How Many Transiting Giant Planets Can JWST Search for Moons and Rotational Oblateness?

Abstract: Observations with the {\it James Webb Space Telescope} (JWST) can, in principle, detect moons and rotational oblateness of giant exoplanets through subtle distortions of transit light curves. The most favorable planets are expected to be on wide orbits ($\gtrsim$0.3~AU) where moons and rapid rotation are more likely to survive tidal evolution. No unambiguous detections have yet been reported. Here, we forecast the number of systems with sufficiently favorable properties to allow for secure detections, using JWST noise models, analytic detectability scalings, giant-planet occurrence rates, and the Gaia star catalog. For planets orbiting 0.9--1.6$\,M_\odot$ stars and a noise model based on demonstrated JWST performance, single-transit observations should be capable of detecting Jupiter-like rotational oblateness in several known systems and of order 10 systems yet to be discovered, if obliquities are typically $\gtrsim$10$<sup>\circ$. A similar number of systems are favorable for Ganymede-sized moons, if such moons are common. The yields can increase to tens or hundreds of systems if lower-mass host stars are included or if JWST can achieve photon-limited performance. Time-correlated noise on 1--10 hr timescales can strongly suppress these yields; a noise floor of a few tens of parts per million is enough to hide oblateness or moons in many otherwise favorable systems. Successful searches will therefore require both a more complete census of long-period transiting giant planets and low levels of instrumental systematics and stellar variability.

• The paper presents an analytic and empirical framework to forecast JWST's detection yield for exomoons and rotational oblateness.
• It leverages transit photometry with both theoretical ETC predictions and empirical noise assessments to establish Δχ² detection thresholds.
• The study predicts yield variations tied to host star mass and survey completeness, outlining potential tens to hundreds of detectable systems.

Quantitative Forecasts for JWST Moon and Oblateness Searches Around Transiting Giant Planets

Motivation and Background

This paper addresses the statistical outlook for the detection of exomoons and rotational oblateness using transit photometry with JWST. Rotational oblateness, indicative of rapid planetary spin, and exomoons, as probes of satellite system formation and stability, are accessible through subtle transit curve distortions (asymmetries and additional dips) on the order of 10–100 ppm. The theoretical context prioritizes wide-orbiting giant planets ($a \gtrsim 0.3~AU$), where neither tidal despinning nor Roche/Hill radius limitations have erased moons or spin. Despite centralized limits on planets, moons, and rings from previous missions, JWST's demonstrated and theoretical photometric precision warrants a population-level forecast for yield expectations.

JWST Photometric Noise Modeling

The authors systematically characterize JWST's photometric noise using both the PandExo ETC predictions and empirical reductions from the literature. The empirical scatter is consistently worse than ETC by factors up to 2.4 in NIRISS/SOSS, primarily due to detector limitations. Optimized observation modes (NIRSpec/G395H for $K<9.5$, PRISM for fainter stars) are integrated into analytic noise models. Critically, the signal integration timescales (0.5 hr for oblateness, up to 10 hr for moons) are longer than the minute-based noise normalization, amplifying vulnerability to time-correlated systematics beyond the white-noise regime.

Detectability Metrics: Rotational Oblateness and Moons

Directly quantifying the signal-to-noise ratio in model selection ($\Delta\chi^2$) for oblateness and moon features, the authors derive geometric and photometric scaling relations:

• For oblateness, the transit ingress-egress asymmetry is maximized for projected obliquity $\sim45^\circ$ and impact parameter $\sim0.7$, yielding a scaling that increases with $R_p$, decreases for larger host stars, and grows for wider orbits.
• For moons, detection probability is a strong function of moon radius and transit duration; longer periods favor detection if the moon's transit does not overlap with that of the planet.

Thresholds for $\Delta\chi^2$ (default = 60) are empirically validated via injection-recovery simulations.

Stellar and Planetary Occurrence Modeling

Star catalogs extracted from Gaia DR3 are filtered for favorable host properties. Within $0.9$–$1.6~M_\odot$ and $0.1$–$K<9.5$0 domains, the search volume is maintained with completeness and binarity considerations (RUWE <1.2). Giant-planet occurrence rates are adopted from the California Legacy Survey, with mass-dependent normalization for low-mass hosts. Monte Carlo simulations populate these stars with planet distributions, assign geometries, and evaluate transit probabilities.

Predicted Yields and Sensitivity Analysis

The paper computes expected yields for oblateness and moon searches under both ETC-based and empirically measured JWST noise models:

Parameter	Baseline Sample (0.9–1.6 $K<9.5$1)	Lower-Mass (0.1–0.9 $K<9.5$2)
Jupiter-like oblateness (Rayleigh $K<9.5$3 obliquity, empirical noise)	10 systems	79 systems
Ganymede-sized moon (empirical noise)	13 systems	172 systems
Yields under photon noise limit (ETC)	higher by factors of $K<9.5$4–$K<9.5$5	higher by factors of $K<9.5$6–$K<9.5$7

The detection yields are highly sensitive to the underlying obliquity distribution (with near-zero yield for Jupiter-like obliquity). Host star mass and photometric noise floor set practical constraints. Red noise analysis indicates that a floor of $K<9.5$8–$K<9.5$9~ppm on relevant timescales suppresses yields substantially.

Survey Completeness and Current Target Population

The cross-check with the NASA Exoplanet Archive and TESS candidate catalog exposes strong deficits in currently known long-period transiting giants and favorable low-mass hosts. Only a small fraction of theoretically favorable systems are represented in confirmed catalogs, underscoring the necessity for longer-baseline survey strategies as planned in PLATO, Earth 2.0, and Gaia follow-up.

Specific top targets (TOI-2449 b, TOI-199 b, Kepler-167 e, etc.) are highlighted as likely to yield detectable oblateness or moon signals, provided schedules, ephemeride accuracy, and noise floors allow.

Limitations and Prospects for Advancement

The forecasts are contingent on several astrophysical and instrumental uncertainties:

• Unknown true distributions for spin obliquity and moon occurrence
• Stellar radii uncertainty and extinction systematics from Gaia
• Overlapping transits and degeneracies between planet and moon parameters not accounted for
• Conservative periastron cuts that may underpredict the true accessible population, especially in young/high-$\Delta\chi^2$0 systems

Scaling relations permit adaptation to other planet or moon sizes (e.g., Earth-sized moons yield factors of $\Delta\chi^2$1 improvement).

Future progress will depend fundamentally on survey completeness for wide-separation giants and photometric stability improvements. JWST's capability may expand with improved data reduction and calibration protocols, and as long-period transit discoveries increase.

Conclusion

This study provides an analytic and empirically validated framework for forecasting JWST's ability to detect rotational oblateness and exomoons via transit photometry (2607.09873). Under current performance assumptions, JWST is sensitive to tens of systems for oblateness and moon searches, pending noise floor and survey completeness constraints. The practical detection yield is most dependent on time-correlated noise levels and the census of wide-separation transiting giants. Systematic improvements in target discovery and photometric stability, alongside expanded mission baselines, are expected to transform these searches from isolated cases to population-level studies, revealing deeper physical characteristics of exoplanetary systems.