Jupiter's moons in fiction smallest of the four Galilean satellites and the second closest to Jupiter. It is theorized to have an ocean of liquid water underneath its icy surface; the thickness of the ice is much debated. The probable presence of the water ocean has made it a favored location for modern fictional speculation about extraterrestrial life in the Solar System. Ganymede is the third of the Galilean moons from Jupiter. It is the largest moon in the Solar System, bigger than the planet Mercury (though less massive), almost 52% larger than the diameter of the Moon and with twice its mass. It
Discovery and exploration of the Solar System announced the discovery of Eris in 2005, a scattered disc object initially thought to be larger than Pluto, which would make it the largest object discovered in orbit around the Sun since Neptune. "New Horizons"' fly-by of Pluto in July 2015 resulted in more-accurate measurements of Pluto, which is slightly larger, though less massive, than Eris. Since the start of the Space Age, a great deal of exploration has been performed by robotic spacecraft missions that have been organized and executed by various space agencies. All planets in the Solar System have now been visited to varying degrees by spacecraft