JWST Has the Sensitivity to Find Exomoons and Measure Planet Oblateness, If the Right Targets Exist
Date: 2026-07-14
Featured image: [Artist’s impression of an exomoon orbiting a gas giant; credit: NASA/JPL-Caltech]
The James Webb Space Telescope has the raw sensitivity to detect exomoons and measure the rotational oblateness of giant exoplanets, but the biggest obstacle is not the instrument. It is the shortage of suitable targets.
A new study by Le-Chris Wang and Joshua Winn of Princeton University, accepted by the Astrophysical Journal Letters, provides the first systematic assessment of how many known transiting giant planets JWST can realistically search for moons and oblateness signals. The answer depends heavily on host star type, noise performance, and the orientation of planetary spin axes, but the paper identifies roughly 10 favorable systems for each measurement among Sun-like stars.
“The bottleneck is not telescope sensitivity,” the authors write. “It is survey incompleteness.”
The Hunt for Exomoons
No unambiguous exomoon has ever been detected for any exoplanet, despite decades of searching. The paper focuses on long-period giant planets in wide orbits, where moons and rapid rotation can survive the tidal forces that would otherwise strip them away.
The detection method is transit photometry: when a moon transits separately from its host planet, it produces an extra dip in starlight. The study assumes an optimistic scenario where the moon transit is fully separated in time from the planetary transit. For a Ganymede-sized moon, JWST could detect the signal in about 9 systems around Sun-like stars using its white-light photometric precision.
Including lower-mass M dwarfs as host stars dramatically expands the yields to approximately 172 favorable systems, because smaller stars make the transit signals relatively larger. The key JWST instruments are NIRISS/SOSS, NIRSpec/PRISM, and NIRSpec/G395H, which provide the photometric precision needed.
Reading a Planet’s Shape
Planetary oblateness, or how much a planet is flattened at its poles due to rapid rotation, can be measured through subtle asymmetries in the transit light curve. When an oblate planet crosses its star, the ingress and egress show characteristic wiggles at the level of 100 to 200 parts per million for a Jupiter-sized planet with Saturn-like flattening.
The oblateness parameter is directly linked to a planet’s rotation rate and internal structure through the Darwin-Radau relation. Measuring it constrains the planet’s moment of inertia and reveals whether it has a core and how centrally condensed its interior is. This is information inaccessible by any other means for transiting exoplanets.
The paper finds about 10 systems around Sun-like stars are favorable for oblateness detection, assuming realistic spin-axis misalignments of 10 degrees or more. If most giant planets have small obliquities like Jupiter’s 3 degrees, the yield drops to zero. Expanding to all host stars pushes the favorable count to about 79.
The Noise Problem
The single biggest technical challenge is time-correlated noise in JWST’s detectors. A noise floor of just a few tens of parts per million on timescales of 1 to 10 hours can completely eliminate detections in otherwise favorable systems.
The study uses an empirical noise model based on 27 published JWST white-light curves. Key finding: NIRISS/SOSS underperforms predictions by a factor of about 2.4 for bright targets. Recent observations of the candidate system Kepler-167e, published by Cassese and Kipping in companion papers, illustrate the problem: exposure-long trends of roughly 600 parts per million over 10-hour exposures prevented definitive confirmation of either oblateness or moons.
What Comes Next
The paper strongly incentivizes wider surveys for long-period transiting giant planets. Most of the best candidates may still be undiscovered. Planned missions such as ESA’s PLATO and China’s Earth 2.0 survey are expected to dramatically increase the catalog of suitable targets.
For now, the most promising systems include TOI-199 b, TOI-2449 b, TOI-4600 c, and Kepler-167 e. The Kepler-167 e system is scheduled for a second JWST transit observation in October 2027, which could break the degeneracy between starspot signals and a possible Roche-skimming exomoon in the earlier ambiguous data.
The paper concludes that JWST is technically capable of delivering two of the most anticipated results in exoplanet science. It simply needs more planets to look at.