section_path: General characteristics/Comparison with extrasolar systems
section_name: Comparison with extrasolar systems
paragraph_text: Comparison with extrasolar systems
  
  Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury. The known Solar System lacks super-Earths, planets between one and ten times as massive as the Earth, although the hypothetical Planet Nine, if it does exist, could be a super-Earth orbiting in the edge of the Solar System.
  
  Uncommonly, it has only small terrestrial and large gas giants; elsewhere planets of intermediate size are typical—both rocky and gas—so there is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). As many of these super-Earths are closer to their respective stars than Mercury is to the Sun, a hypothesis has arisen that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection.
  
  The orbits of Solar System planets are nearly circular. Compared to many other systems, they have smaller orbital eccentricity. Although there are attempts to explain it partly with a bias in the radial-velocity detection method and partly with long interactions of a quite high number of planets, the exact causes remain undetermined.