Planet Plush

The International Astronomical Union defines "planet" as a celestial body that, within the Solar System, more...

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(a) is in orbit around the Sun;

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and

(c) has cleared the neighbourhood around its orbit;

or within another system,

(i) is in orbit around a star or stellar remnants;

(ii) has a mass below the limiting mass for thermonuclear fusion of deuterium; and

(iii) is above the minimum mass/size requirement for planetary status in the Solar System.

Our solar system is thus considered to have eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Under a separate resolution, it is also considered to have three dwarf planets: Ceres, Pluto, and Eris. There have been more than two hundred planets discovered orbiting other stars to date.

Historically, there had been no formal scientific definition of "planet" and without one, the Solar System had been considered to have various planets over the years. This changed when a resolution covering planets within our solar system was formally adopted by the IAU in 2006, limiting the number to eight. However, the IAU's position on those in other systems remains only a working definition in place since 2003, and as such, is easily subject to change. The IAU has not yet taken a position on free-floating objects of planetary mass outside star systems, other than to exclude those in young star clusters.

Etymology

In ancient times, Greek astronomers noted how certain lights moved across the sky in relation to the other stars. These objects were believed to orbit the Earth, which was considered to be stationary. The "wandering" lights were called "πλανήτης" (planētēs), a Greek term meaning "wanderer", and it is from this that the word "planet" was derived.

In near-universal practice in the Western world, the planets in the Solar System are named after Graeco-Roman gods, as, in Europe, it was the Greeks who originally named them. However, because of the influence of the Roman Empire and, later, the Catholic Church, in most countries in the West, the planets are known by their Roman (or Latin) names, rather than the original Greek. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable. When the Romans studied Greek astronomy, they gave the planets their own gods' names. In ancient times, there were seven known planets; each presumed to be circling the Earth according to the complex laws laid out by Claudius Ptolemy in the 2nd century. They were, in increasing order from Earth: the Moon (called Luna by the Romans, and Selene by the Greeks), Mercury (called Hermes by the Greeks), Venus (Aphrodite), the Sun (called Sol by the Romans, Helios by the Greeks), Mars (Ares), Jupiter (Zeus), and Saturn (Kronos). Eventually, the Sun and Moon were removed from the list of planets in accordance with the heliocentric model. However, when subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptune (Poseidon). The Greeks still use their original names for the planets.

Read more at Wikipedia.org