What is the definition of a planet?
The word “planet” originally discribed “wanderers” that moved over the sky in contrast to the “fixed stars”. Only since 2006 there has been an official scientific definition. According to the International Astronomical Union (IAU) to be considered a planet, a celestial body must meet three conditions:
- Be spherical (in hydrostaticequilibrium)
- Turn around its star
- Be in a region where it is alone.
In our solar system there are eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptun.
http://www.iau.org/news/pressreleases/detail/iau0603/
Jupiter is the biggest planet in our solar system. Credit: NASA/JPL/University of Arizona
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