Order of the planets by size
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Order of the Planets by Size
Understanding the order of planets by size is crucial for both planetary science and astronomy. This article synthesizes recent research findings to provide a clear picture of how planets are ordered by size, particularly focusing on exoplanetary systems discovered by the Kepler mission.
Planet Size Distribution and Detection Biases
Detection Biases in Kepler Systems
The apparent size ordering of planets in Kepler systems can be significantly influenced by detection biases. The transit signal-to-noise ratio (S/N) used in Kepler detections varies with stellar properties and orbital periods, leading to different detection thresholds for planetary sizes. This variation can create the illusion of correlated sizes and ordered spacings among planets in the same 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 conditions1.
Relative Sizes of Planets in Multi-Planet Systems
In systems where at least one planet is Neptune-sized or larger, the larger planet is typically the one with the longer orbital period. This size-location correlation is not observed in pairs of planets where both are smaller than Neptune. Specifically, if at least one planet in a pair has a radius of approximately 3 Earth radii (R⊕) or larger, 68% of the pairs have the inner planet smaller than the outer planet. However, no preferred sequential ordering is seen if both planets are smaller than 3 R⊕2.
Size-Ordering and Formation Pathways
Entropy and Size-Ordering
The size-ordering of planets in a system can provide insights into their formation and evolutionary history. Systems with low entropy in their size-ordering suggest a non-random formation process. Kepler systems show a significant deficit in entropy compared to randomly generated populations, indicating that these systems retain some memory of their initial formation conditions. This finding supports the hypothesis that dynamical evolution, while chaotic, does not completely erase the initial size-ordering of planets3.
Planet Radius Gap
The distribution of planet sizes reveals a notable gap between rocky super-Earths and gas-dominated sub-Neptunes. This gap, observed in the radius distribution of Kepler planets, is influenced by factors such as a planet's orbital distance and the mass of its host star. The gap is partially filled, suggesting that photoevaporation of low-density atmospheres plays a significant role in shaping the size distribution of planets4 7.
Composition and Internal Structure
Bimodal Distribution of Planet Sizes
The radii of exoplanets show a bimodal distribution, with peaks corresponding to smaller rocky planets and larger intermediate-size planets. 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. The precise measurements of planet radii and densities help constrain their bulk compositions and internal structures, indicating that many intermediate-size planets are likely water worlds6.
Conclusion
The order of planets by size in exoplanetary systems is influenced by detection biases, formation pathways, and evolutionary processes. While detection biases can create apparent correlations in planet sizes, the intrinsic properties of planets reveal a more complex picture. The size-ordering of planets provides valuable insights into their formation and evolution, with factors such as photoevaporation and host star mass playing crucial roles in shaping the observed size distribution. Understanding these factors helps us better comprehend the diversity and complexity of planetary systems.
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Most relevant research papers on this topic
On the Patterns Observed in Kepler Multi-planet Systems
Kepler planets' physical properties are largely independent of their siblings and parent star, making it difficult to infer their initial formation conditions from observed properties and architecture.
ON THE RELATIVE SIZES OF PLANETS WITHIN KEPLER MULTIPLE-CANDIDATE SYSTEMS
The larger planet in Kepler multiple-candidate systems is often the planet with the longer period, while no such correlation is seen for pairs of planets smaller than Neptune.
Do planets remember how they formed
Kepler systems show a significant deficit in entropy compared to a randomly generated population, suggesting they remember their younger years and the importance of information theory for exoplanetary science.
The California-Kepler Survey. VII. Precise Planet Radii Leveraging Gaia DR2 Reveal the Stellar Mass Dependence of the Planet Radius Gap
The planet size distribution is shaped by factors like orbital distance and host-star mass, with photoevaporation of low-density atmospheres being the dominant evolutionary determinant.
Highly CitedThe Size of the Planets
The size of the planets is determined by their distance from the sun, not their orbital distance.
Growth model interpretation of planet size distribution
Many intermediate-size exoplanets are likely water worlds, with a growth model and Monte Carlo simulations proving this conclusion.
Highly CitedThe California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
The California-Kepler Survey reveals a gap in the radius distribution of small planets, suggesting that close-in planets smaller than Neptune are composed of rocky cores measuring 1.5 or smaller with varying amounts of low-density gas.
Highly CitedThe California-Kepler Survey. V. Peas in a Pod: Planets in a Kepler Multi-planet System Are Similar in Size and Regularly Spaced
Planets in multi-planet systems are similar in size and regularly spaced, with the outer planet being larger in 65% 0.4% of cases, potentially due to photo-evaporation.
Highly CitedOccurrence and core-envelope structure of 1–4× Earth-size planets around Sun-like stars
Small, 1–4 Earth-size planets are extremely common around Sun-like stars, with 26% having them with orbital periods under 100 days, and 11% receiving 1–4 the incident stellar light that warms our Earth.
Highly CitedKepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone
Kepler-62 is a five-planet system with two small planets in the habitable zone of its star, Kepler-62e and -62f, which could be solid, either rocky or composed of mostly solid water in their bulk.
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