The ancient Greeks counted the Sun and Moon as planets, but not Earth
Planet means wanderer. To ancient observers it meant the Sun, the Moon and five bright points drifting against the stars; Earth joined the club only when heliocentrism won out. Since then the list has grown and shrunk, with Pluto demoted in 2006, and thousands of worlds now confirmed around other stars.
The Greek planētai, wanderers, covered seven moving lights: Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. Many cultures linked them to gods, a habit that survives in how newly found bodies are named. Earth was recognised as a planet only when the Sun-centred model displaced the Earth-centred one in the 16th and 17th centuries. Telescopes then broadened the term to include moons, Uranus and Neptune, Ceres and Pluto, until the discovery of Eris and other distant icy bodies forced a formal definition.
In 2006 the International Astronomical Union settled on eight planets: rocky Mercury, Venus, Earth and Mars, and the giants Jupiter, Saturn, Uranus and Neptune. Ceres, Pluto and Eris became dwarf planets, bodies rounded by their own gravity that have not cleared their orbits. Many planetary scientists still use the word more loosely. The range within the eight is enormous: Jupiter weighs 318 Earth masses, Mercury just 0.055. Some moons would count as planets on looks alone; Ganymede and Titan are wider than Mercury, though far less massive, and tiny Mimas is the smallest body generally agreed to be geophysically planet-like.
The leading account of how planets form is the nebular hypothesis. A collapsing cloud produces a young star circled by a disk of gas and dust. Grains stick together, planetesimals grow and pull in more material, and protoplanets collide and merge; once a body passes roughly the mass of Mars it can hold an extended atmosphere. Heat from impacts and radioactivity partly melts young planets, so dense material sinks to form cores. Stars richer in heavy elements appear more likely to host planets.
The first confirmed exoplanets, found in 1992 by Aleksander Wolszczan and Dale Frail, orbit a pulsar. In October 1995 Michel Mayor and Didier Queloz announced 51 Pegasi b, a hot Jupiter circling an ordinary star. By April 2026 some 6,416 exoplanets had been confirmed, and microlensing data suggest at least 1.6 planets per star in the Milky Way on average. Some sit in habitable zones, yet Earth remains the only world known to host life.
Source: Planet
