Limits on the Mass and Initial Entropy of 51 Eri b from Gaia EDR3 Astrometry

Abstract

51 Eri b is one of the only young planets consistent with a wide range of possible initial entropy states, including the cold-start scenario associated with some models of planet formation by core accretion. The most direct way to constrain the initial entropy of a planet is by measuring its luminosity and mass at a sufficiently young age that the initial conditions still matter. We present the tightest upper limit on 51 Eri b's mass yet (M < 11 Mjup at 2$\sigma$) using a cross-calibration of Hipparcos and Gaia EDR3 astrometry and the orbit-fitting code orvara. We also reassess its luminosity using a direct, photometric approach, finding log(Lbol/Lsun) = -5.5$\pm$0.2 dex. Combining this luminosity with the 24$\pm$3 Myr age of the $\beta$ Pic moving group, of which 51 Eri is a member, we derive mass distributions from a grid of evolutionary models that spans a wide range of initial entropies. We find that 51 Eri b is inconsistent with the coldest-start scenarios, requiring an initial entropy of >8 $k_B$/baryon at 97% confidence. This result represents the first observational constraint on the initial entropy of a potentially cold-start planet, and it continues the trend of dynamical masses for directly imaged planets pointing to warm- or hot-start formation scenarios.

Accepted for publication in MNRAS (9 pages, 6 figures)

References in corpus (22)

• Gaia Early Data Release 3: The astrometric solution
• The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
• On the Luminosity of Young Jupiters
• Models of the in situ formation of detected extrasolar giant planets
• A new set of atmosphere and evolution models for cool T-Y brown dwarfs and giant exoplanets
• The Hipparcos-Gaia Catalog of Accelerations: Gaia EDR3 Edition
• Synthetic Spectra and Colors of Young Giant Planet Atmospheres: Effects of Initial Conditions and Atmospheric Metallicity
• Characterizing 51 Eri b from 1-5 $\mu$m: a partly-cloudy exoplanet
• orvara: An Efficient Code to Fit Orbits using Radial Velocity, Absolute, and/or Relative Astrometry
• Detection and Bulk Properties of the HR 8799 Planets with High Resolution Spectroscopy
• Evolution of Gas Giant Entropy During Formation by Runaway Accretion
• A Model-Independent Mass and Moderate Eccentricity for $\beta$ Pic b
• Precise Dynamical Masses and Orbital Fits for $\beta$ Pic b and $\beta$ Pic c
• Identifying and Analysing Protostellar Disc Fragments in Smoothed Particle Hydrodynamics Simulations
• The First Dynamical Mass Measurement in the HR 8799 System
• The Second Discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) Program: A Cold Wide-Orbit Exoplanet around a Young Field M Dwarf at 10.9 pc
• Hint of curvature in the orbital motion of the exoplanet 51 Eridani b using 3 years of VLT/SPHERE monitoring
• A HARPS RV search for planets around young nearby stars
• CO Detected in CI Tau b: Hot Start Implied by Planet Mass and M${}_K$
• Outward Migration of Super-Jupiters
• An updated visual orbit of the directly-imaged exoplanet 51 Eridani b and prospects for a dynamical mass measurement with Gaia
• Dynamical Mass Estimates of the $\beta$ Pictoris Planetary System Through Gaussian Process Stellar Activity Modelling

Cited by in corpus (14)

• Rotation Periods, Inclinations, and Obliquities of Cool Stars Hosting Directly Imaged Substellar Companions: Spin-Orbit Misalignments are Common
• On the Masses, Age, and Architecture of the VHS J1256-1257AB b System
• Revisiting the atmosphere of the exoplanet 51 Eridani b with VLT/SPHERE
• JWST-TST High Contrast: Living on the Wedge, or, NIRCam Bar Coronagraphy Reveals CO${}_2$ in the HR 8799 and 51 Eri Exoplanets' Atmospheres
• Direct-imaging Discovery of a Substellar Companion Orbiting the Accelerating Variable Star, HIP 39017
• HD 83443c: A highly eccentric giant planet on a 22-year orbit
• A High-Resolution Spectroscopic Survey of Directly Imaged Companion Hosts: I. Determination of diagnostic stellar abundances for planet formation and composition
• HD 28185 Revisited: An Outer Planet, Instead of a Brown Dwarf, On a Saturn-like Orbit
• A High-Resolution Spectroscopic Survey of Directly Imaged Companion Hosts: II. Diversity in C/O Ratios among Host Stars
• SCExAO/CHARIS and Gaia Direct Imaging and Astrometric Discovery of a Superjovian Planet 3--4 lambda/D from the Accelerating Star HIP 54515
• Weighing the Darkness III: How Gaia Could, but Probably Won't, Astrometrically Detect Free-Floating Black Holes
• New constraints on the presence of debris disks around G 196-3 B and VHS J125601.92-125723.9 b
• Direct Imaging Discovery of Giant Exoplanet $\beta$ Pictoris d: A Decade-Long Game of Hide-and-Seek
• The Directly-Imaged Exoplanet Host Star 51 Eridani is a Gamma Doradus Pulsator