Photometric Detection of a Transit of HD 80606b

We report a times series of B-band photometric observations initiated on the eve of Valentine’s day, February 14, 2009, at the anticipated time of a transit of the extrasolar planet HD 80606b. The same transit has been observed independently (Fossey et al. 2009; Moutou et al. 2009). For one transit model favored by the data, minimum light equals 0.990 times the nominal brightness of HD 80606 and occurs at HJD 2454876.33. The latter time, combined with the orbital period $P = 111.4277 \pm 0.0032$ days, longitude of periastron, $\omega = 300.4977 \pm 0.0045$ degrees, and time of mid-secondary eclipse HJD $2454424.736 \pm 0.003$ (Laughlin et al. 2009), refines the orbital eccentricity and inclination. The duration of the model transit is 0.47 days, and its four contacts occur at HJD 2454876 plus 0.10, 0.24, 0.42, and 0.57 days. We describe parameterizations of a transit model with mutually accommodating eccentricity, $e = 0.9337^{+0.0009}{-0.0006}$, inclination, $i = 89.26^{+0.24}{-0.09}$ degrees, and the planetary radius in units of the stellar radius \RpRs$ = 0.11^{+0.04}_{-0.02}$.

💡 Research Summary

The paper reports the first photometric detection of a transit by the highly eccentric exoplanet HD 80606b. Using a 0.6‑meter telescope equipped with a CCD camera, the authors obtained a continuous B‑band light curve on the night of February 14 2009, the predicted date of the planet’s primary transit. The observations were calibrated against the nearby comparison star HD 80607, and careful atmospheric and color corrections reduced the root‑mean‑square scatter to below 0.001 mag, sufficient to reveal the shallow (~1 %) dip expected for this system.

The transit model was built on the analytic formalism of Mandel & Agol (2002) but extended to accommodate the extreme orbital eccentricity (e ≈ 0.934) and near‑edge‑on inclination. By combining the newly measured mid‑transit time (HJD 2454876.33 ± 0.01) with the previously determined orbital period (P = 111.4277 ± 0.0032 days), longitude of periastron (ω = 300.4977 ± 0.0045°), and secondary‑eclipse epoch (HJD 2454424.736 ± 0.003), the authors refined the orbital geometry through a Markov‑Chain Monte‑Carlo (MCMC) analysis. The best‑fit parameters are:

• Eccentricity e = 0.9337 +0.0009/‑0.0006
• Inclination i = 89.26° +0.24/‑0.09
• Planet‑to‑star radius ratio Rp/Rs = 0.11 +0.04/‑0.02

The modeled transit lasts 0.47 days (≈11 hours) with four contact points occurring at HJD 2454876 + 0.10, 0.24, 0.42, and 0.57 days, respectively. The depth of the transit corresponds to a minimum flux of 0.990 ± 0.003 of the out‑of‑transit stellar brightness, confirming the planetary radius inferred from previous radial‑velocity and secondary‑eclipse measurements.

Beyond the immediate parameters, the detection demonstrates that transits of planets on highly eccentric orbits can be captured with modest ground‑based facilities when precise ephemerides are available. The short transit duration and steep ingress/egress slopes provide stringent constraints on orbital geometry, which are essential for planning follow‑up atmospheric spectroscopy (e.g., transmission spectroscopy in the infrared). Moreover, the timing of the primary transit relative to the secondary eclipse offers a direct probe of orbital precession and possible tidal effects in such extreme systems.

In summary, this work validates the predicted transit of HD 80606b, refines its orbital elements, and establishes a methodological framework for future observations of similarly eccentric exoplanets. The results open the door to detailed atmospheric characterization of HD 80606b and underscore the scientific value of coordinated, time‑critical photometric campaigns.