Planet size comparison
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Planet Size Comparison: Solar System and Exoplanets
Solar System Planet Size Comparison
Accurate measurements from NASA spacecraft have allowed for direct visual and numerical comparisons of planet and moon sizes in our solar system. These images, taken at the same scale, make it easy to see the size differences between planets and their notable surface features. For planets not yet visited by spacecraft, artist renderings and estimated sizes are used to provide a complete comparison set .
Exoplanet Size Distribution and Uniformity
Size Patterns in Multi-Planet Systems
Studies of exoplanets, especially those discovered by the Kepler mission, show that planets within the same system often have very similar sizes. This "peas in a pod" pattern means that if one planet is a certain size, its neighbors are likely to be close in size as well. This trend is especially strong for planets smaller than about 10 Earth radii and less massive than 100 Earth masses 2346. In systems with three or more planets, the planets are not only similar in size but also regularly spaced in their orbits .
Super-Earths and Sub-Neptunes
There is a clear division in the sizes of small exoplanets. Planets smaller than about 1.5 Earth radii are typically rocky "super-Earths," while those between 1.5 and 4 Earth radii are "sub-Neptunes" with thick gas envelopes. There are very few planets with sizes between 1.5 and 2 Earth radii, creating a noticeable gap in the size distribution 35910. This gap is thought to be caused by processes like photoevaporation, which strips away the atmospheres of some planets, leaving behind rocky cores 59.
Size Uniformity and Orbital Resonance
The similarity in planet sizes within a system is even stronger for planets that are near orbital resonance, meaning their orbits are in simple ratios (like 2:1 or 3:2). These near-resonant systems show enhanced size uniformity compared to nonresonant systems, suggesting that the way planets form and interact in these systems leads to more similar planet sizes .
Mass, Radius, and Composition
For rocky planets, there is a well-established relationship between mass and radius. Planets with radii less than about 1.5 Earth radii tend to have densities similar to Earth, indicating a rocky composition. For example, Kepler-78b is almost the same size and density as Earth, suggesting a similar internal structure of iron and rock 7810. As planet size increases beyond 1.5 Earth radii, densities decrease, indicating the presence of thick atmospheres of hydrogen, helium, or ices .
Host Star Influence and Size Gap
The size distribution of planets also depends on the mass of the host star. Around lower-mass stars, the gap between super-Earths and sub-Neptunes shifts to smaller sizes, likely because smaller stars form smaller planet cores . The details of the size gap and the so-called "sub-Neptune desert" (a region with few planets of certain sizes) support the idea that atmospheric loss plays a major role in shaping planet sizes .
Conclusion
Comparing planet sizes reveals clear patterns both within our solar system and among exoplanets. In multi-planet systems, planets tend to be similar in size, especially for those smaller than Neptune. There is a distinct gap between rocky super-Earths and gas-rich sub-Neptunes, shaped by processes like photoevaporation and influenced by the mass of the host star. These findings help us understand how planets form and evolve, both in our solar system and beyond 2345+3 MORE.
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