Planet, a large, solid heavenly body that revolves around a star and shines by reflected light. The sun has eight known major planets. Many other stars are known to have one or more planets, but very little is known about these bodies. The major planets of the sun, in order of increasing mean, or average, distance from the sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. More than 2,000 smaller planets have been cataloged; most have orbits between the orbits of Mars and Jupiter. Dwarf planets, including Pluto and Ceres and many smaller planetlike bodies, also orbit the sun.

The word planet comes from the Greek word for wanderer. The ancient Greeks considered the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn to be planets because they appear to move among the stars. (They did not know of Uranus, which is difficult to see with the unaided eye, nor of Neptune, which cannot be seen without the aid of a telescope.) The Greeks thought that the planets move around Earth. This idea about planets remained more or less unchanged until the Polish astronomer Copernicus developed his theory, published in 1543, that Earth and the other planets revolve around the sun and that only the moon travels around Earth. Uranus was discovered in 1781 and Neptune in 1846. There is no formal definition for the term planet. Recently, the scholars have come up with a simple classification system that distinguishes the smallest worlds from the largest comets, asteroids, and other bodies.

In 2006, the International Astronomical Union (IAU), a recognized authority in assigning designations to heavenly bodies, defined the term planet. Some astronomers welcomed it while others considered the definition incomplete and therefore refused to adopt it. The IAU standard divides objects that orbit the sun into three major categories: planets, dwarf planets, and small solar system bodies. A planet orbits the sun and no other body. Due to its copious mass it gets compacted into a round shape by its own gravitational pull. A planet has a strong enough gravitational pull to remove most objects from the region of its orbit. A dwarf planet also orbits the sun and is large enough to be round. But it does not have a strong enough gravitational pull to clear its orbit of other objects. Thus many planets, dwarf planets, and other bodies have smaller objects called satellites or moons orbiting them. Smaller solar bodies, including most asteroids and comets, have too little mass for gravity to round their irregular shapes.

Since its discovery in 1930, Pluto was considered a planet. However, because of its small size and irregular orbit scientists debated whether Pluto should be grouped with bodies such as Earth and Jupiter. Pluto is similar in many respects to other icy objects found in a region of the outer solar system called the Kuiper belt. In the early 21st century, astronomers discovered several such Kuiper belt objects (KBOs) comparable in size to Pluto. The IAU thus formulated the dwarf planet classification to describe Pluto and other planet-sized objects.