Q. When is a planet not a planet?

A. When it’s a ‘dwarf’ planet…

…and if that distinction makes any sense then you’re probably an astronomer at a convention in Prague right now.

Today, (Thursday, August 24th 2006) Pluto ceased to be considered a real planet in our solar system. Instead, it has been re-categorised and we are now to call it a “dwarf planet” which might sound like a planet to you and me but isn’t…or is, but smaller…or, erm something.

So, now every text book (aside from those with typos which are now correct) related to astronomy is wrong when it says the solar system contains 9 planets and from this point on we’ll be talking about a solar system with 8 planets…and several dwarf planets, including the demoted Pluto, its satellite Charon (named after the mythological figure who ferried the dead across the river Acheron into underworld), the asteroid Ceres and the newly discovered “UB313″.

The reason the demotion of Pluto came about apparently, is due to the ongoing attempt to re-classify what is and isn’t to be consider a planet following the discovery by the Hubble telescope of many larger objects in our solar system which had astronomers worried we could soon be looking at 50+ planets orbiting the sun if we were not careful.

Some of these bodies are in fact larger than Pluto (as are many of the moon orbiting some of the other remaining planets) leading to the following guidelines for a celestial body to be considered planetary:

 1) Must orbit the sun.

2) Must be large enough to take on a round, or nearly round shape.

3) Its orbit must be clear of other objects.

That last requirement demoted Pluto whose orbit overlaps Neptune’s.

 

 

A few Pluto facts:

1) Pluto was discovered in 1930 by the American Clyde Tombaugh.

2) At just 1467 miles across (2360 kilometres) Pluto is smaller than some of the moons orbiting the other eight traditional planets in the solar system.

3) Pluto was the furthest planet from the sun and took approximately 247.9 years to fully orbit the sun. Now the further planet out is Neptune.

4) Pluto was named after the Roman god of the underworld.

 

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