Universe Guide

Why is Pluto not a Planet?

Pluto was first discovered on 18th February 1930 by American astronomer Clyde Tombaugh whilst studying pictures of the sky in Flagstaff, Arizona. From that date up until to August 2006, it was recognised as the ninth planet of the solar system.

A team led by Mike Brown at Palomar observatory discovered Eris, a kuiper belt object that at the time was considered to be larger than Pluto. It got people wondering what constituted as a planet. Some people were uneasy about having Eros as a planet. It would mean that the Solar System had just grown.

A proposal on what is a planet and what is not a planet was put before delegates at the end of the 2006 International Astronomical Union meeting. The proposal was accepted and Pluto was downgraded to Dwarf Planet status and Ceres was upgraded from asteroid to Dwarf Planet status.

The proposal stated for an object to be a planet, it had to satisfy three conditions.

  1. Be in orbit around a star, in our case the Sun.
  2. Be large enough for its mass to become spherical in shape
  3. Controversially, has cleared its neighbourhood around its orbit.

The third requirement was controversial as that is what demoted Pluto because Pluto lies in an area where the Kuiper Belt lies. The Kuiper Belt is a large area of space at the end of the solar system that contains asteroids. The third rule is technically flawed as no planet in the solar system can satisfy U.S.R.A. .

The third requirement should go and be replaced with a requirement to have an atmosphere to mark the object from being an asteroid to a planet. There are moves to change the requirement but none have gathered much momentum to anything.

What was the point of spending millions sending New Horizons to Pluto when its not a planet. The answer is that at the time the craft was conceived and built, it was a planet, it was downgraded after take-off. If the planet had been downgraded before take-off, the mission would still have gone ahead.

In the television series, The Big Bang Theory, Sheldon expresses his annoyance at Neil deGrasse Tyson as it being downgraded. Neil retorts its not his fault but Neil does support the downgrading of the planet because if it was reinstated, it would insult all the other planets. The Guardian

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