The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC. The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC. The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest. In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the "planets" circled Earth. Ancient Greeks called these lights πλάνητες ἀστέρες ( planētes asteres, "wandering stars") or simply πλανῆται ( planētai, "wanderers"), from which today's word "planet" was derived. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the " fixed stars", which maintained a constant relative position in the sky. The five classical planets of the Solar System, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. It previously encompassed asteroids, moons, and dwarf planets like Pluto, and there continues to be some disagreement today. The consensus as to what counts as a planet, as opposed to other objects, has changed several times. The concept has expanded to include worlds not only in the Solar System, but in multitudes of other extrasolar systems. The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age. Multiple exoplanets have been found to orbit in the habitable zones of their stars, but Earth remains the only planet known to support life.įurther information: History of astronomy and Timeline of Solar System astronomy 1660 illustration of Claudius Ptolemy's geocentric model The discovery of brown dwarfs and planets larger than Jupiter also spurred debate on the definition, regarding where exactly to draw the line between a planet and a star. These often show unusual features that the Solar System planets do not show, such as hot Jupiters-giant planets that orbit close to their parent stars, like 51 Pegasi b-and extremely eccentric orbits, such as HD 20782 b. įurther advances in astronomy led to the discovery of over five thousand planets outside the Solar System, termed exoplanets.
The International Astronomical Union (IAU) adopted a standard by which the four terrestrials and four giants qualify, placing Ceres, Pluto, and Eris in the category of dwarf planet, although many planetary scientists have continued to apply the term planet more broadly. The discovery of other large objects in the Kuiper belt, particularly Eris, spurred debate about how exactly to define a planet. With the development of the telescope, the meaning of planet broadened to include objects only visible with assistance: the moons of the planets beyond Earth the ice giants Uranus and Neptune Ceres and other bodies later recognized to be part of the asteroid belt and Pluto, later found to be the largest member of the collection of icy bodies known as the Kuiper belt. Earth itself was recognized as a planet when heliocentrism supplanted geocentrism during the 16th and 17th centuries. Planets have historically had religious associations: multiple cultures identified celestial bodies with gods, and these connections with mythology and folklore persist in the schemes for naming newly discovered Solar System bodies.
In antiquity, this word referred to the Sun, Moon, and five points of light visible by the naked eye that moved across the background of the stars-namely, Mercury, Venus, Mars, Jupiter, and Saturn. The word planet probably comes from the Greek planḗtai, meaning "wanderers". The Solar System has at least eight planets: the terrestrial planets Mercury, Venus, Earth, and Mars, and the giant planets Jupiter, Saturn, Uranus, and Neptune.
Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. The eight planets of the Solar System with size to scale (up to down, left to right): Saturn, Jupiter, Uranus, Neptune (outer planets), Earth, Venus, Mars, and Mercury (inner planets)Ī planet is a large, rounded astronomical body that is neither a star nor its remnant. For other uses, see Planet (disambiguation). This article is about the astronomical object.