Planet

Image of the Solar System planets. Top image shows the rocky planets and the bottom image shows the giant planets.

A planet is a large object such as Venus or Earth that orbits a star. Planets do not make light but can reflect light. Planets are spherical (that is, ball-shaped), because their gravity compresses them into a sphere.

Jupiter is the biggest planet in the Solar System, while the smallest planet in the Solar System is Mercury.

Planets are shaped like a slightly squashed ball (called a spheroid). Objects that orbit planets are called satellites. A star and everything which orbits it are called a star system.

There are eight planets in the Solar System. Pluto used to be called a planet, but in August 2006, the International Astronomical Union decided it was a dwarf planet instead. Because of its similar size to other things in space that were not planets. There are eight more known dwarf planets in the Solar System, Ceres, Eris, Makemake, Haumea, Quaoar, Orcus, Gonggong, and Sedna.

The name "planet" is from the Greek word πλανήτης (planetes), meaning "wanderers", or "things that move".

Until the 1990s, people only knew the planets in the Solar System. Since then 4,905 extrasolar planets (exoplanets) have been discovered in 3,629 planetary systems (January 2022 data). The count includes 808 multi-planetary systems. Known exoplanets range in size from gas giants about twice as large as Jupiter down to just over the size of the Moon. About 100 of these planets are roughly the size as Earth. Nine of these orbit in the habitable zone of their star.^[1][2] Also extrasolar moons (moons orbiting extrasolar planets) have been observed.^[3]

1. ↑ "NASA discovery doubles the number of known planets". USA TODAY. 10 May 2016. Retrieved 10 May 2016.
2. ↑ Schneider, Jean (16 January 2013). "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopaedia. Retrieved 2013-01-15.
3. ↑ https://www.nature.com/articles/s41550-022-01631-0