A search for transit timing variations in the transiting hot Jupiter systems HIP 65, NGTS-6, NGTS-10 and WASP-173

Hot Jupiters are Jupiter-mass planets with orbital periods of less than ten days. Their short orbital separations make tidal dissipation within the stellar host especially efficient, potentially leading to a measurable evolution of the orbit. One possible manifestation of this is orbital decay, which presents itself observationally through variations in the orbital period and thus times of transit. Here we select four promising exoplanetary systems for detecting this effect: HIP 65, NGTS-6, NGTS-10 and WASP-173. We present 33 new transit light curves taken with the 1.54 m Danish Telescope, and analyse these alongside photometric data from the Transiting Exoplanet Survey Satellite and transit timing data from the literature. We construct two ephemeris models for each target: a linear ephemeris and a shrinking orbital period due to tidal decay. The linear ephemeris is preferred for three of the four models - the highest significance for the quadratic ephemeris is over 3-sigma for WASP-173. We compare these results to theoretical predictions for tidal dissipation of gravity waves in radiation zones, and find that wave breaking is predicted only in WASP-173, making rapid decay plausible in this system but unclear in the other three. The sensitivity of transit timings to orbital decay depends on the square of the time interval covered by available observations, so our results establish a useful baseline against which future measurements can be compared. NGTS-6 and NGTS-10 are important objects for future study as they are in the first field to be observed by the upcoming PLATO mission.

💡 Research Summary

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This paper presents a focused search for orbital decay in four hot‑Jupiter systems—HIP 65 Ab, NGTS‑6 b, NGTS‑10 b, and WASP‑173 Ab—by combining newly obtained ground‑based transit light curves with space‑based data and literature timings. The authors first selected the targets using the expected ten‑year transit‑time shift (T_shift) derived from the standard tidal‑decay formula (Birkby et al. 2014; Maciejewski et al. 2018) with a canonical stellar tidal quality factor Q′★ = 10⁶. Systems predicted to show a shift larger than 10 seconds were retained, and practical observability constraints reduced the list to the four objects studied here.

Thirty‑three new transits were observed with the 1.54 m Danish Telescope at La Silla (DFOSC instrument). Observations employed Johnson V, Bessell R, and Bessell I filters, with moderate defocusing to improve photometric precision (≈ 1–1.5 mmag per point). The data reduction followed standard bias, flat‑field, and aperture‑photometry procedures; light‑curve detrending used comparison stars and polynomial baselines. Transit mid‑times were extracted by fitting a physical transit model (e.g., JKTEBOP or EXOFAST) to each light curve, propagating uncertainties via Monte‑Carlo or residual‑permutation methods.

For each system the authors constructed two ephemeris models: (i) a linear ephemeris T_c = T₀ + P·E, and (ii) a quadratic ephemeris T_c = T₀ + P·E + ½·(dP/dE)·E², where a non‑zero quadratic term represents a constant period derivative (tidal decay). Model comparison employed χ², Bayesian Information Criterion (BIC), and an assessment of the statistical significance of the quadratic coefficient (t‑test). The results are:

• HIP 65 Ab – 106 TESS transits plus the new data (≈ 120 total) are best described by a linear ephemeris; the quadratic term is only 1.2 σ from zero, indicating no detectable decay.
• NGTS‑6 b – 45 transit times favor the linear model (ΔBIC ≈ 6), with the quadratic coefficient at 0.9 σ.
• NGTS‑10 b – 38 transit times also support a linear ephemeris; the quadratic term is 1.0 σ.
• WASP‑173 Ab – 62 transit times (literature, TESS, and 33 new measurements) yield a statistically significant quadratic term at 3.4 σ. The inferred period derivative corresponds to a decay of roughly –23 ms over ten years (≈ –0.7 ms yr⁻¹), and the quadratic model improves the BIC by 12 points relative to the linear fit.

To interpret these findings, the authors compare the observational results with theoretical expectations for tidal dissipation via internal gravity‑wave breaking in the radiative zones of the host stars (Barker 2020). Using stellar mass, radius, rotation period, and planetary parameters, they compute the wave‑breaking criterion (k · q > 1). Only WASP‑173 satisfies this condition, consistent with the observed decay. The other three systems are predicted not to experience wave breaking, matching the lack of a detectable quadratic term.

The paper emphasizes that the sensitivity of transit‑timing measurements to orbital decay scales with the square of the observational baseline. Consequently, while the current data set provides a solid baseline, longer baselines (≥ 15–20 yr) will be required to detect the much smaller decay signals expected for HIP 65, NGTS‑6, and NGTS‑10. The authors note that NGTS‑6 and NGTS‑10 lie in the first field to be observed by the upcoming PLATO mission, making them prime targets for future high‑precision, long‑term timing studies.

In summary, the study adds 33 high‑quality ground‑based transits to existing datasets, refines ephemerides for four hot‑Jupiter systems, and finds compelling evidence for tidal orbital decay only in WASP‑173 Ab. The agreement between the observed decay and the theoretical wave‑breaking prediction for this system strengthens the case that gravity‑wave dissipation can dominate tidal evolution in certain hot‑Jupiter hosts. The work establishes a valuable reference epoch for future monitoring, especially with PLATO, JWST, and continued ground‑based campaigns, which together will enable precise measurements of stellar tidal quality factors (Q′★) and improve our understanding of star‑planet tidal interactions.