Order of planets by size
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Order of Planets by Size
Understanding the order of planets by size is crucial for comprehending planetary formation and evolution. This article synthesizes recent research findings on the relative sizes of planets, particularly those discovered by the Kepler mission.
Planet Size Distribution and Detection Biases
The Kepler mission has significantly contributed to our understanding of planetary sizes. However, detection biases play a crucial role in the observed size distribution of planets. The transit signal-to-noise ratio (S/N) used in Kepler detections varies with stellar properties and orbital periods, leading to apparent correlations in planet sizes and their ordering within a system. When these biases are accounted for, the physical properties of Kepler planets appear largely independent of their siblings and parent star, suggesting that dynamical evolution has erased initial formation conditions .
Size Ordering in Multi-Planet Systems
Studies have shown that in multi-planet systems, there is a tendency for larger planets to be located further from their star, particularly when at least one planet in the pair is Neptune-sized or larger. Specifically, 68% of such planet pairs have the inner planet smaller than the outer planet. However, this size-location correlation is not observed for pairs of planets smaller than Neptune . This indicates that the size ordering effect is more pronounced in systems with larger planets.
Entropy and Size-Ordering
The concept of entropy has been applied to understand the size-ordering of planets in multi-planet systems. Kepler systems exhibit a significant deficit in entropy compared to randomly generated populations, suggesting that these systems retain some memory of their initial formation conditions. This finding is robust even when accounting for potentially missing planets, indicating that the observed size-ordering is not entirely random and may reflect underlying formation processes .
Planet Size Distribution and Evolution
The distribution of planet sizes reveals important details about their formation and evolution. Precise measurements from Gaia parallaxes and Kepler photometry have identified a bimodal distribution of planet sizes, separating rocky super-Earths from gas-dominated sub-Neptunes. This distribution is influenced by factors such as a planet's orbital distance and its host star's mass, which are related to the planet's X-ray/UV irradiation history. For lower-mass stars, the distribution shifts to smaller sizes, supporting the view that photoevaporation of low-density atmospheres is a dominant factor in shaping planet sizes .
Composition of Intermediate-Size Planets
Planets with radii between 2 and 4 Earth radii (R⊕) have compositions that are still debated. These planets could either be gas dwarfs with rocky cores and H2-rich envelopes or water worlds with significant amounts of H2O-dominated fluid/ice. Monte Carlo simulations suggest that many of these intermediate-size planets are likely water worlds, with their radii matching the second peak of the exoplanet radius bimodal distribution .
Conclusion
The order of planets by size is influenced by a combination of detection biases, dynamical evolution, and intrinsic formation processes. Larger planets tend to be located further from their star, especially in systems with Neptune-sized or larger planets. The bimodal distribution of planet sizes and the composition of intermediate-size planets provide insights into the complex processes governing planetary formation and evolution. Understanding these factors is essential for interpreting the observed architectures of planetary systems.
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