Scientific Premise

A planet's shape encodes its rotational state and interior density profile. A rapidly rotating or tidally distorted planet is oblate, and that oblateness subtly alters the ingress and egress morphology of its transit light curve. Theoretical predictions have long suggested that Kepler-class photometry should be sensitive to oblateness at the level of a few percent for inflated hot Jupiters, and recent results are finally beginning to deliver: HIP 41378f yielded the first robust oblateness constraint in 2025 (f ≤ 0.889 at 95% confidence), and Kepler-427b shows depth variations consistent with f = 0.19 at ~90% significance. A recent PNAS review (Ragozzine et al., 2025) highlighted that tidal distortion, rotational oblateness, and obliquity remain among the least-explored dynamical signals accessible to JWST and existing photometric archives — the community has been so focused on atmospheric characterization that shape-based science has been left on the table.

These early detections demonstrate that the signal is there, but the field remains in a one-planet-at-a-time mode. The key insight is that oblateness is not just a curiosity — it is a direct probe of interior rotation and tidal dissipation efficiency, two quantities that are extremely difficult to measure by any other means. A systematic oblateness census across the known hot Jupiter population could distinguish between tidal synchronization models, constrain the tidal quality factor Q', and identify planets with anomalously high obliquities (potentially maintained by secular spin-orbit resonances in multi-planet systems).

Target Datasets

• Kepler long-cadence and short-cadence light curves for ~300 confirmed hot Jupiters and inflated sub-Saturns (MAST archive)
• TESS 2-minute and 20-second cadence data for the brightest transit hosts observed across multiple sectors
• JWST NIRSpec/NIRISS transit observations already in the archive (GO Cycles 1–3) where ingress/egress are well-sampled at high SNR
• Spitzer 4.5 µm phase curves (archival) for thermal emission asymmetry cross-checks

Novelty

While individual studies have now achieved detections for a small number of planets (e.g., HIP 41378f, Kepler-427b), and earlier work placed upper limits on systems like Kepler-7b, no one has performed a population-level oblateness survey using uniform methodology across hundreds of planets. The statistical power of a population approach is that even non-detections become informative: an ensemble of upper limits on oblateness across a range of orbital periods, planetary radii, and stellar types would place the tightest constraints to date on tidal synchronization timescales and interior viscosity models. This is distinct from the one-planet-at-a-time paradigm that has dominated so far.

Concrete Workflow

1. Construct a target list of all transiting planets with Rp > 0.5 RJ and P < 10 days from the NASA Exoplanet Archive, cross-matched with Kepler, TESS, and JWST archival coverage.
2. Develop a custom transit model incorporating planetary oblateness (parameterized as f = (Req - Rpole)/Req) and projected obliquity (θ), building on the Seager & Hui (2002) and Carter & Winn (2010) formalisms.
3. Fit each system's phase-folded light curve with both a spherical model and an oblate model using nested sampling (e.g., dynesty) to compute Bayesian evidence ratios.
4. For systems with marginal detections (ΔlnZ > 2), stack residuals from similar-parameter planets to search for a population-level signal.
5. Compare the resulting oblateness–period distribution against tidal evolution models (e.g., Leconte et al. 2010; Millholland & Laughlin 2019) to constrain Q' as a function of planet mass and radius.

Expected Signal / Observable

For a planet with f ~ 0.05 (comparable to Saturn) and an obliquity of 30°, the transit depth asymmetry between ingress and egress is on the order of 50–100 ppm for a Jupiter-sized planet around a Sun-like star. This is below the noise floor for individual Kepler long-cadence transits, but stacking 50+ transits for the best-observed planets (or using TESS 20-second cadence for bright hosts) brings the effective sensitivity into the 10–30 ppm range, where detections become plausible for the most inflated, rapidly rotating cases.

Possible False Positives

• Starspot crossings during ingress/egress can mimic asymmetric transit shapes. Mitigation: multi-epoch analysis (spot patterns change, oblateness does not), and infrared observations where spot contrast is reduced.
• Gravity darkening of the host star (if rapidly rotating or tidally distorted by the planet) can produce asymmetric transits. Mitigation: restrict the sample to slow rotators (v sin i < 5 km/s) or model gravity darkening jointly.
• Transit timing variations (TTVs) can shift ingress/egress if not properly detrended. Mitigation: fit individual transits rather than relying solely on phase-folded data.

Why This Could Matter

A population-level constraint on planetary oblateness would be a first. It would directly measure how efficiently tidal forces synchronize planetary rotation — a process that is central to understanding the thermal evolution, atmospheric dynamics (day-night circulation is rotation-dependent), and long-term stability of close-in planets. It could also identify "smoking gun" cases of planets in spin-orbit resonance with obliquities preserved at high values, opening a new window on multi-planet dynamical histories.