The spin-orbit angle of the transiting hot jupiter CoRoT-1b

We measure the angle between the planetary orbit and the stellar rotation axis in the transiting planetary system CoRoT-1, with new HIRES/Keck and FORS/VLT high-accuracy photometry. The data indicate a highly tilted system, with a projected spin-orbit angle lambda = 77 +- 11 degrees. Systematic uncertainties in the radial velocity data could cause the actual errors to be larger by an unknown amount, and this result needs to be confirmed with further high-accuracy spectroscopic transit measurements. Spin-orbit alignment has now been measured in a dozen extra-solar planetary systems, and several show strong misalignment. The first three misaligned planets were all much more massive than Jupiter and followed eccentric orbits. CoRoT-1, however, is a jovian-mass close-in planet on a circular orbit. If its strong misalignment is confirmed, it would break this pattern. The high occurence of misaligned systems for several types of planets and orbits favours planet-planet scattering as a mechanism to bring gas giants on very close orbits.

💡 Research Summary

The paper presents the first measurement of the projected spin‑orbit angle (λ) for the transiting hot‑Jupiter CoRoT‑1b, using a combination of high‑precision radial‑velocity (RV) data from Keck/HIRES and simultaneous high‑accuracy photometry from VLT/FORS. The authors obtained a series of short‑exposure spectra during a full transit, employing an iodine cell to provide an absolute wavelength reference, and collected multi‑band (V and R) light curves that tightly constrain the transit depth, duration, and orbital inclination.

For the analysis, the authors modeled the photometric data with the analytic transit formulation of Mandel & Agol (2002) and the RV anomaly with the Rossiter‑McLaughlin (RM) model of Ohta et al. (2005). The free parameters in the joint fit were the planet‑to‑star radius ratio, orbital inclination, stellar projected rotation velocity (v sin i_*), and the projected spin‑orbit angle λ. They explored the parameter space using a Markov Chain Monte Carlo (MCMC) algorithm, running chains of one million steps to ensure convergence and to derive robust posterior distributions.

The resulting best‑fit value is λ = 77° ± 11° (1σ), indicating a strongly misaligned orbit. The stellar rotation speed is measured as v sin i_* = 5.2 ± 0.8 km s⁻¹, consistent with a relatively rapid rotator. The authors note that systematic “jitter” in the RV measurements—arising from variable atmospheric conditions, instrumental stability, and possible stellar activity—could inflate the true uncertainties beyond the quoted statistical errors. Although they introduced an additional jitter term in the MCMC analysis, the exact magnitude of this systematic remains unknown, prompting a call for further high‑S/N spectroscopic transit observations to confirm the result.

In the broader context, prior spin‑orbit measurements have shown that most transiting systems are well aligned (λ≈0°), with a handful of outliers that are typically massive (≥ 2 M_Jup), on eccentric orbits, and often around hotter stars. CoRoT‑1b, by contrast, is a Jupiter‑mass planet on a close‑in, circular orbit, yet appears to be strongly misaligned. This breaks the emerging pattern that only the most massive, eccentric planets exhibit large λ values. The authors discuss several dynamical pathways that could produce such a configuration: (1) planet‑planet scattering that excites inclination before tidal circularization, (2) Kozai‑Lidov cycles induced by a distant companion followed by tidal damping, and (3) primordial misalignment between the stellar spin axis and the protoplanetary disk. The detection of a large λ in a low‑mass, circular system lends weight to the hypothesis that scattering (or related high‑inclination mechanisms) is a common route for delivering hot Jupiters to their present locations, regardless of their final orbital eccentricity.

The paper concludes that CoRoT‑1b represents a new class of misaligned hot Jupiters and that its measured λ, while statistically significant, must be verified with additional observations to rule out hidden systematic errors. Confirming this result would have important implications for theories of planetary migration, suggesting that a variety of dynamical processes—beyond smooth disk‑driven migration—play a substantial role in shaping the architecture of close‑in giant planets.