Ganymede (moon) has an ionosphere associated with its atmosphere is unresolved. Ganymede's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610. The satellite's name was soon suggested by astronomer Simon Marius, after the mythological Ganymede, cupbearer of the Greek gods, kidnapped by Zeus for the purpose. Beginning with "Pioneer 10", several spacecraft have explored Ganymede. The Voyager probes, "Voyager 1" and "Voyager 2", refined measurements of its size, while "Galileo" discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer
Planet Nine and proposed a possible orbit for the planet. This hypothesis could also explain TNOs with orbits perpendicular to the inner planets and others with extreme inclinations, as well as the tilt of the Sun's axis. Planet Nine is hypothesized to follow a highly elliptical orbit around the Sun with a period lasting 10,000–20,000 years. The planet's semi-major axis is estimated to be , roughly 20 times the distance from Neptune to the Sun, and its inclination to the ecliptic, the plane of the Solar System, to be about . The high eccentricity of Planet Nine's orbit could bring it as