The eight planets of the Solar System with size to scale: Saturn, Jupiter, Uranus, Neptune, Earth, Venus, Mars, and Mercury
A planet is a large, rounded astronomical body in orbit around a star (or in our Solar System, the Sun). All the planets in our Solar System have moons except Mercury and Venus. The home planet of the human civilization is Earth.
History[]
In ancient times, the planets were regarded as Mercury, Venus, Mars, Jupiter, and Saturn. Earth was recognized as a planet when heliocentrism was accepted. With the telescope, these were discovered: moons of the planets beyond Earth; the ice giants Uranus and Neptune; Ceres and other asteroids; and Pluto.
In 2006, the International Astronomical Union (IAU) redefined the planet, which is a celestial body that:
- is in orbit around the Sun;
- has sufficient mass to assume its nearly round shape;
- has "cleared the neighbourhood" around its orbit.
Thereafter, Ceres, Pluto, and Eris became dwarf planets and the solar system was left with eight planets (Pluto had been the ninth since 1930).
Solar System[]
| Mercury | The smallest and fastest planet, Mercury is a cratered, airless world closest to the Sun. |  |
| Venus | A scorching planet with a thick, toxic atmosphere and runaway greenhouse effect, making it the hottest in the solar system. |  |
| Earth | The only known planet to support life, Earth has vast oceans, a breathable atmosphere, and a dynamic climate. |   |
| Mars | A cold, desert-like planet with the tallest volcano and deepest canyon, often called the "Red Planet" due to its iron-rich soil. |   |
| Jupiter | The largest planet, Jupiter is a gas giant with a powerful magnetic field and the iconic Great Red Spot storm. |   |
| Saturn | Known for its spectacular ring system, Saturn is a gas giant with dozens of moons, including the potentially habitable Titan. |   |
| Uranus | A pale blue ice giant that rotates on its side, Uranus has a frigid atmosphere and faint rings. |   |
| Neptune | The windiest planet, Neptune is a deep blue ice giant with powerful storms and the distant, icy moon Triton. |   |
Honorable mention[]
| Pluto | A dwarf planet in the Kuiper belt. Considered a planet since its discovery in 1930, its classification changed in 2006. |   |
Further advances in astronomy led to the discovery of planets outside the Solar System, termed exoplanets.
Exoplanets[]
In 2011, the first Earth-sized exoplanets were discovered orbiting a Sun-like star, Kepler-20e and Kepler-20f. One in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone, which suggests that the nearest would be expected to be within 12 light-years distance from Earth. The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation.
Rogue planet[]
A rogue planet is an interstellar object of planetary mass which is not gravitationally bound to any star or brown dwarf. Rogue planets may originate from planetary systems in which they are formed and later ejected, or they can also form on their own, outside a planetary system. The Milky Way alone may have billions to trillions of rogue planets.