by Raphy Mendoza

Created on: March 25, 2007   Last Updated: May 02, 2007

Pluto: A planet or not?

This diagram (left) shows that Pluto, what we thought to be the furthest planet from our Sun was actually closer to it than Neptune in the year 1989 (Circled) when the two planets' orbits intertwined.

Pluto is said to be made of mainly rock and ice. Through a telescope, it has a brownish colour, this is because the thin atmosphere around it is composed of nitrogen, methane and carbon dioxide.

A planet's satellite (moon) often orbits around the planet, Pluto's relation with Charon (its moon) however is considered a "binary system" as both objects are almost as big as each other and rather than Charon orbiting around Pluto, both objects seem to orbit around each other; the centre of their orbits do not lie within either object. For example, our (Earth's) moon's centre of orbit is the Earth, therefore it orbits around that
point/object. Charon's centre of orbit however does not lie within Pluto's point, and neither does Pluto's orbit lie within its moon. They therefore move around a space between them.

This raised another question as to what defines a planet, and whether Pluto should be considered as one; is Pluto's size to insignificant?

As mentioned above, a planet must have enough gravitational pull within itself to be able to hold a sphere/round shape and appearance, it must orbit around A sun, and has cleared it neighbourhood. ("clearing its neighbourhood" I PRESUME means have a clear space around the planet, like the other 8 official planets have, whilst Pluto a "dwarf planet" (see above) is found in the Kuiper Belt and has therefore NOT cleared its neighbourhood). This definition is given by the International Astronomical Union.

So why Pluto should remain a planet?

Pluto being classified as a "dwarf" planet already differentiates it with the rest of the more obvious planets, but still emphasises the fact that this object is nonetheless a planet. The change of Pluto's status therefore is unnecessary as it falls nicely into the category of "dwarf planet".

There is also no official definition for a planet that states anything about where the barycentre of a planet-moon system is. As mentioned above, Charon's centre of orbit does not lie in the centre point of Pluto and that's why they are considered to have a "binary-system". This happens through course of nature and after a few million (billion?) years the same thing may happen to Earth's system. This is not enough of a characteristic to demote Pluto as a planet.

If crossing another planet's

• 1
• 2
• 3
• Next Page >>