A planet is an astronomical body that orbits a star. Planets are small enough so that nuclear fusion does not take place, rounded by their own gravity, and have cleared planetesimals in their neighboring region.

A planet is a body in space that orbits a star. Earth, Mars, and Neptune are all examples of planets in our solar system. In order to be defined as a planet by the International Astronomical Union (IAU), the body needs to meet three criteria. The first is that the body isn’t so large that nuclear fusion reactions occur in its core. Bodies that are large enough so that fusion reactions take place are more commonly known as stars. The body also needs to be large enough so that its gravitational force has rounded it into a rough sphere shape. Rounding doesn’t occur with asteroids, even though they also orbit the Sun. Finally, the body needs to have cleared all the planetesimals in its neighboring area. There are eight planets in our solar system.

There were nine planets recognized by the IAU until 2006 when Pluto was downgraded from a planet to a dwarf planet. While Pluto meets most of the criteria of being a planet, it hasn’t cleared the neighborhood around its orbit. This means along Pluto’s orbit there are still a large amount of other, smaller objects.

Planets can be divided into two main groups depending on their composition: gas giants and rocky planets. In our solar system, the rocky planets are Mercury, Venus, Earth, and Mars. The gas giants are Jupiter, Saturn, Uranus, and Neptune, although the last two are sometimes also called the ice giants due to their very cold temperatures.

In the 21st century, scientists started to search for extrasolar planets (a.k.a. exoplanets). The first confirmed discovery of a planet outside of our solar system occurred in 1992. Since then, there have been thousands of discoveries of exoplanets in planet systems, with hundreds of systems having more than one planet. Scientists are now becoming more interested in the possibility of life on one of these far away planets. They have found some planets that appear to have similar conditions to Earth.

Planets In Our Solar System

  • Mercury
  • Venus
  • Earth
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune

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Learn more about the stars and other celestial bodies in our Picture Encyclopedia of Astronomy Terms!
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