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May062011

How many planets are there in the Solar System?

How many planets are there in the Solar System?

(Traditional Solar System with 9 planets - http://www.nasa.gov/)

Several years ago, the answer to this question would be simple: 9 planets. For decades it was accepted that Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto were the 9 planets within our Solar System. A few years ago our definition of what a planet is changed. Consequently, so did the answer to this question.

Due to a new set of guidelines for the definition of a planet in the Solar System, as voted by the International Astronomical Union (IAU) in August 2006, Pluto is no longer a planet. There are now 8 planets in our Solar System!

Our view of the Solar System's construct began to change in August 1992. At that time, the very first Trans-Neptunian Object (TNO) was discovered. A TNO is any object orbiting the Sun in a region of space just beyond Neptune. This region of space, often referred to as the Trans-Neptunian region, contains thousands of known Trans-Neptunian Objects as well as ex-planet, Pluto! It was in January 2005 that Eris (initially named 2003 UB313) was identified in this same Trans-Neptunian region. Eris was more massive than Pluto and also had its own natural satellite.

Would Eris be the 10th planet in our Solar System? It would not. As mentioned earlier, the IAU established a new set of rules for determining if a celestial body in the Solar System is in fact a planet:

  • It needs to be in orbit around the Sun.
  • It needs to have sufficient mass so that, due to its self-gravity, it assumes a hydrostatic equilibrium (nearly round) shape.
  • It needs to have cleared all other objects from its orbit.


While Pluto meets both the first and second requirements, it has failed to clear some objects from its orbit. So like Eris, this is why Pluto is not a planet.

It was at this time in 2006, that the IAU further established a three-way categorization of all celestial bodies orbiting the Sun: planets, dwarf planets, and small solar system bodies. Eris and Pluto make up two-thirds of the accepted dwarf planets in the Solar System today.

So, how many planets are in our Solar System? There are 8 planets, 3 dwarf planets, and also thousands of small solar system bodies!

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