Worlds Next Door. IV. Mapping the Late Stages of Giant Planet Evolution with a Precise Dynamical Mass and Luminosity for $ε$ Ind Ab

Abstract

We present new JWST/NIRCam 4-5 $μ$m (F410M, F430M) and JWST/MIRI 18-25 $μ$m (F1800W, F2100W, F2550W) imaging detections of the nearby (3.6 pc) cold (275 K) gas giant exoplanet $ε$ Ind Ab. The F2550W detection of $ε$ Ind Ab constitutes the longest wavelength image of an exoplanet acquired to date. Combining three decades of radial velocity monitoring, Gaia-Hipparcos absolute astrometry, and relative astrometry from direct imaging (including the new NIRCam astrometry), we conduct a comprehensive re-analysis of $ε$ Ind Ab's orbit and obtain a dynamical mass $M_{\rm Ab} = 6.5^{+0.7}_{-0.6}\;M_{\rm Jup}$. Using $ε$ Ind Ab's NIRCam and MIRI photometry, we assemble the first 4-25 $μ$m spectral energy distribution (SED) of a cold gas giant outside the Solar System. The NIRCam photometry supports a metal-enriched atmosphere for $ε$ Ind Ab based on analysis with atmospheric model grids, consistent with predictions from the giant planet mass-metallicity relation. While the current data do not provide definitive evidence for or against the presence of water ice clouds, we tentatively find that the H$_2$O vapor absorption-dominated F2550W photometry is systematically brighter ($>1σ$, but $<2σ$) than predictions from cloud-free/rainout chemistry models and better explained by a cloudy model. We calculate a bolometric luminosity of $\log L_{\rm bol}/L_\odot = -7.23 \pm 0.03$ dex by directly integrating $ε$ Ind Ab's SED. Combining this with the planet's dynamical mass and age ($3.5 \pm 1.0$ Gyr), we demonstrate excellent agreement with evolutionary model predictions in a new regime of low luminosities, low masses, and old ages. Our results establish $ε$ Ind Ab as a benchmark system for planetary evolution studies and set the stage for the detailed atmospheric characterization of this frigid extrasolar world.

Figures (16)

• Figure 1: Gallery of PSF-subtracted images centered on $\epsilon$ Ind A in NIRCam coronagraphic (top row) and MIRI non-coronagraphic (center row) imaging filters. All images are oriented North up and East left. Exoplanet $\epsilon$ Ind Ab is detected in all filters except F200W (expected location marked with a '$+$'). An inset image (bottom right corner) shows the residuals in the aperture centered on $\epsilon$ Ind Ab after the best-fit planet forward model is subtracted. The aperture encompasses the six-lobed structure of the planet PSF in the NIRCam images and has a radius = 1 FWHM in the MIRI images. The F2550W image of $\epsilon$ Ind Ab is the longest wavelength image of an exoplanet taken to date. The bottom panel shows the complete 4--25 $\mu$m spectral energy distribution of $\epsilon$ Ind Ab. Boxes with error bars represent the measured photometry and inverted triangles represent upper limits, with the filter profiles plotted below (no filter profile available for VLT/NEAR). Solid gray horizontal lines mark the dominant opacity source in the corresponding wavelength region.
• Figure 2: A comparison between posteriors from the seven converged (out of eight) orbit models considered for $\epsilon$ Ind Ab, listed in the legend. All contours in the corner plot encompass the volume contained within the 1$\sigma$ 2D Gaussian equivalent. The final adopted model is in black and yields a dynamical mass $6.5^{+0.7}_{-0.6}\;M_{\rm Jup}$. A key takeaway is that the new JWST/NIRCam astrometry provides evidence for a moderately eccentric orbit (yellow vs black posteriors).
• Figure 3: Summary of orbit fit for the complete relative astrometry + RV + HGCA model. Top: The top sub-panel shows the full RV timeseries with the model fit (dark blue) and residuals. Data from distinct instruments are represented with different colors and symbols. An orange line shows the effect of secular acceleration. The colored line connecting the points is the GP model and the colored band shows the uncertainty from the GP model. The bottom sub-panel shows the phase-folded model RV curve in blue. The binned data are shown as red points. Bottom left: The top sub-panel shows 250 randomly sampled sky-projected orbits with the measured relative astrometry. A mean anomaly of zero corresponds to periastron passage. The bottom sub-panel shows the corresponding projected separation and position angle vs time for the sampled orbits. Bottom right: The top sub-panel shows 500 random samples of the proper motion (PM) in R.A. and Decl. vs time with measurements from HGCA. The bottom sub-panel shows the covariances between R.A. and Decl. velocity at each PM epoch. The "H" epoch is the net PM at the Hipparcos epoch calibrated to the DR3 velocity reference by the HGCA, "G-H" is the long-term PM derived from the HGCA, and "G" is the Gaia DR3 PM.
• Figure 4: A comparison of the RV residuals obtained after fitting models without (top) and with (bottom) a GP, for the maximum posterior orbit. The GP explains variations in the RV time series and improves the residuals.
• Figure 5: Comparison between $\epsilon$ Ind Ab's and $\sim$285 K brown dwarf WISE 0855's spectral energy distributions. The colored open boxes with error bars represent $\epsilon$ Ind Ab's measured photometry. WISE 0855's spectrum (flux re-scaled to match the distance to $\epsilon$ Ind A and binned to $R\sim 50$) is shown in solid gray and the corresponding synthetic photometry is shown as colored filled circles. A reference blackbody spectrum is shown in dashed gray. The filter profiles are plotted below the photometry (no profile available for VLT/NEAR, constant transmission assumed between $\approx$10--12.5 $\mu$m). The 4.1--4.3 $\mu$m CO$_2$ absorption region is marked with a solid horizontal black line.