm Neptune K2-55b
• K2-280b -- a low density warm sub-Saturn around a mildly evolved star
• TOI-5108 b and TOI 5786 b: Two transiting sub-Saturns detected and characterized with TESS, MaHPS and SOPHIE
• Exploiting periodic orbits as dynamical clues for Kepler and K2 systems
• Exploiting the transit timing capabilities of Ariel
• Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396
• Discovery and characterization of a dense sub-Saturn TOI-6651b
• Mapping the structure of the planetary 2:1 mean motion resonance. The TOI-216, K2-24, and HD27894 systems
• The K2-24 planetary system revisited by CHEOPS
• Evidence of a gap in the envelope mass fraction of sub-Saturns
• The Occurrence-weighted Median Planets Discovered by Transit Surveys Orbiting Solar-type Stars and Their Implications for Planet Formation and Evolution
• Orbital migration and Resonance Offset of the Kepler-25 and K2-24 systems
• Constraining stellar rotation and planetary atmospheric evolution of a dozen systems hosting sub-Neptunes and super-Earths