Stellar Spin-Orbit Misalignment in a Multiplanet System (1310.4503v2)

Abstract: Stars hosting hot Jupiters are often observed to have high obliquities, whereas stars with multiple co-planar planets have been seen to have low obliquities. This has been interpreted as evidence that hot-Jupiter formation is linked to dynamical disruption, as opposed to planet migration through a protoplanetary disk. We used asteroseismology to measure a large obliquity for Kepler-56, a red giant star hosting two transiting co-planar planets. These observations show that spin-orbit misalignments are not confined to hot-Jupiter systems. Misalignments in a broader class of systems had been predicted as a consequence of torques from wide-orbiting companions, and indeed radial-velocity measurements revealed a third companion in a wide orbit in the Kepler-56 system.

• The paper demonstrates that asteroseismology reveals a ~45° spin-orbit misalignment in Kepler-56, challenging traditional views focused on hot Jupiter systems.
• It employs radial velocity and transit observations to identify a distant massive companion exerting significant torque on the inner planetary orbits.
• Dynamical simulations confirm that low mutual inclinations in the inner planets are crucial for long-term orbital stability despite the complex gravitational interactions.

Stellar Spin-Orbit Misalignment in a Multiplanet System

The research presented in the paper explores the dynamics and characteristics of the Kepler-56 system, an evolved star hosting multiple planets, and offers critical insight into stellar spin-orbit misalignment. Specifically, it examines the obliquity in star-planet systems traditionally observed with such misalignments, typically associated with hot Jupiters. This paper extends the understanding of spin-orbit misalignment to multiplanet systems by employing asteroseismology and radial velocity measurements, marking a significant contribution to the field of astrophysics.

Key Findings

1. Asteroseismic Measurements: The paper utilizes oscillations in the red giant star Kepler-56 detected by the Kepler mission to determine its stellar properties. These oscillations deliver precise stellar parameters, revealing Kepler-56's mass and size to be significantly greater than the Sun's, vital for understanding the mechanics of its planetary system.
2. Spin-Orbit Misalignment: A stark revelation from the asteroseismic measurements was the inclination of Kepler-56's spin axis. The analysis led to measurements indicating that the star's rotational axis is misaligned with the planetary orbits by approximately 45 degrees, challenging existing paradigms that primarily restrict spin-orbit misalignments to systems hosting hot Jupiters.
3. Orbit Dynamics and System Architecture: Kepler-56's planetary system includes two closely orbiting transiting planets exhibiting a near 2:1 resonance. The meticulously derived orbital inclinations and radial velocity data identified a third, distant massive companion, potentially a brown dwarf or a giant planet. This third body likely introduces a substantial torque affecting the orbital plane of the inner planets, thereby contributing to the observed obliquity of the system.
4. Dynamical Stability Considerations: Through extensive dynamical simulations, the paper found that configurations with high mutual inclinations between the inner planets are dynamically unstable, leading to rapid catastrophic outcomes. Conversely, it demonstrated that present low mutual inclination orbits are stable over long timescales, aligning well with observational data.
5. Implications of Findings: Several plausible scenarios were examined to account for the observed misalignment, including dynamical mechanisms such as the Kozai effect from the outward companion's gravitational perturbations and planet-planet scattering. These theories emphasize the potential complexity and variability in planet-star orientation beyond traditional migration models.

Theoretical and Practical Implications

The findings of the paper have substantial implications for the theory of planetary system formation and evolution, suggesting that spin-orbit misalignment can occur in systems beyond the previously typical hot Jupiters. This perspective necessitates reevaluation of existing models of planetary migration and the impact of distant companions in planetary systems. Moreover, the research sets a precedent for using asteroseismology alongside radial velocity and planetary transit data, showcasing a powerful multimodal approach to characterizing exoplanetary systems.

Future Directions

Building on these findings, future research should aim to explore additional multiplanet systems with similar observational techniques to assess the prevalence of spin-orbit misalignment across various stellar types and evolutionary stages. Furthermore, more longitudinal data and enhanced simulations may provide a deeper understanding of the long-term dynamical behaviors of such celestial arrangements, potentially shaping the future discourse on planetary formation and dynamical evolution theories.

In conclusion, this paper marks a notable advancement in astrophysical research, indicating that spin-orbit misalignment is a more widespread phenomenon than previously recognized and warrants further investigation across an array of exoplanetary systems.