Orbits of planets
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Understanding the Orbits of Planets: Insights from Recent Research
Orbital Architecture of Giant Planets in the Solar System
The formation theories of giant planets suggest that they initially formed on circular and coplanar orbits. However, the current eccentricities of Jupiter, Saturn, and Uranus are 6%, 9%, and 8%, respectively, with inclinations of up to 2 degrees for Saturn, Uranus, and Neptune relative to Jupiter's mean orbital plane. This discrepancy can be explained by the crossing of the 1:2 orbital resonance between Jupiter and Saturn during their migration, influenced by interactions with a disk of planetesimals. This model successfully reproduces the current semimajor axes, eccentricities, and mutual inclinations of the giant planets1.
Eccentricity in Small Planet Systems
The eccentricities of small Kepler planets have been studied using asteroseismology and transit light-curve analysis. Systems with a single transiting planet exhibit a different eccentricity distribution compared to those with multiple transiting planets. Single-transiting systems often have moderate eccentricities, which may be self-excited in closely packed planetary systems or influenced by long-period giant companion planets. There is no significant correlation between eccentricity and stellar metallicity or the presence of companion stars2.
Neighboring Orbits and Dissimilar Densities: The Case of Kepler-36
The Kepler-36 system challenges traditional planet formation theories by hosting two planets with vastly different densities but similar orbital periods. One planet is rocky and Earth-like, while the other resembles Neptune. This finding suggests that planets' orbits can change significantly after formation, leading to close-in giant planets around other stars and violating the typical orbit-composition pattern observed in our solar system3.
Migration History of Giant Planets
A comprehensive study of 231 transiting giant planets reveals that their orbital parameters are influenced by tides raised by their host stars. High-eccentricity migration (HEM) scenarios, such as planet-planet scattering and Kozai-Lidov perturbations, play a significant role in shaping these orbits. The distribution of semimajor axes relative to the Roche limit supports the HEM theory, although some planets may have migrated through disc-planet interactions4.
Direct Imaging of Exoplanetary Systems
Direct imaging techniques have successfully revealed multiple planets orbiting the star HR 8799, with separations of 24, 38, and 68 astronomical units. These planets, with masses between 5 and 13 times that of Jupiter, exhibit counterclockwise orbital motion. This system resembles a scaled-up version of the outer solar system, providing valuable insights into the dynamics of wide-orbit planets5.
Eccentric Orbits of Close-in Giant Planets Around Evolved Stars
Recent radial velocity measurements indicate that close-in giant planets orbiting evolved stars tend to have more eccentric orbits compared to those around main sequence stars. This suggests that these planets undergo a transient, moderately eccentric phase where their orbits shrink faster than they circularize due to tidal interactions with their evolved host stars6.
Co-orbital Configurations and Stability
Studies on the stability of two planets in co-orbital configurations reveal various stable regions and periodic orbits, including quasi-satellite orbits and classical Lagrangian points. New asymmetric periodic solutions have been identified, which vary with the mass ratio and orbital eccentricities of the planets. These findings enhance our understanding of the dynamics and stability of co-orbital planetary systems7.
Dynamics of Close Planetary Systems
The dynamics of two close planets orbiting a star have been explored, showing that systems with initially circular orbits can remain Hill stable if their fractional orbital separation exceeds a certain threshold. This stability condition is supported by numerical integrations and has implications for planetary accretion and the stability of current planetary systems8.
Resonant Orbits in Multi-Planet Systems
The TOI-178 system, initially observed as having three planets, was found to host six planets with five in a resonant chain. These planets, ranging in size between Earth and Neptune, have short orbits with high surface temperatures. Such resonant chains provide valuable information on the formation and migration processes of planetary systems9.
Orbital Dynamics of Circumbinary Planets
The dynamics of circumbinary planets, which orbit around two stars, have been studied through numerical simulations. These studies reveal conditions for orbital libration versus circulation and the stationary inclination states for both prograde and retrograde orbits. The results align well with analytic models and have significant implications for the formation and evolution of circumbinary planets10.
Conclusion
Recent research has significantly advanced our understanding of planetary orbits, from the eccentricities of small planets to the complex dynamics of multi-planet and circumbinary systems. These studies highlight the diverse mechanisms influencing planetary orbits, including resonance crossings, tidal interactions, and migration processes, providing a comprehensive view of planetary dynamics across different types of systems.
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Most relevant research papers on this topic
Origin of the orbital architecture of the giant planets of the Solar System
The orbital architecture of the giant planets in the Solar System could have evolved from initial quasi-circular, coplanar orbits if Jupiter and Saturn crossed their 1:2 orbital resonance, interacting with a disk of planetesimals.
Highly CitedThe Orbital Eccentricity of Small Planet Systems
Small planet systems with only one transiting planet have a different orbital eccentricity distribution than systems with multiple transiting planets, potentially reflecting differences in formation pathways.
Highly CitedKepler-36: A Pair of Planets with Neighboring Orbits and Dissimilar Densities
Two planets with different densities orbiting the same star, one a rocky "super-Earth" and the other akin to Neptune, violate the orbit-composition pattern and are 20 times more closely spaced than any adjacent pair in the solar system.
Highly CitedThe GAPS Programme with HARPS-N at TNG - XIV. Investigating giant planet migration history via improved eccentricity and mass determination for 231 transiting planets
The orbital parameters of transiting giant planets in non-compact systems are shaped by tides raised by their host stars, with some planets potentially migrating through disc-planet interactions instead of high-eccentricity migration.
Highly CitedDirect Imaging of Multiple Planets Orbiting the Star HR 8799
Three planets orbiting the star HR 8799 have been identified, with masses between 5 and 13 times that of Jupiter, indicating a scaled-up version of our solar system.
Highly CitedDo Close-in Giant Planets Orbiting Evolved Stars Prefer Eccentric Orbits?
Close-in giant planets orbiting evolved stars tend to have more eccentric orbits than those around main sequence stars, possibly due to tides raised on evolved host stars.
Dynamics of two planets in co-orbital motion
Asymmetric periodic solutions can be found for two planets in co-orbital motion, with stability preserving as long as mass variations are adiabatic.
Dynamics of Systems of Two Close Planets
The dynamics of two close planets can be stable against close approaches if the fractional orbital separation > 2.4(1 + 2) 1/3, with implications for planetary accretion, the current Solar System, and the pulsar planet system PSR 1257 + 12.
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