Hot Jupiter · orbital decay · extreme irradiation

WASP-12 b

A severely irradiated giant spiralling toward its star, framed as a careful TESS timing analysis where ephemeris drift is itself part of the science.

REAL TESS DATATIMING-AWARE FITPRIMARY-SOURCE CONTEXTTESTED PIPELINE

AI-generated artist’s interpretation · not a direct image

What the corrected fit shows

Transit supportSUPPORTED (ΔBIC ≥ 10)
Fitted midpoint shift-0.011 h
Model depth15714.4 ± 125.2 ppm
ΔBIC (null − transit)17136.0

The timing-adjusted transit is strongly preferred by ΔBIC = 17136.0. Its fitted midpoint is -0.011 hours from the historical prediction; the model's mid-transit depth is 15714.4 ± 125.2 ppm. The support decision uses ΔBIC ≥ 10; timing-search p-values are not presented as blind-detection probabilities.

Timing-adjusted limb-darkened transit fit for WASP-12 b
The dotted vertical line is the historical archive prediction; the dashed colored line marks the fitted midpoint (-0.011 h). The smooth curve fits timing, radius ratio, impact parameter, and a local linear baseline.

Explicit model comparison

Linear nullχ² 19856.94 / 11383 dof

BIC = 19875.62

Limb-darkened transitχ² 2692.96 / 11380 dof

BIC = 2739.66

The null fits a local intercept and slope. The transit adds bounded midpoint, radius-ratio, and impact-parameter freedom while retaining the same baseline terms. Fixed limb darkening and circular geometry make this a diagnostic fit rather than a full system retrieval.

Multi-sector robustness and noise

Supported combined depth15429.6 +/- 55.8 ppm
Supported / fitted sectors3 / 3
Residual beta range1.22-1.67

The archive prediction was timing-adjusted independently in 3 fitted sector(s) (S20, S43, S45), of which 3 meet Delta BIC >= 10. Formal depth errors were inflated by sqrt(max(reduced chi-square, 1)) times the residual time-averaging beta factor (observed range 1.22-1.67). The robust inverse-variance model depth across supported sectors is 15429.6 +/- 55.8 ppm; Cochran Q = 5.45 for 2 dof (p = 0.0656). These scaled errors address underestimated scatter and short-timescale correlation, but they are not a full Gaussian-process or physical limb-darkened transit fit.

Independent sector transit fits for WASP-12 b
Independent sector transit fits. Reproduced from the committed public TESS SPOC files.
Sector depth consistency for WASP-12 b
Sector depth consistency. Reproduced from the committed public TESS SPOC files.
Residual RMS time-averaging diagnostic for WASP-12 b
Residual RMS time-averaging diagnostic. Reproduced from the committed public TESS SPOC files.

Orbital decay: the baseline matters

Published timing events158
Recalculated period derivative−29.28 ± 2.03 ms yr⁻¹
ΔBIC (linear − quadratic)202.5
Formal decay timescale3.22 Myr

The three committed TESS sectors alone do not establish quadratic curvature: 62 supported individual transit times give ΔBIClinear−quadratic = −1.9. When the 158-event published transit-and-occultation baseline is fitted, the quadratic ephemeris is strongly favoured and yields Ṗ = −29.28 ± 2.03 ms yr⁻¹. This reproduces the published decay scale; it is not presented as a new discovery.

WASP-12 b observed-minus-calculated timing diagram comparing published long-baseline and TESS-only fits
Top: the long-baseline timing compilation and quadratic-minus-linear ephemeris. Bottom: the separate TESS-only sensitivity check, which cannot support a curvature claim.

Machine-readable outputs: published timing input, independently fitted TESS transits, and model comparison.

The system

Radius22.03 Earth radii
Mass467.21 Earth masses
Orbital period1.091419 days
Transit duration3.001 hours
Semi-major axis0.0234 AU
Equilibrium temperature2601 K
Host and distanceWASP-12 · 427.25 pc
Discovery2008 · Transit · SuperWASP

Values are from the exact saved NASA Exoplanet Archive pscomppars row in data/system_parameters.csv.

Data and reproducibility

The observed photometry is the public MAST file tess2019357164649-s0020-0000000086396382-0165-s_lc.fits, TESS Sector 20, collection DOI 10.17909/t9-nmc8-f686. It is stored unmodified. Exact archive URI, URL, retrieval date, and checksum are in data/SOURCE.md.

Run python scripts/analyze_transit.py, python scripts/analyze_multisector.py, and python scripts/analyze_orbital_decay.py to regenerate the figures and CSVs, then pytest tests/ -v to check the documented values against the real files.

Limitations

  • The orbit is circular and the quadratic limb-darkening coefficients are fixed representative values.
  • The scaled semi-major axis comes from saved composite values whose uncertainties are not propagated.
  • The midpoint is searched within a bounded window; ΔBIC is used as the support gate.
  • PDCSAP processing, dilution, stellar variability, timing variations, and covariance can bias the geometry.
  • Published global fits with physical priors, instrument-specific detrending, and simultaneous sectors remain authoritative.

References

  1. Hebb et al. 2009 — discovery reference listed by the NASA Exoplanet Archive.
  2. Ricker et al. (2015), Transiting Exoplanet Survey Satellite (TESS), doi:10.1117/1.JATIS.1.1.014003.
  3. TESS Team, TESS Light Curves — All Sectors, MAST, doi:10.17909/t9-nmc8-f686; Sector 20 used here.
  4. NASA Exoplanet Archive, saved TAP row retrieved 2026-08-15.
  5. Yee et al. (2020), The Orbit of WASP-12b Is Decaying, doi:10.3847/2041-8213/ab5c16.
  6. Wong et al. (2022), TESS Revisits WASP-12, doi:10.3847/1538-3881/ac5680.