Solar System 7 light-years away. Compared to many other planetary systems, the Solar System stands out in lacking planets interior to the orbit of Mercury. The known Solar System also lacks super-Earths (Planet Nine could be a super-Earth beyond the known Solar System). Uncommonly, it has only small rocky planets 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). Also, these super-Earths have closer orbits than Mercury. This led to hypothesis that all planetary systems start
Extraterrestrial liquid water Should planetary mass objects be present, a single, gas giant planet, with or without planetary mass moons, orbiting close to the circumstellar habitable zone, could prevent the necessary conditions from occurring in the system. However, it would mean that planetary mass objects, such as the icy bodies of the solar system, could have abundant quantities of liquid within them. Lunar maria are vast basaltic plains on the Moon that were thought to be bodies of water by early astronomers, who referred to them as "seas". Galileo expressed some doubt about the lunar 'seas' in his "Dialogue Concerning the Two Chief