HIP 54515 b: A Superjovian Exoplanet

• HIP 54515 b is a superjovian exoplanet bridging the gap between planetary and brown dwarf regimes, identified using combined astrometry and direct imaging methods.
• High-contrast imaging and MCMC dynamical modeling have precisely constrained its mass (~17.7 M_Jup) and eccentric orbit, reinforcing evolutionary models.
• Its distinctive observational parameters make the planet a pivotal target for testing the performance of advanced coronagraphs on the Roman Space Telescope.

HIP 54515 b is a recently discovered superjovian exoplanet whose unique properties and orbital configuration position it as a crucial benchmark for both planetary population studies and the capabilities of upcoming direct imaging instrumentation. Identified via synergy between high-precision astrometry from Hipparcos–Gaia data and extreme-AO high-contrast imaging with Subaru/SCExAO and CHARIS, HIP 54515 b demonstrates features at the interface of the planetary and brown dwarf mass regimes, with direct implications for models of planetary evolution and the technical frontier of faint-star, small-inner working angle coronagraphy (Currie et al., 8 Dec 2025, Currie et al., 1 Dec 2025).

1. Host Star and Planetary System Properties

The host star, HIP 54515 (HD 96855), is cataloged as a mid-A to late-G/early-K main-sequence star, with a V-band magnitude of 6.8, placing it at the practical faint-star limit for the Roman Coronagraph Instrument (CGI) (Currie et al., 8 Dec 2025). The system lies at a distance of 83.2 pc.

HIP 54515 b, residing in the mass regime designated “superjovian,” has its mass dynamically modeled as $$M_{\mathrm{sec}} = 17.7_{-4.9}^{+7.6}\,M_{\mathrm{Jup}}$$, corresponding to a mass ratio $q = 0.0090_{-0.0024}^{+0.0036}$ relative to the host (Currie et al., 1 Dec 2025). The semimajor axis is $a = 24.8_{-4.7}^{+7.2}$ au, with a moderately eccentric orbit ($e = 0.42_{-0.14}^{+0.13}$, $P \sim 91^{+45}_{-26}$ yr), and an inclination $i = 129.6^\circ_{-5.9^\circ}^{+6.7^\circ}$. Observationally, the planet appears at a projected separation of $0.145''$–$0.192''$ (CHARIS, 2023-2025) and is predicted to be at $0.23''$ in early 2028, well-placed for direct coronagraphic detection in the small-IWA regime (Currie et al., 8 Dec 2025).

Spectral characterization places HIP 54515 b at the M/L transition (best-fit $T_{\mathrm{eff}} = 2348 \pm 218$ K, $\log g = 4.46 \pm 0.81$), with $R = 1.08 \pm 0.23~R_{\mathrm{Jup}}$ and $\log(L/L_\odot) = -3.52 \pm 0.03$ (Currie et al., 1 Dec 2025). Near-IR photometry yields $J = 15.99 \pm 0.14$, $H = 15.38 \pm 0.11$, and $K = 15.08 \pm 0.17$ (MKO). Contrast predictions at 575 nm are $4.7 \times 10^{-8}$ to $2.5 \times 10^{-7}$, derived from state-of-the-art self-luminous atmosphere models (Currie et al., 8 Dec 2025).

2. Discovery Techniques and Parameter Determination

HIP 54515 b was discovered by integrating two independent techniques: proper motion acceleration from the Hipparcos–Gaia Catalogue of Accelerations (HGCA) and direct imaging observations. The HGCA data revealed significant proper-motion acceleration, indicative of a 10–20 $M_{\mathrm{Jup}}$ companion at tens of au. SCExAO/CHARIS targeted HIP 54515 in five pupil-tracking epochs (2022–2025), employing a Lyot coronagraph (0.113" radius) and DM-generated satellite spots for calibration. The planet was detected at SNRs between 5.4 and 16.2 (finite-sample-corrected) across these multiple epochs. Orbital motion was unambiguously established via time-resolved astrometry, tracing decreasing position angles ($64.8^\circ$ to $47.3^\circ$, clockwise rotation) (Currie et al., 1 Dec 2025).

Dynamical modeling utilized joint MCMC fits (orvara framework) to relative astrometry and HGCA accelerations. Priors included log-uniform distributions in companion mass, geometric probability distributions for orbital elements, and marginalized posteriors for $M_{\mathrm{sec}}$, $a$, $e$, and $i$. This approach robustly constrained both the mass and three-dimensional orbit, confirming HIP 54515 b’s planetary nature on demographic grounds.

3. Spectrophotometric and Atmospheric Characterization

High-contrast near-IR spectroscopy with CHARIS (R ≈ 18) shows a characteristically “sawtooth” spectral shape, consistent with late-M to early-L dwarfs. Cross-match with the Montreal Spectral Library and BT-Settl models positions HIP 54515 b near the M8–L2 boundary, in a regime pivotal for calibrating cooling tracks across the M/L transition. Effective temperature, gravity, and radius were determined jointly from the spectrum. The bolometric luminosity, $L = 4\pi R^2 \sigma T^4$, derived from $T_{\mathrm{eff}}$ and $R$, is consistent with that inferred from synthetic MKO photometry and bolometric correction ($BC_K \approx 3.2$ mag) (Currie et al., 1 Dec 2025).

Predicted optical contrasts for HIP 54515 b at 575 nm are sensitive to the planet’s cloud properties (silicate/iron grain composition, modal particle size 5–10 μm), bracketing model contrasts in the $7 \times 10^{-8}$ to $2.5 \times 10^{-7}$ range—providing a uniquely stringent target for faint-star coronagraphy and exoplanet atmosphere model validation (Currie et al., 8 Dec 2025).

