Planet

A planet is a large celestial body that orbits a star, such as the Sun. A planet has sufficient mass for gravity to pull it into a spherical or nearly spherical shape, but not enough mass for nuclear fusion to occur in its interior, as happens in stars. Planets do not emit light; they are visible by reflecting the light of their star.

The solar system contains eight planets. In order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The first four are called terrestrial planets, while the last four are large planets composed of gas and ice.

Scientific definition

In 2006, the International Astronomical Union (IAU) defined a planet as a celestial body that: (1) orbits the Sun, (2) has sufficient mass to be pulled into a spherical shape, and (3) has cleared its orbital neighborhood of other debris. For this reason, Pluto was removed from the list of major planets and reclassified as a dwarf planet.

Orbit and rotation

Each planet orbits the Sun in an elliptical path. The time it takes to complete an orbit is its year: Earth takes approximately 365 days, while Neptune takes several hundred Earth years. Planets also rotate on their axes; this rotation creates day and night.

Moons and rings

Many planets have moons, which are small bodies that orbit the planet. Earth has one moon; Mars has two; Jupiter and Saturn have many moons. Mercury and Venus have no moons. Saturn is known for its rings, made of ice and rock; the other large planets also have fainter rings.

Planets of other stars

Planets that orbit other stars are called exoplanets. Since the 1990s, astronomers have discovered thousands of such planets, often by measuring how the star's light dims as the planet passes in front of it, or by detecting the star's motion caused by the planet's gravitational pull.