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What Makes a Planet?

StarBase 118 Staff

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Since its discovery in 1930, Pluto has been touted as the ninth planet in our solar system, though its always been an tenuous title. From the beginning, doubts about whether or not it should be considered a major planet have been in discussion by**astronomers. That status seemed even more in jeopardy with the discovery of Charon, Pluto’s largest moon that could almost be considered a planet in its own right, along with other icy objects *near the kuiper belt *that seemed to share similar characteristics to Pluto*. *Some of those objects are even more massive than the tiny planet. It wasn’t until 2006, however, that it’s grasp on the title of ‘major planet’ was lost.

The International Astronomical Union (or IAU) officially sat down in 2006 to discuss what characteristics defined a body so that it could be categorized as a ‘planet’. Before that, there was no specific definition for a ‘planet’. So what is a planet? According to the IAU, a planet is a heavenly object that orbits the sun and is round thanks to its personal force of gravity. That’s not all – a planet also has to ‘dominate’ its own neighborhood. One of the sticking points seems to be the size of Pluto in relation to its moon. Most planets dwarf their moons, but Pluto is only about twice the size of its largest moon. Another definition is that planets keep their neighborhood clean by ‘sweeping up’ debris that enters their orbit. Pluto’s neighborhood has some work to do when it comes to that.

This definition is still under scrutiny by many astronomers by those who don’t agree. A proposal that would have sethe bar lower and allowed Pluto to retain its status – but also would have meant reclassifying dozens of other bodies that should be called ‘planets’ under its definition. Despite protests, however, the IAU stands behind its decision to reclassify what was once our smallest planet. Mike Brown, Professor of Planetary Astronomy at the California Institute of Technology states that by keeping the definition so narrow, “Finding a new planet will really mean something.” You can learn more about the decision to officially definite a planet here:

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