4. Technical Role in Roman CGI and Faint-Star/Small-IWA Regime

HIP 54515 b’s brightness, separation, and contrast make it a primary technical demonstration target for the Nancy Grace Roman Space Telescope’s Coronagraph Instrument (CGI). The relevant CGI configuration is the Hybrid Lyot Coronagraph (HLC) with a 575 nm narrow-band filter (“Band 1”). The inner working angle (IWA) is $3\lambda/D \approx 0.15''$ for the Roman 2.4 m aperture. HIP 54515 b, at $0.23''$ (corresponding to $1.5\lambda/D$), is positioned just outside the IWA but inside the most challenging performance regime, probing the small-IWA, faint-target limit (Currie et al., 8 Dec 2025).

Total throughput ($\eta$) is estimated as 10–20%, and the residual wavefront error (WFE) goal is $\lesssim 10$ pm rms, setting the contrast floor $$C_{\mathrm{floor}} \sim (2\pi\,\mathrm{WFE}/\lambda)^2$$. Representative signal-to-noise for point-source detection in the “dark hole” is given by

$$\mathrm{SNR} = \frac{N_s}{\sqrt{N_s + N_{\mathrm{res}} + N_z + N_{ez} + N_{\mathrm{dark}} + N_{\mathrm{read}}}},$$

where $N_s$ is the planetary photon signal and other $N$ terms represent residual starlight and background noise contributions. Target exposure times for a $5\sigma$, $10^{-7}$ contrast detection are 2.35 hr (optimistic) to 5.96 hr (conservative), with end-to-end duty cycle leading to total observation durations of 4.0–10.2 hr per science+reference pair (including reference-differential imaging) (Currie et al., 8 Dec 2025).

5. Observational Strategies and Thermally-Induced Contrast Stability

The campaign employs both Angular Differential Imaging (ADI) through $\pm 15^\circ$ telescope rolls and Reference Differential Imaging (RDI) against two vetted PSF reference stars (HIP 54872—$21^\circ$ from HIP 54515, HIP 57632—$19^\circ$ from HIP 54515). These reference stars enable systematic mapping of CGI performance as a function of $\Delta$ pitch angle, defined as the Sun–target angle difference between science and reference observations. This parameter directly affects the telescope thermal state, which in turn induces wavefront drift and impacts achievable contrast (Currie et al., 8 Dec 2025).

Contrast floors show a dependence on $\Delta$ pitch angle: larger values ($>3^\circ$) can degrade attainable contrast by factors of 2–3 due to increased WFE. By bracketing observations across low and high $\Delta$ pitch scenarios ($\Delta$ pitch $<3^\circ$ for HIP 57632; $>10^\circ$ for HIP 54872), the program maps contrast stability and informs requirements for wavefront control in both Roman and future missions. Achieving SNR $>$10 in $\lesssim$6 hr on HIP 54515 b at the faint star/IWA performance boundary constitutes a key proof-of-capability.

6. Demographic and Instrumentation Context

HIP 54515 b represents a pivotal addition to the small population of superjovian planets with well-characterized orbits, bridging the gap between radial velocity/astrometry and direct imaging detection domains. Its properties are aligned with other recently reported superjovian companions (HIP 99770 b, AF Lep b). Comparative analyses (e.g., companion mass vs. semimajor axis and mass ratio vs. semimajor axis) identify a planet population extending to $\sim 25\,M_{\mathrm{Jup}}$ ($q \lesssim 0.025$) distinct from the brown dwarf regime at higher masses/ratios (Currie et al., 1 Dec 2025).

The statistical distribution of moderate eccentricities ($e \sim 0.4$) and cooling rates at the M/L transition are refined by inclusion of HIP 54515 b. A plausible implication is strengthened empirical calibration of luminosity evolution models and mass demarcation at the upper planetary boundary.

Integration with the Roman CGI program provides direct feedback to mission planning for the “faint-star, small-IWA” regime—particularly relevant for projected follow-on observatories such as the Habitable Worlds Observatory. Demonstrated control of wavefront error and contrast stability at the HIP 54515 b performance boundary yields actionable data for operational planning and technology development targeting analogs to Jupiter and, ultimately, terrestrial exoplanets around K and early-M dwarfs in the solar neighborhood.

Summary Table of Key Parameters

Parameter	Value	Source
Host star V (mag)	6.8	(Currie et al., 8 Dec 2025)
Spectral type (host)	Late-G/early-K main sequence	(Currie et al., 8 Dec 2025)
Distance (pc)	83.2	(Currie et al., 1 Dec 2025)
$M_{\mathrm{sec}}$ ($M_{\mathrm{Jup}}$)	$17.7^{+7.6}_{-4.9}$	(Currie et al., 1 Dec 2025)
Semimajor axis (au)	$24.8^{+7.2}_{-4.7}$	(Currie et al., 1 Dec 2025)
Eccentricity $e$	$0.42^{+0.13}_{-0.14}$	(Currie et al., 1 Dec 2025)
Projected separation ($''$)	$0.23$ at 2028 (pred.)	(Currie et al., 8 Dec 2025)
$T_{\mathrm{eff}}$ (K)	$2348 \pm 218$	(Currie et al., 1 Dec 2025)
$\log(L/L_\odot)$	$-3.52 \pm 0.03$	(Currie et al., 1 Dec 2025)
Estimated contrast at 575 nm	$4.7 \times 10^{-8}-2.5 \times 10^{-7}$	(Currie et al., 8 Dec 2025)

HIP 54515 b thus constitutes a technical and astrophysical touchstone: a fully characterized superjovian gas giant challenging coronagraph performance, refining models of giant planet demographics, and directly shaping strategies for imaging cold giant and terrestrial planets with the next generation of space telescopes.