SCExAO/CHARIS and Gaia Direct Imaging and Astrometric Discovery of a Superjovian Planet 3--4 lambda/D from the Accelerating Star HIP 54515

TL;DR

This study reports the first exoplanet discovery from the Observing Accelerators with SCExAO Imaging Survey (OASIS): HIP 54515 b, a superjovian planet at a small angular separation detected by combining Hipparcos–Gaia absolute astrometry with direct imaging from SCExAO/CHARIS. The planet’s dynamical mass is tightly constrained to about 18–19 $M_{Jup}$ with a semi-major axis near 25 au and an eccentric orbit ($e \sim 0.4$), placing it in the planet-like region of companion demographics despite its high mass. Spectroscopic analysis places HIP 54515 b at the M/L transition with $T_{eff} \approx 2350$ K and $\log L/L_{\odot} \approx -3.52$, enabling atmospheric characterization prospects with JWST and Roman CGI. The results demonstrate the power of combining high-contrast imaging with precision absolute astrometry to obtain dynamical masses for young, massive planets, and identify HIP 54515 b as a valuable target for testing planet formation theories and exoplanet atmospheric evolution, as well as a potential Roman CGI demonstration object.

Abstract

We present the discovery of a superjovian planet around the young A5 star HIP 54515, detected using precision astrometry from the Hipparcos Gaia Catalogue of Accelerations and high-contrast imaging with SCExAO/CHARIS from the recently-commenced OASIS program. SCExAO/CHARIS detects HIP 54515 b in five epochs 0\farcs{}145--0\farcs{}192 from the star ($\sim$3--4 $λ$/D at 1.65 $μm$), exhibiting clockwise orbital motion. HIP 54515 b lies near the M/L transition with a luminosity of log(L/L$_{\rm \odot}$) $\sim$ -3.52 $\pm$ 0.03. Dynamical modeling constrains its mass and mass ratio to be ${17.7}_{-4.9}^{+7.6}$ $M_{\rm Jup}$ and ${0.0090}_{-0.0024}^{+0.0036}$ and favors a $\sim$25 au semimajor axis. HIP 54515 b adds to a growing list of superjovian planets with moderate eccentricities (e $\approx$ 0.4). Now the third planet discovered from surveys combining high-contrast extreme adaptive optics imaging with precision astrometry, HIP 54515 b should help improve empirical constraints on the luminosity evolution and eccentricity distribution of the most massive planets. It may also provide a key technical test of the Roman Space Telescope Coronagraph Instrument's performance in the low stellar flux, small angular separation limit and a demonstration of its ability to yield constrainable planet spectral properties.

Figures (7)

• Figure 1: (left) The Gaia color-magnitude diagram for the Pleiades (gray) and Hyades (dark red) compared to PARSEC isochrones for 112 Myr and 750 Myr (roughly equal to the clusters' estimated ages), young stars with interferometrically measured ages $\lesssim$ 150 Myr, stars in the 400 Myr old Ursa Majoris Moving group, and other young stars with imaged substellar companions Jones2015Jones2016Jones2016PhDCurrie2023aTobin2024. (right) Minimum mass vs. projected separation needed to explain HIP 54515's proper motion anomaly, following the approach of Kervella2022 with the stellar/substellar boundary overplotted as a red dotted line and approximate brown dwarf/planet boundary overplotted as a black dotted line. The shaded region denotes the minimum mass 68% confidence interval derived from uncertainties in the stellar mass and the proper motion.
• Figure 2: Wavelength-collapsed CHARIS images over five epochs showing detections of HIP 54515 b. The color scale is adjusted to saturate the signal within a FWHM-sized region, except for the April 2024 epoch where the scaling is adjusted to better differentiate the planet from residual speckle noise. The companion PSF is exterior to the coronagraph mask edge in all epochs (dashed circle).
• Figure 3: Epoch-averaged HIP 54515 b spectrum compared to spectra extracted from individual epochs (left) and to selected best-fitting substellar objects in the Montreal Spectral Library (middle) and BT-Settl atmosphere models (right).
• Figure 4: Corner plot of the downsampled posterior distributions for HIP 54515 b assuming a log-uniform prior on the companion mass. The plot shows marginalized 1D and 2D posteriors for the primary mass ($M_{\rm pri}$), companion mass ($M_{\rm sec}$), semi-major axis ($a$), eccentricity ($e$), and inclination ($i$). The contours represent the 68%, 95%, and 99.7% credible regions. Note that the downsampling uniformly selects 10,000 random samples from the flattened, post-burn-in posterior to produce corner plots without altering the underlying distribution.
• Figure 5: Orbital fit for HIP 54515 b assuming a log-uniform prior on companion mass using orvara. (Left) Ensemble of orbital solutions with the maximum likelihood orbit shown in black and 70 randomly drawn posterior samples overplotted and color-coded by companion mass. Filled red circles indicate our new CHARIS astrometric data, while open circles mark the predicted locations at each epoch. (Center) Position angle as a function of time. (Right) Angular separation versus epoch. All panels reflect the range of plausible orbits consistent with the data and inferred parameter uncertainties.