What the corrected fit shows
Lower-margin within this portfolio
The timing-adjusted transit is strongly preferred by ΔBIC = 28.2. Its fitted midpoint is +0.032 hours from the historical prediction; the model's mid-transit depth is 491.5 ± 78.7 ppm. The support decision uses ΔBIC ≥ 10; timing-search p-values are not presented as blind-detection probabilities.
Explicit model comparison
BIC = 571.17
BIC = 542.99
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) (S42), 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 1.01-1.01). The robust inverse-variance model depth across supported sectors is 491.5 +/- 79.8 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.
The system
| Radius | 1.98 Earth radii |
|---|---|
| Mass | 3.02 Earth masses |
| Orbital period | 6.201830 days |
| Transit duration | 1.226 hours |
| Semi-major axis | 0.0530 AU |
| Equilibrium temperature | 675 K |
| Host and distance | GJ 9827 · 29.66 pc |
| Discovery | 2017 · Transit · K2 |
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 tess2021232031932-s0042-0000000301289516-0213-s_lc.fits, TESS Sector 42, 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 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.
- This is a lower-margin supported result within this portfolio. Its support is more sensitive than the very large-ΔBIC cases to the fixed outlier rule, fitting window, and limb-darkening coefficients.
- 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
- Niraula et al. 2017 — 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 42 used here.
- NASA Exoplanet Archive, saved TAP row retrieved 2026-08-15.