What the corrected fit shows
The timing-adjusted transit is strongly preferred by ΔBIC = 454334.4. Its fitted midpoint is +0.075 hours from the historical prediction; the model's mid-transit depth is 11032.1 ± 18.6 ppm. The support decision uses ΔBIC ≥ 10; timing-search p-values are not presented as blind-detection probabilities.
Explicit model comparison
BIC = 467155.46
BIC = 12821.04
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
The archive prediction was timing-adjusted independently in 1 fitted sector(s) (S2), of which 1 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 2.36-2.36). The robust inverse-variance model depth across supported sectors is 11032.1 +/- 43.8 ppm; a sector-to-sector Q test requires at least two supported sectors. These scaled errors address underestimated scatter and short-timescale correlation, but they are not a full Gaussian-process or physical limb-darkened transit fit.
Published planetary spectrum
Four independent published NIRISS/SOSS reductions are shown. Each is tested against its own weighted-flat eclipse spectrum, while the comparison plot exposes reduction spread; this is not an atmospheric retrieval.
Atmospheric evidence: detection, limit, or unknown?
Four independent reductions in the repository show highly significant spectral structure. The species-level statements come from the published retrieval and opacity-removal tests.
| Species | Status | Evidence | Basis |
|---|---|---|---|
| H2O | reported detection | >6 sigma | three water emission features |
| H- / TiO / VO | reported evidence | 3.8 sigma combined | optical-opacity interpretation |
| O2 | no evidence | not reported | no molecular-oxygen inference |
Primary source: Coulombe et al. 2023, Nature. Oxygen-bearing molecules are not detections of molecular oxygen (O2) and are not, by themselves, biosignatures.
The system
| Radius | 13.90 Earth radii |
|---|---|
| Mass | 3241.85 Earth masses |
| Orbital period | 0.941452 days |
| Transit duration | 2.210 hours |
| Semi-major axis | 0.0202 AU |
| Equilibrium temperature | 2429 K |
| Host and distance | WASP-18 · 123.48 pc |
| Discovery | 2009 · 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 tess2018234235059-s0002-0000000100100827-0121-s_lc.fits, TESS Sector 2, 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 and python scripts/analyze_multisector.py and python scripts/analyze_atmospheric_evidence.py plus python scripts/analyze_spectrum.py to regenerate the figures and CSVs, then pytest tests/ -v to check the documented values against the real file.
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
- Hellier et al. 2009 — discovery reference listed by the NASA Exoplanet Archive.
- Ricker et al. (2015), Transiting Exoplanet Survey Satellite (TESS), doi:10.1117/1.JATIS.1.1.014003.
- TESS Team, TESS Light Curves — All Sectors, MAST, doi:10.17909/t9-nmc8-f686; Sector 2 used here.
- NASA Exoplanet Archive, saved TAP row retrieved 2026-08-15.