Why Pluto is not a planet
We’ve all grown up learning that there were nine planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.
Perhaps you even learned sayings like Many Visitors Eat Mouldy Jam Sandwiches Until Nearly Puffed to remember them but on 24th August 2006 the International Astronomical Union (IAU) decided that Pluto wasn’t a planet any more.
Things are changing in science all the time, but rarely does something as large as a planet go missing.
To understand a decision which shocked and upset many both within and without the scientific community, we need to go back to the start of the story.
The story of Pluto is rather a sad one really. The more we’ve learned about it, the less significant it seems.
Pluto was discovered in 1930 at the Lowell Observatory in Arizona by Clyde Tombaugh who was just 23 at the time.
It was named after Roman God of the underworld due to its mysterious circumstances and dark and cold nature. The name was chosen by Venetia Burney, an 11-year-old schoolgirl, who received £5 as a reward which was a fortune then.
Pluto is/was the most distant of the planets from the sun and hence the coldest.
Pluto is also the smallest of the planets. When it was first discovered, it was thought to be the same mass as the earth, but we’ve since discovered it’s very small (about 1/450th of the Earth’s mass) and very reflective (more so than other planets)
So it just appeared bigger than it was because it’s so bright. This is because Pluto is covered in ice.
Pluto’s orbit is very different from the other planets. They have orbits which are nearly circular whereas Pluto’s is elliptical (like a comet)
The bottom diagram shows most of the planets orbit on a flat plane while Pluto’s orbit is at a steep angle.
This is evidence that, unlike the other planets, Pluto doesn’t come from our solar system but was drawn into orbit around the sun.
Other evidence for this is the largely icy nature of Pluto’s make-up, quite unlike any of the other large gaseous planets in the outer solar system.
Despite this Pluto was considered the ninth planet of the Solar System, because it orbits the sun and is heavy enough to have formed into a sphere under its own gravity.
What first started to threaten Pluto’s status as the Solar System’s ninth planet was the discovery beginning in 1992 of objects of similar size, composition and orbit nearby in the outer region of the solar system known as The Kuiper Belt.
In 2005, a planetoid called Eris larger than Pluto was discovered nearby. So no longer was Pluto seen as a unique body in space, but just one of a group of tiny, far-flung planetoids – and it wasn’t even the biggest.
If Pluto is a planet, then there was no reason that these newly discovered planetoids shouldn’t also be given planet status.
Most astronomers don’t want to add lots of extra, tiny planets to the solar system.
In 2006 the IAU formally defined the criteria for planet status. A planet must orbit the sun and be heavy enough to have formed a spheroid (nearly a sphere) under its own gravity. Finally, a planet must be heavy enough to have ‘cleared the neighbourhood’ around its orbit. Pluto does not qualify on this count because there is a far greater mass than the planet sharing its orbit.
For the first time since 1930, the solar system has only eight planets. The new category of Dwarf Planets, separate from Planets proper was created for Pluto, Eris and other minor planets.
The decision proved highly controversial because the third criterion is thought ambiguous by some.
A lot of people are fond of Pluto and feel that for historic reasons it ‘deserves’ a place in the Solar System.
Pluto has simply fallen victim to advances in science – the more we learned, the less unique we realised it is but for most people it will always hold a special place in the solar system.