[Paper Review] Observation of the full 12-hour-long transit of the exoplanet HD80606b. Warm-Spitzer photometry and SOPHIE spectroscopy

G. Hébrard, J. -M. Désert+25 more|arXiv (Cornell University)|Apr 6, 2010

Stellar, planetary, and galactic studies85 references81 citations

TL;DR

This study presents the first complete 12-hour transit observation of the highly eccentric exoplanet HD 80606b using simultaneous warm-Spitzer infrared photometry and ground-based SOPHIE radial velocity spectroscopy. The data refine the planet-to-star radius ratio to 0.1001 ± 0.0006, confirm a significant spin-orbit misalignment (λ = 42° ± 8°), and reveal possible transit timing variations and stellar spot features, suggesting distinct evolutionary pathways for massive planets compared to Jupiter-mass planets.

ABSTRACT

We present new observations of a transit of the 111-day-period exoplanet HD80606b. Using the Spitzer Space Telescope and its IRAC camera on the post-cryogenic mission, we performed a 19-hour-long photometric observation of HD80606 that covers the full transit of 13-14 January 2010. We complement this photometric data by new spectroscopic observations that we simultaneously performed with SOPHIE at Haute-Provence Observatory. This provides radial velocity measurements of the first half of the transit that was previously uncovered with spectroscopy. This new data set allows the parameters of this singular planetary system to be significantly refined. We obtained a planet-to-star radius ratio R_p/R_* = 0.1001 +/- 0.0006 that is slightly lower than the one measured from previous ground observations. We detected a feature in the Spitzer light curve that could be due to a stellar spot. We also found a transit timing about 20 minutes earlier than the ephemeris prediction; this could be caused by actual TTVs due to an additional body in the system or by underestimated systematic uncertainties. The sky-projected angle between the spin-axis of HD80606 and the normal to the planetary orbital plane is found to be lambda = 42 +/- 8 degrees thanks to the fit of the Rossiter-McLaughlin anomaly. This allows scenarios with aligned spin-orbit to be definitively rejected. Over the twenty planetary systems with measured spin-orbit angles, a few of them are misaligned; this is probably the signature of two different evolution scenarios for misaligned and aligned systems, depending if they experienced or not gravitational interaction with a third body. As in the case of HD80606b, most of the planetary systems including a massive planet are tilted; this could be the signature of a separate evolution scenario for massive planets in comparison with Jupiter-mass planets.

Motivation & Objective

• To observe the complete 12-hour transit of HD 80606b, which is exceptionally long due to its high orbital eccentricity (e = 0.9).
• To improve the precision of planetary and stellar parameters through simultaneous high-accuracy space-based photometry and ground-based radial velocity measurements.
• To investigate the spin-orbit alignment of the system and assess whether the observed misalignment is consistent with dynamical migration scenarios.
• To search for anomalies such as transit timing variations or stellar spots that could indicate additional bodies or stellar activity.

Proposed method

• Conducted a 19-hour warm-Spitzer IRAC photometric observation covering the full 12-hour transit of HD 80606b on January 13–14, 2010.
• Performed simultaneous radial velocity measurements using the SOPHIE spectrograph at the Haute-Provence Observatory during the first half of the transit.
• Applied combined photometric and spectroscopic modeling to refine planetary and stellar parameters, including the planet-to-star radius ratio and orbital inclination.
• Fitted the Rossiter-McLaughlin anomaly to measure the sky-projected spin-orbit angle λ.
• Used systematic error modeling to assess potential biases in transit timing and light curve shape.
• Compared infrared and optical radius measurements to investigate discrepancies possibly due to systematic errors or astrophysical effects.

Experimental results

Research questions

• RQ1 What is the true value of the planet-to-star radius ratio R_p/R_* for HD 80606b, and how does it compare between infrared and optical observations?
• RQ2 Is the observed transit timing consistent with the ephemeris, or are there signs of transit timing variations indicating a third body?
• RQ3 What is the true spin-orbit angle λ, and does it confirm or reject the hypothesis of aligned spin-orbit systems for massive planets?
• RQ4 Are there detectable stellar features such as spots influencing the transit light curve?
• RQ5 Do the observed discrepancies between optical and infrared radii stem from systematic errors or physical effects?

Key findings

• The planet-to-star radius ratio was refined to R_p/R_* = 0.1001 ± 0.0006, which is more precise but slightly lower than previous optical measurements.
• No astrophysical explanation was found for the discrepancy between optical and infrared radii; systematic uncertainties in ground-based data are considered the most likely cause.
• A possible stellar spot feature was detected in the Spitzer light curve, suggesting surface inhomogeneities on the otherwise inactive star.
• The observed transit occurred approximately 20 minutes earlier than predicted by the ephemeris, indicating potential transit timing variations that may point to an additional perturbing body.
• The spin-orbit angle was measured as λ = 42° ± 8°, confirming a significant misalignment and ruling out aligned spin-orbit configurations.
• The results support a scenario in which massive planets like HD 80606b undergo different dynamical evolution—likely involving strong gravitational interactions—compared to lower-mass Jupiter-like planets.