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.

Accepted in AJ, 25 pages, 10 figures, 5 tables

References in corpus (44)

• The Gaia mission
• The Transiting Exoplanet Survey Satellite
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• 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
• Mass-Radius Relationships for Solid Exoplanets
• EXOFAST: A fast exoplanetary fitting suite in IDL
• A Technique for Extracting Highly Precise Photometry for the Two-Wheeled Kepler Mission
• ESPRESSO@VLT -- On-sky performance and first results
• The Science Performance of JWST as Characterized in Commissioning
• Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation
• On the age of the $\beta$ Pictoris moving group
• Gaia Early Data Release 3: The Gaia Catalogue of Nearby Stars
• Improving Stellar and Planetary Parameters of Transiting Planet Systems: The Case of TrES-2
• A Sub-Neptune Exoplanet with a Low-Metallicity Methane-Depleted Atmosphere and Mie-Scattering Clouds
• 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
• A Kepler Study of Starspot Lifetimes with Respect to Light Curve Amplitude and Spectral Type
• K2 rotation periods for low-mass Hyads and a quantitative comparison of the distribution of slow rotators in the Hyades and Praesepe
• Three years of Sun-as-a-star radial-velocity observations on the approach to solar minimum
• 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?
• 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
• 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
• The Habitable-zone Planet Finder Reveals A High Mass and a Low Obliquity for the Young Neptune K2-25b
• 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
• 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
• 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