Planet sizes comparison
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Planet Size Distribution and Classification
Research shows that exoplanets, especially those found in multi-planet systems, tend to fall into two main size categories: "super-Earths" (rocky planets with radii around 1.4 Earth radii) and "sub-Neptunes" or "mini-Neptunes" (planets with radii around 2.4 Earth radii) Izidoro2022Fulton2018. There is a noticeable gap, called the "radius valley," between about 1.5 and 2 Earth radii, with relatively few planets found in this range Izidoro2022Fulton2018. This gap is thought to separate rocky planets from those with significant gaseous envelopes Izidoro2022Fulton2018.
Intra-System Size Uniformity ("Peas in a Pod")
Planets within the same multi-planet system are often very similar in size, more so than would be expected by chance Weiss2017Millholland2021Goyal2023+1 MORE. This "peas in a pod" pattern is especially strong when comparing planets within the same size category (either all super-Earths or all sub-Neptunes) . Near-resonant planetary systems—where planets have orbital periods close to simple ratios—show even greater size uniformity than nonresonant systems . However, this uniformity is less pronounced for planets larger than about 10 Earth radii or more massive than about 100 Earth masses .
Size Comparisons Within Systems
In most multi-planet systems, the outer planet is larger than the inner one about 65% of the time, and the size difference tends to increase with the temperature difference between the planets, possibly due to atmospheric loss from the inner, hotter planets . There is also a modest trend for planets with wider orbits to be larger, even within the same size category .
Mass, Density, and Composition
While planet sizes within a system are often similar, their masses can vary more widely . For small planets (up to about 8 Earth masses), a semi-empirical mass–radius relationship suggests that their composition is similar to Earth's, with a mix of rock and iron . Some Earth-sized exoplanets, such as Kepler-78b, have been found to have densities and compositions very similar to Earth, indicating a rocky nature Pepe2013Howard2013. However, planets in the 2–4 Earth radii range can have a wide range of densities, suggesting a diversity of compositions, from rocky to gas-rich .
Host Star Influence and Evolution
The distribution of planet sizes is also influenced by the mass of the host star and the planet's distance from the star. For lower-mass stars, the typical planet size shifts smaller, likely because these stars produce smaller planet cores . The dominant process shaping the planet size distribution appears to be photoevaporation, where high-energy radiation from the star strips away the atmospheres of close-in planets, especially those with low-density envelopes Fulton2018Izidoro2022.
Prevalence of Earth-Sized Planets
Earth-sized planets (within 25% of Earth's radius) are common, with about 22% of Sun-like stars estimated to host an Earth-sized planet in the habitable zone . However, not all Earth-sized planets are necessarily Earth-like in composition, as mass and density measurements are needed to confirm a rocky nature Pepe2013Howard2013.
Conclusion
In summary, exoplanet research reveals that planet sizes cluster into two main groups—super-Earths and sub-Neptunes—with a clear gap between them. Planets within the same system are often similar in size, especially within the same size category, and this uniformity is strongest in near-resonant systems. While size similarity is common, mass and density can vary, leading to a diversity of planetary compositions. The size distribution is shaped by both the properties of the host star and evolutionary processes like photoevaporation. Earth-sized planets are widespread, but confirming their Earth-like nature requires detailed measurements of both size and mass.
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