Pluto, Photo : NASA
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In 2006 the International Astronomical Union (IAU) decided that to be a planet an object has to:
- Orbit the sun.
- Have enough gravity to pull itself into a more or less spherical shape.
- And have cleared everything else out of its orbital path.
- Pluto’s orbit is tilted by 17 degrees compared to the other planets of the solar system.
- Pluto’s orbit intersects with Neptune’s orbit.
- Pluto’s moon Charon is relatively so big that rather than Charon orbiting Pluto, they both orbit around a common center of gravity.
Pluto satisfies the first two conditions but falls short on the 3rd one. There are more than 70,000 large objects (in the Kuiper belt) in Pluto’s orbit. One of them Eris, discovered in 2005, is even bigger than Pluto. This is why Pluto is called a “dwarf planet”.
There are some abnormal features of Pluto. For example:
Here two videos on this with cool animations:
Video by Fraser Cain:
Video by CGP Grey:
Sources:
1. Wikipedia
2. nasa.gov
3. Fraser Cain
4. CGP Grey