Zodiacal Exoplanets in Time (ZEIT) IX: a flat transmission spectrum and a highly eccentric orbit for the young Neptune K2-25b as revealed by Spitzer

Abstract

Transiting planets in nearby young clusters offer the opportunity to study the atmospheres and dynamics of planets during their formative years. To this end, we focused on K2-25b -- a close-in ($P$=3.48 days), Neptune-sized exoplanet orbiting a M4.5 dwarf in the 650Myr Hyades cluster. We combined photometric observations of K2-25 covering a total of 44 transits and spanning >2 yr, drawn from a mix of space-based telescopes (Spitzer Space Telescope and K2) and ground-based facilities (Las Cumbres Observatory Global Telescope network and MEarth). The transit photometry spanned 0.6--4.5$\mu$m, which enabled our study of K2-25b's transmission spectrum. We combined and fit each dataset at a common wavelength within a Markov Chain Monte Carlo framework, yielding consistent planet parameters. The resulting transit depths ruled out a solar-composition atmosphere for K2-25b for the range of expected planetary masses and equilibrium temperature at a $>4\sigma$ confidence level, and are consistent with a flat transmission spectrum. Mass constraints and transit observations at a finer grid of wavelengths (e.g., from the Hubble Space Telescope) are needed to make more definitive statements about the presence of clouds or an atmosphere of high mean molecular weight. Our precise measurements of K2-25b's transit duration also enabled new constraints on the eccentricity of K2-25's orbit. We find K2-25b's orbit to be eccentric ($e>0.20$) for all reasonable stellar densities and independent of the observation wavelength or instrument. The high eccentricity is suggestive of a complex dynamical history and motivates future searches for additional planets or stellar companions.

arXiv commentv2 updated for proofs

References in corpus (90)

