Size in order of planets
Pro Research Analysisby 
Searched over 200M research papers
Size Order of Planets: Insights from Kepler Multi-Planet Systems
Introduction to Planet Size Distribution
The study of planet sizes within multi-planet systems, particularly those discovered by the Kepler mission, has revealed intriguing patterns and correlations. These findings help us understand the formation and evolution of planetary systems.
Relative Sizes of Planets in Kepler Systems
Size-Location Correlation
Research indicates that in systems where at least one planet is Neptune-sized or larger, the larger planet tends to be the one with the longer orbital period. Specifically, in pairs where one or both planets have a radius of approximately 3 Earth radii (R⊕) or larger, 68% of the pairs have the inner planet smaller than the outer planet. However, this size-location correlation does not hold for pairs where both planets are smaller than 3 R⊕.
Detection Biases and Size Ordering
The apparent patterns in planet sizes and their ordering within systems can largely be attributed to detection biases. The Kepler mission's detection capabilities are influenced by the transit signal-to-noise ratio, which varies with stellar properties and orbital periods. This variation can create the illusion of correlated sizes and ordered spacing among planets. When these biases are accounted for, the evidence for intra-system uniformity and size ordering diminishes, suggesting that the physical properties of Kepler planets are largely independent of their siblings and parent star.
Patterns in Multi-Planet Systems
Similar Sizes and Regular Spacing
In systems with three or more planets, there is a tendency for planets to have similar sizes and regular spacing. The orbital period ratios of adjacent planet pairs are correlated, and this correlation is stronger in systems with smaller planets. Additionally, the outer planet is larger in 65% of cases, and the size ratio between outer and inner planets is positively correlated with the temperature difference between them, possibly due to photo-evaporation effects.
Bimodal Size Distribution
The distribution of planet sizes shows a bimodal pattern, with peaks corresponding to smaller rocky planets and larger intermediate-size planets. This bimodal distribution is influenced by factors such as a planet's orbital distance and the mass of its host star. For lower-mass stars, the distribution shifts to smaller sizes, consistent with the formation of smaller planet cores .
Composition and Evolution of Planets
Water Worlds and Gas Dwarfs
Planets with radii between 2 and 4 R⊕ are thought to be either gas dwarfs with rocky cores and H2-rich envelopes or water worlds with significant amounts of H2O-dominated fluid/ice. The precise measurements of planet radii and densities help constrain their compositions, with many intermediate-size planets likely being water worlds.
Photo-Evaporation and Planet Size Gap
The gap in the radius distribution of small planets, particularly between 1.5 and 2.0 R⊕, supports the idea that close-in planets smaller than Neptune are composed of rocky cores with varying amounts of low-density gas. This gap is partially filled, indicating that photo-evaporation of low-density atmospheres plays a significant role in shaping the planet size distribution .
Conclusion
The study of planet sizes within Kepler multi-planet systems reveals complex patterns influenced by detection biases, formation processes, and evolutionary dynamics. While larger planets tend to be located further from their stars, the overall size distribution is shaped by factors such as photo-evaporation and the mass of the host star. Understanding these patterns provides valuable insights into the formation and evolution of planetary systems.
Sources and full results
Most relevant research papers on this topic