24th of August, 2006: Over 2500 astronomers participated in the vote at the General Assembly of the International Astronomical Union (IAU) in Prague, The Czech Republic.
A celestial body can now be classed as a planet if:
- it is in orbit around the Sun
- it has enough mass to take on a nearly round shape
- it has cleared the neighbourhood around its orbit.
Since Pluto does not meet these criteria anymore, because its highly eccentric orbit overlaps with that of Neptune, the IAU’s decision means that the Solar System now consists of just eight major planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Pluto is not a planet anymore, but a ‘dwarf-planet’.
Quote from wikipedia
As of 2008, the IAU has classified five celestial bodies as dwarf planets. Two of these, Ceres and Pluto, are known to qualify as dwarf planets through direct observation. The other three, Eris, Haumea, and Makemake, are thought to be dwarf planets from mathematical modeling—or in the case of Eris, because it is larger than Pluto—and qualify for the classification under IAU naming rules based on their magnitudes.
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