Hyades Member K2-136c: The Smallest Planet in an Open Cluster with a Precisely Measured Mass

Abstract

K2-136 is a late-K dwarf ($0.742\pm 0.039$ M${}_{\odot }$) in the Hyades open cluster with three known, transiting planets and an age of $650\pm 70$ Myr. Analyzing K2 photometry, we found that planets K2-136b, c, and d have periods of $8.0$, $17.3$, and $25.6$ days and radii of $1.014\pm 0.050$ R${}_{\oplus }$, $3.00\pm 0.13$ R${}_{\oplus }$, and $1.565\pm 0.077$ R${}_{\oplus }$, respectively. We collected 93 radial velocity measurements (RVs) with the HARPS-N spectrograph (TNG) and 22 RVs with the ESPRESSO spectrograph (VLT). Analyzing HARPS-N and ESPRESSO data jointly, we found K2-136c induced a semi-amplitude of $5.49\pm 0.53$ m s${}^{-1}$, corresponding to a mass of $18.1\pm 1.9$ M${}_{\oplus }$. We also placed $95$% upper mass limits on K2-136b and d of $4.3$ and $3.0$ M${}_{\oplus }$, respectively. Further, we analyzed HST and XMM-Newton observations to establish the planetary high-energy environment and investigate possible atmospheric loss. K2-136c is now the smallest planet to have a measured mass in an open cluster and one of the youngest planets ever with a mass measurement. K2-136c has $\sim$75% the radius of Neptune but is similar in mass, yielding a density of $3.69_{-0.56}^{+0.67}$ g cm${}^{-3}$ ($\sim$2-3 times denser than Neptune). Mass estimates for K2-136b (and possibly d) may be feasible with more RV observations, and insights into all three planets' atmospheres through transmission spectroscopy would be challenging but potentially fruitful. This research and future mass measurements of young planets are critical for investigating the compositions and characteristics of small exoplanets at very early stages of their lives and providing insights into how exoplanets evolve with time.

arXiv commentAccepted in AJ, 25 pages, 10 figures, 5 tables

References in corpus (90)