• emcee: The MCMC Hammer
• The Infrared Array Camera (IRAC) for the Spitzer Space Telescope
• The Spitzer Space Telescope Mission
• Analytic Lightcurves for Planetary Transit Searches
• Gaia Data Release 2: The astrometric solution
• A new extensive library of PHOENIX stellar atmospheres and synthetic spectra
• The K2 Mission: Characterization and Early results
• New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit
• Shrinking binary and planetary orbits by Kozai cycles with tidal friction
• Las Cumbres Observatory Global Telescope Network
• batman: BAsic Transit Model cAlculatioN in Python
• Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b
• Dynamical Outcomes of Planet-Planet Scattering
• Achieving better than 1 minute accuracy in the Heliocentric and Barycentric Julian Dates
• 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
• Most 1.6 Earth-Radius Planets are not Rocky
• How to Constrain Your M Dwarf: measuring effective temperature, bolometric luminosity, mass, and radius
• Stellar Diameters and Temperatures II. Main Sequence K & M Stars
• Dynamical Origin of Extrasolar Planet Eccentricity Distribution
• Discovery and spectroscopy of the young Jovian planet 51 Eri b with the Gemini Planet Imager
• Line and Mean Opacities for Ultracool Dwarfs and Extrasolar Planets
• Design Considerations for a Ground-based Transit Search for Habitable Planets Orbiting M dwarfs
• Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology
• A ground-based transmission spectrum of the super-Earth exoplanet GJ1214b
• The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets
• Gaseous Mean Opacities for Giant Planet and Ultracool Dwarf Atmospheres over a Range of Metallicities and Temperatures
• How to Constrain Your M dwarf II: the mass-luminosity-metallicity relation from 0.075 to 0.70$M_{\odot }$
• The Flat Transmission Spectrum of the Super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope
• Fundamental Properties of Kepler Planet-Candidate Host Stars using Asteroseismology
• Eccentricity from transit photometry: small planets in Kepler multi-planet systems have low eccentricities
• Spitzer Secondary Eclipses of the Dense, Modestly-irradiated, Giant Exoplanet HAT-P-20b Using Pixel-Level Decorrelation
• WASP-47: A Hot Jupiter System with Two Additional Planets Discovered by K2
• The rotation and Galactic kinematics of mid M dwarfs in the Solar Neighborhood
• The Impact of Stellar Multiplicity on Planetary Systems, I.: The Ruinous Influence of Close Binary Companions
• Probabilistic Mass-Radius Relationship for Sub-Neptune-Sized Planets
• The orbital eccentricity of small planet systems
• Real-time processing of the imaging data from the network of Las Cumbres Observatory Telescopes using BANZAI
• LDTk: Limb Darkening Toolkit
• Density and Eccentricity of Kepler Planets
• Kepler Planet Masses and Eccentricities from TTV Analysis
• Spectro-Thermometry of M dwarfs and their candidate planets: too hot, too cool, or just right?
• They are Small Worlds After All: Revised Properties of Kepler M Dwarf Stars and their Planets
• A Neptune-sized transiting planet closely orbiting a 5-10-million-year-old star
• Orbital Phase Variations of the Eccentric Giant Planet HAT-P-2b
• Planetary System Disruption by Galactic Perturbations to Wide Binary Stars
• Zodiacal Exoplanets in Time (ZEIT) III: A short-period planet orbiting a pre-main-sequence star in the Upper Scorpius OB Association
• ACCESS I: An Optical Transmission Spectrum of GJ 1214b Reveals a Heterogeneous Stellar Photosphere
• Zodiacal Exoplanets In Time (ZEIT) I: A Neptune-sized planet orbiting an M4.5 dwarf in the Hyades Star Cluster
• The Kepler Dichotomy among the M Dwarfs: Half of Systems Contain Five or More Coplanar Planets
• The Distribution of Transit Durations for Kepler Planet Candidates and Implications for their Orbital Eccentricities
• The Photoeccentric Effect and Proto-Hot Jupiters I. Measuring photometric eccentricities of individual transiting planets
• The Atmospheres of Earth-like Planets after Giant Impact Events
• Trends in Atmospheric Properties of Neptune-Size Exoplanets
• The thermal emission of the young and massive planet CoRoT-2b at 4.5 and 8 microns
• Placing the spotted T Tauri star LkCa 4 on an HR diagram
• Exo-Transmit: An Open-Source Code for Calculating Transmission Spectra for Exoplanet Atmospheres of Varied Composition
• Characterizing the Orbital Eccentricities of Transiting Extrasolar Planets with Photometric Observations
• An Empirical Correction for Activity Effects on the Temperatures, Radii, and Estimated Masses of Low-Mass Stars and Brown Dwarfs
• Measuring the Recoverability of Close Binaries in Gaia DR2 with the Robo-AO Kepler Survey
• The Monitor project: Data processing and lightcurve production
• The Hyades open cluster is chemically inhomogeneous
• Accounting for Incompleteness due to Transit Multiplicity in Kepler Planet Occurrence Rates
• Repeatability and Accuracy of Exoplanet Eclipse Depths Measured with Post-Cryogenic Spitzer
• That's How We Roll: The NASA K2 Mission Science Products and Their Performance Metrics
• Zodiacal Exoplanets in Time (ZEIT) V: A Uniform Search for Transiting Planets in Young Clusters Observed by K2
• Zodiacal Exoplanets in Time (ZEIT) VIII: A Two Planet System in Praesepe from K2 Campaign 16
• Magnetic Inflation and Stellar Mass II: On the Radii of Single, Rapidly Rotating, Fully Convective M Dwarf Stars
• K2 Discovers a Busy Bee: An Unusual Transiting Neptune Found in the Beehive Cluster
• Consistent metallicity scale for cool dwarfs and giants. A benchmark test using the Hyades
• Dusty outflows in planetary atmospheres: Understanding "super-puffs" and transmission spectra of sub-Neptunes
• Kepler-93b: A Terrestrial World Measured to within 120 km, and a Test Case for a New Spitzer Observing Mode
• Constraints on Planet Occurrence around Nearby Mid-to-Late M Dwarfs from the MEarth Project
• New Pleiades Eclipsing Binaries and a Hyades Transiting System Identified by K2
• A discontinuity in the $T_{\mathrm{eff}}$-radius relation of M-dwarfs
• Ground-Based Submillimagnitude CCD Photometry of Bright Stars Using Snapshot Observations
• The Broadband and Spectrally-Resolved H-band Eclipse of KELT-1b and the Role of Surface Gravity in Stratospheric Inversions in Hot Jupiters
• A Novel Method to Photometrically Constrain Orbital Eccentricities: Multibody Asterodensity Profiling (MAP)
• Calibration of the MEarth Photometric System: Optical Magnitudes and Photometric Metallicity Estimates for 1802 Nearby M-dwarfs
• A Multi-Planet System Transiting the $V$ = 9 Rapidly Rotating F-Star HD 106315
• The Factory and the Beehive III: PTFEB132.707+19.810, a Low-Mass Eclipsing Binary in Praesepe Observed by PTF and K2
• The Effects of Stellar Companions on Exoplanet Radius Distributions
• The Gold Standard: Accurate Stellar and Planetary Parameters for Eight Kepler M dwarf Systems Enabled by Parallaxes
• A search for radius inflation among active M-dwarfs in Praesepe
• Spitzer Secondary Eclipse Depths with Multiple Intrapixel Sensitivity Correction Methods: Observations of WASP-13b, WASP-15b, WASP-16b, WASP-62b, and HAT-P-22b
• Repeatability of Spitzer/IRAC exoplanetary eclipses with Independent Component Analysis
• Radius Inflation at Low Rossby Number in the Hyades Cluster
• Orbital migration of giant planets induced by gravitationally unstable gaps: the effect of planet mass
• Knot a Bad Idea: Testing BLISS Mapping for Spitzer Space Telescope Photometry
• Temperate super-Earths/mini-Neptunes around M/K dwarfs Consist of 2 Populations Distinguished by Their Atmospheres

