Definition of planet Earth's own orbit around the Sun—looking down on the ecliptic, the Moon never actually loops back on itself, and in essence it orbits the Sun in its own right. Also many moons, even those that do not orbit the Sun directly, often exhibit features in common with true planets. There are 19 moons in the Solar System that have achieved hydrostatic equilibrium and would be considered planets if only the physical parameters are considered. Both Jupiter's moon Ganymede and Saturn's moon Titan are larger than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as
The Ganymede Club The Ganymede Club The Ganymede Club is a science fiction novel by American writer Charles Sheffield, published in 1995. A mystery and a thriller, the story unravels in the same universe that Sheffield imagined in "Cold as Ice". Shortly after humanity begins colonisation of the solar system, a trade war sets off vicious civil war that kills billions. The book received favorable reviews from "Library Journal" and "Kirkus Reviews", as well as in the science-fiction press. It was ranked #14 in SF novels in the 1996 "Locus" awards. The novel has been translated into Italian and was published as "Memoria