List of planets
Planets in the Solar SystemEdit
- Terrestrial planets
- Mercury – The planet with the second highest temperature in the Solar System and the closest planet to the Sun.
- Venus – The warmest planet. Sometimes called "Earth's twin" because Venus and Earth are very similar.
- Earth – The only planet that is known to have life. It has one natural satellite, the Moon.
- Mars – Sometimes called the "red planet" and "the brother of Earth".
- Phaeton or Astra – a planet that some think broke apart to form the asteroid belt, though most astronomers think it never formed.
- Pluto – the smallest planet, Pluto is now considered a "Dwarf planet" by the International Astronomical Union (IAU) since August 24, 2006.
- Theia – Hypothetical planet which some think crashed into Earth and created the Moon.
- Planet Nine – a hypothetical planet beyond Neptune. It has not been found, but some astronomers think its gravity pulls on the orbits of many dwarf planets. It is thought to be a gas giant.
Definition of a planetEdit
Technically, there was never a scientific definition of the term planet before 2006. When the Greeks observed the sky thousands of years ago, they discovered objects that acted differently than stars. These points of light seemed to wander around the sky throughout the year. The term "planet" derives from the Greek word "planets" - wanderer.
In 2006, the International Astronomical Union (IAU) convened a Planet Definition Committee, deliberated, and ultimately reached a consensus on a new definition of a planet which leaves us with the eight planets we today consider to comprise the Solar System (thus the exit of Pluto). That new definition: " A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit [meaning: 'there are no other bodies in its path that it must sweep up as it goes around the Sun'.]"