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Have you ever wondered what Pluto could have possibly done to lose it’s position as the ninth planet from the Sun? Well, it’s all about gaining a better understanding of our solar system.

Pluto, once dubbed Planet X, was originally discovered in 1930 by Clyde Tombaugh and was then later officially classified as the ninth planet from the Sun. As time went on Pluto continued to be considered to be the largest object past Neptune, which justified it’s planetary title, but as technology improved we began to find more and more objects in the outer solar system.

These objects in the outer solar system all orbit the Sun and are collectively known as the Kuiper Belt, which was officially discovered in 1992. The objects in the Kuiper Belt are numerous and made up of mostly of rock, metal, and ice. With more and more objects being discovered, it was likely that we would soon find a Kuiper Belt object that was larger than Pluto, which was eventually found in 2005. The object found was discovered by Mike Brown and his team and was later named Eris, which is 2,600 km across compared to Pluto’s 2,400 km. However, this information may be inaccurate as recent measurements have shown Eris to be much smaller than initially stated, in some cases it may actually be smaller than Pluto.

With all of these new discoveries it was becoming harder and harder to justify Pluto being our ninth planet. What really helped put an end to Pluto’s planet-hood was when we compared Pluto to the IAU’s (International Astronomical Union) definition of a planet which states that in order for an object to be considered a planet, it must meet the following requirements:

1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has “cleared the neighborhood” around its orbit. ( This means it must be the largest and most gravitationally dominant object in it’s orbital path)

Pluto runs into a big problem with the third requirement as the would-be planet is less than 1% of the mass of any other object in it’s orbit. This officially makes Pluto a Dwarf Planet and thus no longer the ninth planet in our solar system. Regardless of Pluto’s new title, we will always have a place in our hearts for the tiny cold rock. In fact, we will be visiting Pluto in 2015 as NASA’s Horizons spacecraft will be reaching the Dwarf Planet after a 3 billion mile, 10 year journey to explore Pluto and the Kuiper Belt.

Also, it’s important to note that many disagree with these recent changes to Pluto’s classification and are advocating the overturning of the IAU definition of a planet that caused this debate and downgrade of Pluto. You can read more information about this effort on LaurelsPlutoBlog.