The retrograde orbit of the HAT-P-6b exoplanet (1101.5009v1)

Abstract: We observed with the SOPHIE spectrograph (OHP, France) the transit of the HAT-P-6b exoplanet across its host star. The resulting stellar radial velocities display the Rossiter-McLaughlin anomaly and reveal a retrograde orbit: the planetary orbital spin and the stellar rotational spin point towards approximately opposite directions. A fit to the anomaly measures a sky-projected angle lambda = 166 +/- 10 degrees between these two spin axes. All seven known retrograde planets are hot jupiters with masses M_p < 3 M_Jup. About two thirds of the planets in this mass range however are prograde and aligned (lambda ~ 0). By contrast, most of the more massive planets (M_p > 4 M_Jup) are prograde but misaligned. Different mechanisms may therefore be responsible for planetary obliquities above and below ~3.5 M_Jup.

Analysis of the Retrograde Orbit of the \cibleb\ Exoplanet

The paper presents comprehensive spectroscopic observations of the \cibleb\ exoplanet and highlights its retrograde orbital nature relative to its host star, HAT-P-6. Conducted using the \sophie\ spectrograph, the research examines the Rossiter-McLaughlin (RM) effect to measure the sky-projected angle between the planetary orbital spin and the stellar rotational spin, a method pivotal for assessing spin-orbit alignment in exoplanet systems.

Key Findings

1. Retrograde Orbit Identification: The research identifies a retrograde orbit for \cibleb\ with a sky-projected angle $\lambda = 166^{\circ} \pm 10^{\circ}$. This indicates a significant deviation from the typical prograde alignment observed in many exoplanetary systems.
2. Technique Utilization and Accuracy: The utilization of high-precision radial velocity measurements facilitated by the \sophie\ spectrograph solidified the findings. Additionally, the RM effect was instrumental in detecting the misalignment, providing a reliable technique for orbit characterization.
3. Planetary Mass Influence: The findings contribute to existing literature suggesting the impact of planetary mass on spin-orbit alignment. Notably, \cibleb\ joins other mass-comparable exoplanets exhibiting similar retrograde or polar configurations, strengthening the argument for mass-dependent obliquity mechanisms.

Implications

The detection of a retrograde orbit has profound implications for theories surrounding planet formation and migration. Traditionally, aligned orbits were thought to result from migration through a protoplanetary disk. The presence of misaligned or retrograde orbits challenges these models, suggesting that alternatives like planet-planet scattering, Kozai migration, or tidal interactions may be at play.

Furthermore, the correlation between stellar characteristics, particularly effective temperature, and orbital alignment raises questions about the influence of host star properties on planetary orbits. This trend of misalignment in systems with hotter stars presents a fertile ground for further exploration into how stellar evolution might affect planetary dynamics.

Future Directions

Future research should prioritize increasing the sample size of planets with determined obliquities to refine statistical models correlating planetary mass with spin-orbit alignment characteristics. Enhancing the observational techniques to reduce uncertainties in $i_s$ and $\psi$ will also be crucial for precise measurements of true spin-orbit angles, enabling a more comprehensive understanding of these systems.

In summary, the documentation of a retrograde orbit in the \cibleb\ system enriches the current understanding of exoplanetary dynamics and suggests directions for both theoretical and observational advancements in the paper of planetary system evolution.