Cited by in corpus (21)

• Aerosols in Exoplanet Atmospheres
• Atmospheric Regimes and Trends on Exoplanets and Brown Dwarfs
• TESS Hunt for Young and Maturing Exoplanets (THYME) III: a two-planet system in the 400 Myr Ursa Major Group
• The Warm Neptune GJ 3470b has a Polar Orbit
• TESS Hunt for Young and Maturing Exoplanets (THYME) VI: an 11 Myr giant planet transiting a very low-mass star in Lower Centaurus Crux
• The Habitable-zone Planet Finder Reveals A High Mass and a Low Obliquity for the Young Neptune K2-25b
• Updated Parameters and a New Transmission Spectrum of HD 97658b
• Evidence for disequilibrium chemistry from vertical mixing in hot Jupiter atmospheres. A comprehensive survey of transiting close-in gas giant exoplanets with warm-Spitzer/IRAC
• Constraints on the mass and atmospheric composition and evolution of the low-density young planet DS Tuc A b
• A giant planet transiting a 3-Myr protostar with a misaligned disk
• Zodiacal Exoplanets in Time. XI. The Orbit and Radiation Environment of the Young M Dwarf-Hosted Planet K2-25b
• A Lyman-alpha transit left undetected: the environment and atmospheric behavior of K2-25b
• Probing atmospheric escape through metastable He I triplet lines in 15 exoplanets observed with SPIRou
• Characterization of starspots on a young M-dwarf K2-25: multi-band observations of stellar photometric variability and planetary transits
• Hazy with a chance of star spots: constraining the atmosphere of the young planet, K2-33b
• The young planetary system K2-25: constraints on companions and starspots
• Reflected-light Phase Curves with PICASO: A Kepler-7b Case Study
• Discovering planets with PLATO: Comparison of algorithms for stellar activity filtering
• The Effects of Cuboid Particle Scattering on Reflected Light Phase Curves: Insights from Laboratory Data and Theory
• TOI-1201 b: A mini-Neptune transiting a bright and moderately young M dwarf
• Grain Growth in Escaping Atmospheres: Implications for the Radius Inflation of Super-Puffs