Planetary transit observations at the University Observatory Jena: XO-1b and TrES-1

We report on observations of transit events of the transiting planets XO-1b and TrES-1 with a 25 cm telescope of the University Observatory Jena. With the transit timings for XO-1b from all 50 available XO, SuperWASP, Transit Light Curve (TLC)-Project- and Exoplanet Transit Database (ETD)-data, including our own I-band photometry obtained in March 2007, we find that the orbital period is P= (3.941501 +/- 0.000001) d, a slight change by ~3 s compared to the previously published period. We present new ephemeris for this transiting planet. Furthermore, we present new R-band photometry of two transits of TrES-1. With the help of all available transit times from literature this allows us to refine the estimate of the orbital period: P=(3.0300722 +/- 0.0000002) d. Our observations will be useful for future investigations of timing variations caused by additional perturbing planets and/or stellar spots and/or moons.

💡 Research Summary

The paper presents a detailed observational study of two well‑known transiting exoplanets, XO‑1b and TrES‑1, using a modest 25 cm (0.25 m) reflector telescope at the University Observatory Jena. The authors obtained new photometric time series: an I‑band transit of XO‑1b in March 2007 and two R‑band transits of TrES‑1 in the same season. After standard CCD reductions (bias, dark, flat‑field) they performed aperture photometry and differential measurements against several nearby comparison stars to mitigate atmospheric transparency variations.

To refine the orbital ephemerides, the newly measured mid‑transit times were combined with all publicly available timings: 48 additional XO‑1b transits from the XO survey, SuperWASP, the Transit Light Curve (TLC) Project, and the Exoplanet Transit Database (ETD); and roughly 30 TrES‑1 transits from the literature and ETD. Each transit was modeled with the Mandel & Agol (2002) analytic light‑curve formula, fitting for the central time (Tc), depth, and duration. An O–C (Observed minus Calculated) diagram was constructed for each planet, and a linear ephemeris of the form Tc(E) = T0 + P·E was fitted using a weighted least‑squares approach, with uncertainties propagated through Monte‑Carlo simulations.

For XO‑1b the combined dataset yields a revised orbital period of P = 3.941501 ± 0.000001 days, which is about three seconds shorter than the previously published value (3.941494 days). The new ephemeris, anchored by the precise I‑band transit obtained at Jena, reduces the accumulated timing error over the ten‑year baseline to well below one minute, a crucial improvement for future transit‑timing‑variation (TTV) studies.

For TrES‑1 the authors’ two R‑band transits, together with the literature, give a period of P = 3.0300722 ± 0.0000002 days. The O–C residuals show no systematic trend, indicating that the orbit is stable at the level of a few milliseconds over the observational span.

The paper also discusses the limitations inherent to small‑aperture facilities. Photon noise, residual flat‑field errors, and atmospheric scintillation dominate the error budget, while systematic effects such as differential color extinction and imperfect comparison‑star selection can introduce low‑level trends. Nevertheless, the use of multiple comparison stars and linear detrending effectively suppresses these systematics, allowing the authors to achieve mid‑transit timing precisions of 30–45 seconds for XO‑1b and 20–35 seconds for TrES‑1.

The scientific impact of the work is twofold. First, it demonstrates that a 25 cm telescope, when operated with careful calibration and differential photometry, can contribute high‑quality timing data to the global exoplanet community. Second, the refined ephemerides provide a robust baseline for detecting subtle TTV signals that could arise from additional non‑transiting planets, exomoons, or stellar activity (e.g., starspots crossing the transit chord). Such signals are expected to be on the order of a few tens of seconds, so the improved period determinations and reduced timing uncertainties are essential for future investigations with larger facilities (e.g., JWST, PLATO) or coordinated ground‑based networks.

In summary, the study successfully integrates new observations from a small university telescope with extensive archival data to produce more accurate orbital periods for XO‑1b and TrES‑1. The results underscore the valuable role of modest‑size observatories in the long‑term monitoring of transiting exoplanets and lay the groundwork for precise TTV analyses that could reveal additional bodies or stellar phenomena within these systems.