Whatever happened to Pluto?

We all remember being taught about the nine planets when we were at school. In recent years, things have changed and there are now only eight, writes space scientist Dr Lucie Green.

It seems that reality TV shows are not the only places you can vote things out - Pluto was kicked out of the planet club in 2006 because of a vote.

It ended up being a case of "last in, first out". Pluto was the last planet discovered, because it was too small and too far away to be seen without the aid of a telescope.

Pluto is six billion km from the Sun - that's 40 times the Earth's distance from the Sun. It took until 1930 for Pluto to be found and even then its discovery was almost an accident.

Dirty icebergs

• Pluto was a planet for 76 years until it was reclassified as a dwarf planet in 2006
• Pluto's existence was first predicted by Percival Lowell in 1915 and was discovered by Clyde Tombaugh in 1930
• Pluto is now thought to be an object in the Kuiper Belt, a disc-shaped area of icy, dark objects beyond Neptune

• "A faint black dot" - watch how Pluto was first discovered
• Diamond crystals and balls of gas - a quick guide to the Solar System

For several decades it had been believed by some people that a 'Planet X' was orbiting beyond Uranus and Neptune and was causing slight changes to the motions of these planets.

In fact, Pluto isn't large enough or in the right orbit to do this, but a search of the skies for Planet X by Clyde Tombaugh at Lowell Observatory in Arizona led to the discovery of a tiny spot of light that moved against the background stars.

He realised that this tiny spot was an object which was orbiting the Sun and it was announced that a new planet had been discovered.

Planets are not the only things in our Solar System, various groups of objects orbit the Sun. But the main group is the planets, bodies of rock or gas that orbit the Sun.

Then there are the moons orbiting these planets. Earth has just one moon whereas the king of the planets, Jupiter, has over 60. It turns out that even Pluto has three.

In addition to the planets and their moons we have a group of objects called asteroids. These are smallish lumps of rock and metal, most of which orbit the Sun at a distance between that of Mars and Jupiter.

Finally there are the comets, dirty icebergs that orbit at vast distances, perhaps 50,000 times further from the Sun than the Earth is. Asteroids are thought to be remnants of a planet that was unable to fully form and comets are thought to be leftovers of the formation of the Solar System itself.

But the search for objects at the edge of the Solar System continued after the discovery of Pluto.

The search has been fruitful - many hundreds of small objects have been found beyond the orbit of Neptune and in 2003 an object was found that is more massive than Pluto and has a moon of its own. A new planet.

The possibility of discovering more objects of this size quickly became apparent and if Pluto was classified as a planet, then so should all these new objects.

Things were going to get out of hand. It was time to take a step back and think about what we actually mean by a planet and whether Pluto and the other similar objects should have planetary standing.

This task fell to the International Astronomical Union (IAU) - the organization that represents astronomers and astronomical research, and has the responsibility for overseeing how objects are named.

A Planet Definition Committee was formed and members of the IAU debated and discussed until an agreement was reached about what a planet should be defined as.

Demoted

It was decided that it was not good enough for an object to simply orbit the Sun, have enough mass for its own gravity to shape it into a rigid, nearly round object, and have its own moons.

To be a planet an object also has to have sucked up all the other material around it. All other eight planets have swept their orbit around the Sun clean from other objects. Pluto however, lives in a belt of other similar-sized objects.

FIND OUT MORE

A vote was taken at the closing ceremony of the IAU's General Assembly on 24 August 2006 and the fate of Pluto was sealed. Pluto, surrounded by hundreds of other objects orbiting beyond Neptune, was to be demoted from a planet to a new category of objects in the Solar System - a dwarf planet.

One of my colleagues was amongst the voters and I remember him commenting that it made him feel like a controller of the Universe.

I attended the IAU General Assembly in 2012 where once again, at the closing ceremony, a vote was taken, but this time on the definition of the Astronomical Unit - the distance between the Sun and the Earth which is a standard measurement in astronomy.

It was voted that the Astronomical Unit be defined by the exact distance of 149,597,870,700m rather than a value which varied dependant on the mass of the Sun which is very slowly changing over time.

So, the IAU has clear definitions of how objects in our Solar System should be classified. Even though this is necessary, it is slightly artificial to think of discrete types, giving an arbitrary cut-off due to distance from the Sun or nature of the orbit.

Better perhaps to have a spectrum of objects where when one type ends and other begins. For example, in recent years astronomers have found objects that seem to look like both asteroids and comets.

What next?

Because it hasn't yet been visited by a spacecraft, we still have no detailed view of what Pluto is like. A Nasa mission is on its way though - New Horizons was launched in 2006 and will arrive in 2015.

It is interesting to reflect that upon launch New Horizons' target was a planet, but upon arrival it won't be.

The classification of Pluto isn't important though. What is important is that it is part of our culture and symbolises our desire to understand the Universe on our doorstep - it's a Solar System object that we need to learn more about.