Why isn't Pluto a planet?

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Pluto was discovered back in 1930. At that time, it was called the ninth planet in our Solar System, and it stayed that way up until 2006. Nothing changed about Pluto to make it not a planet. The definition of what makes a planet was changed, and Pluto no longer fits the definition.

Astronomers (the scientists who study outer space) are always making new discoveries. One recent discovery was Eris, an object far out in our Solar System slightly larger than Pluto.  But is Eris a planet?  Scientists met in 2006 and decided to come up with new rules about what makes a planet.  

There are three rules for an object in space to be a planet. Number 1: A planet must be round, pulled into that shape by gravity. Number 2: A planet must orbit the Sun. Number 3: A planet must have cleared the neighborhood of its orbit. That means that as a planet travels, its gravity sweeps the space around it clean of other objects. Some of those objects may crash into the planet or even become moons.

Pluto and Eris both follow the first two rules, but they have not cleared their neighborhoods in space. They share space with other objects that were recently discovered. Because they don’t follow all three rules, they aren’t planets. They have their own special category called “dwarf planets.”

by   (whyzz writer)
  • Exploration

    Meet the Dwarf Planets

    Can you name all eight planets in the Solar System? There’s Mercury, Venus, Earth (that’s our home!), Mars, Jupiter, Saturn, Uranus and Neptune.  A lot of people are familiar with those, but do you know about the Dwarf Planets.

    There are currently 5 official dwarf planets, but many more are expected to be found.
    Pluto used to be the 9th planet, located beyond Neptune.
    Makemake, Haumea and Eris are three other dwarf planets located beyond Neptune.
    Ceres is the fifth, and it’s in an interesting location. It’s in the Asteroid Belt, between Mars and Jupiter!

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