Fifth planet (hypothetical) Fifth planet (hypothetical) In the history of astronomy, a handful of Solar System bodies have been counted as the fifth planet from the Sun. Under the present definition of a planet, Jupiter is counted as the fifth. There are three main ideas regarding hypothetical planets between Mars and Jupiter. During the early 19th century, as asteroids were discovered, they were considered planets. Jupiter became the sixth planet with the discovery of Ceres in 1801. Soon, three more asteroids, Pallas (1802), Juno (1804), and Vesta (1807) were discovered. They were counted as separate planets, despite the fact that they shared an
Solar eclipses on Jupiter the variations were caused by the varying distance between Earth and Jupiter as the two planets moved in their orbits around the Sun. Spacecraft can be used to observe the solar eclipses on Jupiter; these include "Pioneer 10" and "Pioneer 11" (1973 and 1974), "Voyager 1" and "Voyager 2" (1979), "Galileo" orbiter (1995–2003), "Cassini–Huygens" (2000) and "New Horizons" (2007) observed the transits of their moons and its shadows. The mean angular diameter of the Sun as viewed from Jupiter is 372 arc-seconds, or 6' 12" (about that of the Sun as viewed from Earth), varying slightly from 381" at perihelion