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A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star. A mass that becomes massive enough to undergo nuclear reactions is considered a star, not a planet. The name comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. Based on historical consensus, the International Astronomical Union (IAU) lists nine planets in our solar system. However, since the term "planet" has no precise scientific definition, many astronomers contest that figure. Some say it should be lowered to eight by removing Pluto from the list, while others claim it should be raised to ten or even higher depending on how planets are categorized.

Planetary formation
It is not known with certainty how planets are formed. The prevailing theory is that they are formed from those remnants of a nebula that do not condense under gravity to form a protostar. Instead, these remnants become a thin disc of dust and gas revolving around the protostar and begin to condense about local concentrations of mass within the disc. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets. When the protostar has grown such that it ignites to form a star, its solar wind blows away most of the disc's remaining material. Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb. Meanwhile, protoplanets that have avoided collisions may become moons of larger planets.

With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account.

Within our solar system
The process of naming planets and their features is known as planetary nomenclature. All the currently accepted planets in the solar system are named after Roman gods, except for Uranus (named after a Greek god) and the Earth, which was not seen as a planet by the ancients but rather the centre of the universe. The original number of planets was seven: Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn, in orbit order outwards according to the Ptolemaic system. These were all seen as regular "wanderers" in the sky. Developments in Astronomy removed the Sun and the Moon, and added the currently accepted planetary members of the Solar System.

Some Romans believed that the seven gods that the planets were named after, took hourly shifts in looking after affairs on Earth, in Ptolemaic orbit order listed inwards (the reverse of the above list). As a result, a list of which god has charge of the first hour in each day, comes out as Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order: Sunday, Mo(o)nday, French mardi, mercredi, jeudi, vendredi, Satur(n)day.

The designated planetary names are near-universal in the Western world, but some non-European languages, such as Chinese, use their own. A western exception is, naturally, Greece which uses the equivalent Greek gods' names: Hermes (Mercury), Aphrodite (Venus), Gaia (Earth), Ares (Mars), Zeus (Jupiter), Cronus (Saturn), Ouranos (Uranus), Poseidon (Neptune), Pluton—not the expected Hades—(Pluto). Moons are also named after gods and characters from classical mythology, or, in the case of Uranus, after characters from English literature. Asteroids can be named after anybody or anything at the discretion of their discoverers, subject to approval by the IAU's nomenclature panel.