Gravitational effects limit the total mass of gas planet satellites
A simulation may have solved the mystery of why the total mass of the moons of gaseous planets is amazingly constant at one ten-thousandth the mass of the home planet, while it is significantly greater for Earth-like planets.
Robin Canup and William Ward from the Southwest Research Institute in San Antonio had simulated the formation history of the gas planets and the moons orbiting them. The latter grow in a disc of dust and gas around the planet, which is initially supplied with more matter from outside. The gravitational pull of the chunks increases with the size, which then triggers spiral waves in the surrounding gas. Interactions between these waves and the satellite mean that the latter eventually pull their orbit ever tighter around the mother planet until they collide with it. The balance between growing satellites due to the replenishment of matter and the loss of large chunks due to collisions is ultimately responsible for the observed maximum mass, the researchers explain.
With their model, the scientists were able to reproduce the satellites orbiting Saturn, Jupiter and Uranus today. These appear to be the last generation of several that preceded them - left over after the planets stopped growing and the dust-gas disk disappeared. If these results also apply to extrasolar planets, moons around Jupiter-sized planets are likely to reach the maximum size of the moon or Mars - an Earth-like planet is therefore unlikely, according to the researchers.
