The Occurrence and Architecture of Exoplanetary Systems (1410.4199v4)

Abstract: The basic geometry of the Solar System -- the shapes, spacings, and orientations of the planetary orbits -- has long been a subject of fascination as well as inspiration for planet formation theories. For exoplanetary systems, those same properties have only recently come into focus. Here we review our current knowledge of the occurrence of planets around other stars, their orbital distances and eccentricities, the orbital spacings and mutual inclinations in multiplanet systems, the orientation of the host star's rotation axis, and the properties of planets in binary-star systems.

• The paper reveals that nearly 10% of Sun-like stars host giant planets while 50% exhibit compact, short-period planetary systems.
• The authors compare exoplanet orbital properties and alignments to those of the Solar System, highlighting both similarities and key differences.
• They propose that post-formation dynamics, such as migration and scattering events, are critical in shaping diverse planetary architectures.

An Overview of Exoplanetary System Occurrence and Architecture

The paper "The Occurrence and Architecture of Exoplanetary Systems" by Joshua N. Winn and Daniel C. Fabrycky provides a comprehensive review of the current understanding of the geometric properties of exoplanetary systems. It juxtaposes these properties with those of the Solar System, contributing significantly to the discourse on planet formation theories.

Summary of Exoplanetary Properties

The paper begins by outlining centuries of astronomical observations of the Solar System's structure: nearly circular planetary orbits, low mutual inclinations, and low stellar obliquity. These properties have historically supported theories of planet formation within a flat, protoplanetary disk.

Occurrence Rates

The authors present data indicating that exoplanets are prevalent, with studies using techniques such as Doppler spectroscopy, transits, and microlensing revealing a wide variety of planetary configurations. Some key findings include:

• Roughly 10% of Sun-like stars host giant planets within a few astronomical units, while about 50% have smaller, compactly arranged planets with periods under a year.
• Planet occurrence is strongly linked to host star metallicity, particularly for giant planets.
• Systems with multiple planets typically exhibit near-circular orbits and low mutual inclinations.

Architectural Insights

In examining exoplanetary architectures, the authors highlight the diverse dynamical landscapes of observed systems:

• Orbital Eccentricity: Many giant exoplanets exhibit high eccentricities, suggesting interactions or scattering events in their evolutionary past. In contrast, planets in compact systems show lower eccentricities.
• Mutual Inclination and Period Ratios: There is evidence of low mutual inclinations in compact systems of smaller planets, aligning with the largely coplanar organization of Solar System planets. Notably, giant planets are more frequently found in resonances than their smaller counterparts.

Theoretical Implications

The paper underscores the limitations of early planet-formation theories, which largely anticipated low eccentricities and inclinations. The detection of high eccentricities and various orbital misalignments has led to an expanded focus on post-formation dynamics, such as planet-disk interactions, migration, and planet-planet scattering, to explain these features. Furthermore, the apparent scarcity of planets in close binary systems suggests significant dynamical interactions affecting planetary orbits.

Stellar Rotation and Obliquity

One intriguing aspect covered is the stellar rotation and obliquity in planetary systems. Observed spin-orbit misalignments, particularly among hot Jupiters around hotter stars, challenge the presumed alignment shared by the Sun and the Solar System planets. This points to complex angular momentum interactions and evolutionary histories within these systems.

Observational Outlook

The prospect of forthcoming observational advancements, such as those from missions like Gaia and improved microlensing networks, promises an enriched understanding of the exoplanetary architectures. Future studies aim to bridge the gap between statistical occurrences and detailed planetary characteristics, unveiling further insights into the fundamental processes shaping planetary systems.

Conclusion

The paper elegantly contrasts the architectural properties of exoplanetary systems with those of our Solar System, underscoring both commonalities and intriguing disparities. These insights are pivotal in refining our understanding of planet formation and evolution, guiding theoretical models towards a more comprehensive depiction of planetary system genesis, diversity, and dynamical evolution.