Fifth planet

Fifth planet may refer to:

Astronomy

 * Jupiter, fifth planet from the Sun in our Solar System
 * Fifth planet (hypothetical), any of various hypothetical planets thought to have existed
 * Planet V, 2002 scientific proposal for a destroyed fifth planet
 * Mars, fifth planet from Earth in the Ptolemaic geocentric model
 * Ceres (dwarf planet), dwarf planet between Mars and Jupiter in the asteroid belt

Fiction

 * Fifth Planet (novel), science fiction novel by Fred and Geoffrey Hoyle