• The Wide-field Infrared Survey Explorer (WISE): Mission Description and Initial On-orbit Performance
• The Gaia mission
• The Transiting Exoplanet Survey Satellite
• MESA Isochrones and Stellar Tracks (MIST). I: Solar-Scaled Models
• The Galaxy Evolution Explorer: A Space Ultraviolet Survey Mission
• Analytic Lightcurves for Planetary Transit Searches
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• MESA Isochrones and Stellar Tracks (MIST) 0: Methods for the construction of stellar isochrones
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• The Dartmouth Stellar Evolution Database
• New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• A Non-Gray Theory of Extrasolar Giant Planets and Brown Dwarfs
• The Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4)
• batman: BAsic Transit Model cAlculatioN in Python
• Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits
• Importance Nested Sampling and the MultiNest Algorithm
• 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
• Mass-Radius Relationships for Solid Exoplanets
• No planet for HD 166435
• EXOFAST: A fast exoplanetary fitting suite in IDL
• Understanding the Mass-Radius Relation for Sub-Neptunes: Radius as a Proxy for Composition
• A Technique for Extracting Highly Precise Photometry for the Two-Wheeled Kepler Mission
• A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization
• ESPRESSO@VLT -- On-sky performance and first results
• RadVel: The Radial Velocity Modeling Toolkit
• An asteroseismic view of the radius valley: stripped cores, not born rocky
• The Role of Core Mass in Controlling Evaporation: the Kepler Radius Distribution and the Kepler-36 Density Dichotomy
• The Cosmic Origins Spectrograph
• The Science Performance of JWST as Characterized in Commissioning
• Mass-Radius Relation for Rocky Planets based on PREM
• A Gaussian process framework for modelling stellar activity signals in radial velocity data
• Estimation of the XUV radiation onto close planets and their evaporation
• How Thermal Evolution and Mass Loss Sculpt Populations of Super-Earths and Sub-Neptunes: Application to the Kepler-11 System and Beyond
• Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation
• Dearth of short-period Neptunian exoplanets - a desert in period-mass and period-radius planes
• On the age of the $\beta$ Pictoris moving group
• Gaia Early Data Release 3: The Gaia Catalogue of Nearby Stars
• Improved spectroscopic parameters for transiting planet hosts
• Improving Stellar and Planetary Parameters of Transiting Planet Systems: The Case of TrES-2
• Understanding The Effects Of Stellar Multiplicity On The Derived Planet Radii From Transit Surveys: Implications for Kepler, K2, and TESS
• Water Vapor and Clouds on the Habitable-Zone Sub-Neptune Exoplanet K2-18b
• The stellar rotation--activity relationship in fully convective M dwarfs
• Probabilistic Mass-Radius Relationship for Sub-Neptune-Sized Planets
• The Dependence of Differential Rotation on Temperature and Rotation
• A Detailed Model Grid for Solid Planets from 0.1 through 100 Earth Masses
• A Sub-Neptune Exoplanet with a Low-Metallicity Methane-Depleted Atmosphere and Mie-Scattering Clouds
• Planetary Candidates from the First Year of the K2 Mission
• A Guide to Realistic Uncertainties on Fundamental Properties of Solar-Type Exoplanet Host Stars
• Testing the recovery of stellar rotation signals from Kepler light curves using a blind hare-and-hounds exercise
• Deriving precise parameters for cool solar-type stars. Optimizing the iron line list
• A Kepler Study of Starspot Lifetimes with Respect to Light Curve Amplitude and Spectral Type
• Zodiacal Exoplanets In Time (ZEIT) I: A Neptune-sized planet orbiting an M4.5 dwarf in the Hyades Star Cluster
• K2 rotation periods for low-mass Hyads and a quantitative comparison of the distribution of slow rotators in the Hyades and Praesepe
• Age Determinations of the Hyades, Praesepe, and Pleiades via MESA Models with Rotation
• Three years of Sun-as-a-star radial-velocity observations on the approach to solar minimum
• TESS Spots a Compact System of Super-Earths around the Naked-Eye Star HR 858
• Bayesian Ages for Early-Type Stars from Isochrones Including Rotation, and a Possible Old Age for the Hyades
• Irradiated ocean planets bridge super-Earth and sub-Neptune populations
• The HARPS search for southern extra-solar planets XXXV. The interesting case of HD41248: stellar activity, no planets?
• Transit Timing Variation of Near-Resonance Planetary Pairs. II. Confirmation of 30 planets in 15 Multiple Planet Systems
• New and updated stellar parameters for 71 evolved planet hosts. On the metallicity - giant planet connection
• Predicting the long-term stability of compact multiplanet systems
• Warm terrestrial planet with half the mass of Venus transiting a nearby star
• Correcting the spectroscopic surface gravity using transits and asteroseismology. No significant effect on temperatures or metallicities with ARES+MOOG in LTE
• Mass-Radius Relations and Core-Envelope Decompositions of Super-Earths and Sub-Neptunes
• The lithium depletion boundary and the age of the Hyades cluster
• Investigating the young AU~Mic system with SPIRou: large-scale stellar magnetic field and close-in planet mass
• Zodiacal Exoplanets in Time (ZEIT) VI: a three-planet system in the Hyades cluster including an Earth-sized planet
• Planet Hunters VII. Discovery of a New Low-Mass, Low-Density Planet (PH3 c) Orbiting Kepler-289 with Mass Measurements of Two Additional Planets (PH3 b and d)
• A Bayesian periodogram finds evidence for three planets in HD 11964
• K2-136: A Hyades Binary Star with a Neptune-sized Planet
• The Habitable-zone Planet Finder Reveals A High Mass and a Low Obliquity for the Young Neptune K2-25b
• Radial velocity confirmation of K2-100b: a young, highly irradiated, and low density transiting hot Neptune
• Kepler-411: a four-planet system with an active host star
• Information Content Analysis for Selection of Optimal JWST Observing Modes for Transiting Exoplanet Atmospheres
• Homogeneous spectroscopic parameters for bright planet host stars from the northern hemisphere
• The young HD 73583 (TOI-560) planetary system: Two 10-M${}_{\oplus }$ mini-Neptunes transiting a 500-Myr-old, bright, and active K dwarf
• Constraints on the mass and atmospheric composition and evolution of the low-density young planet DS Tuc A b
• Three Small Planets Transiting a Hyades Star
• Impact of stellar companions on precise radial velocities
• Exoplanet X-ray irradiation and evaporation rates with eROSITA
• HAZMAT. V. The Ultraviolet and X-ray Evolution of K Stars
• The Factory and the Beehive. IV. A Comprehensive Study of the Rotation X-ray Activity Relation in Praesepe and the Hyades
• An 11 Earth-Mass, Long-Period Sub-Neptune Orbiting a Sun-like Star
• Multi-Mask Least-Squares Deconvolution: Extracting RVs using tailored masks
• A Framework for Optimizing Exoplanet Target Selection for the James Webb Space Telescope
• Revisiting the Architecture of the KOI-89 System
• X-ray irradiation of three planets around Hyades star K2-136

Cited by in corpus (4)

• Shallower radius valley around low-mass hosts: Evidence for icy planets, collisions or high-energy radiation scatter
• Resonant sub-Neptunes are puffier
• A study of the frequency and characteristics of stellar companions and Jupiter-like planets in nearby open clusters
• Helium escape signatures are generally strongest during younger ages but this age dependence is lost in the diversity of observed exoplanets