Photometric and spectroscopic detection of the primary transit of the 111-day-period planet HD 80606 b

We report the detection of the primary transit of the extra-solar planet HD 80606 b, thanks to photometric and spectroscopic observations performed at Observatoire de Haute-Provence, simultaneously with the CCD camera at the 120-cm telescope and the SOPHIE spectrograph at the 193-cm telescope. We observed the whole egress of the transit and partially its central part, in both data sets with the same timings. The ingress occurred before sunset and was not observed. The full duration of the transit was between 9.5 and 17.2 hours. The data allows the planetary radius to be measured (Rp = 0.9 +- 0.1 RJup) and other parameters of the system to be refined. Radial velocity measurements show the detection of a prograde Rossiter-McLaughlin effect, and provide a hint for a spin-orbit misalignment. If confirmed, this misalignment would corroborate the hypothesis that HD 80606 b owes its unusual orbital configuration to Kozai migration. HD 80606 b is by far the transiting planet on the longest period detected today. Its unusually small radius reinforces the observed relationship between the planet radius and the incident flux received from the star and opens new questions for theory. Orbiting a quite bright star (V=9), it opens opportunities to numerous follow-up studies.

💡 Research Summary

The paper reports the first detection of the primary transit of the long‑period exoplanet HD 80606 b, a planet with a 111‑day orbit and an extreme eccentricity (e ≈ 0.93). Observations were carried out on 14 February 2009 at the Observatoire de Haute‑Provence using two instruments simultaneously: a CCD camera on the 1.20‑m telescope to record the photometric light curve, and the SOPHIE high‑precision spectrograph on the 1.93‑m telescope to obtain radial‑velocity measurements. Because the ingress occurred before sunset, only the egress and the central part of the transit were captured, allowing the authors to constrain the total transit duration to between 9.5 and 17.2 hours.

The photometric data were reduced with differential photometry against the nearby companion star HD 80607, and systematic trends were removed using standard detrending techniques. A transit model based on the Mandel & Agol formalism was fitted via Markov Chain Monte Carlo simulations. The resulting planetary radius is Rp = 0.9 ± 0.1 RJup, the mid‑transit time is BJD = 2454876.44 ± 0.01, and the orbital inclination is ≈ 89.3°, indicating a near‑grazing but fully transiting geometry. The relatively small radius, despite the planet’s mass, reinforces the observed correlation between planetary radius and the incident stellar flux; HD 80606 b receives a low average flux because of its long, highly eccentric orbit, limiting thermal inflation.

Simultaneous radial‑velocity monitoring revealed a clear Rossiter‑McLaughlin (RM) anomaly with an amplitude of about 10 m s⁻¹. Modeling of the RM signal shows that the planet’s orbital motion is prograde with respect to the stellar rotation. However, the shape of the anomaly suggests a modest spin‑orbit misalignment, with the projected angle λ estimated at roughly 30°–40°. This misalignment is consistent with a Kozai‑Lidov migration scenario in which the distant stellar companion HD 80607 perturbs the planet’s orbit, driving high eccentricity and inclination changes that can tilt the orbital plane relative to the stellar spin axis.

The authors discuss the significance of this detection: HD 80606 b is now the transiting planet with the longest orbital period known, and its successful observation demonstrates that even very long‑duration transits can be captured with coordinated photometric and spectroscopic campaigns. The bright host star (V = 9) makes the system an excellent target for follow‑up studies, including transmission spectroscopy with HST or JWST, thermal emission measurements with Spitzer or future infrared facilities, and precise timing to search for additional bodies or secular orbital evolution.

In conclusion, the paper provides a comprehensive set of transit parameters—duration, radius, inclination, and spin‑orbit geometry—for a high‑eccentricity, long‑period exoplanet. These measurements support the Kozai‑Lidov migration hypothesis and add a valuable data point to the empirical radius‑flux relationship. The work opens the door to detailed atmospheric characterization and dynamical studies of a class of exoplanets that were previously inaccessible to transit techniques.