Ceres, Dwarf Planet or Asteroid?

In the previous post I made a mistake about Ceres and its classification as an asteroid or dwarf planet. It’s been a topic of debate for some time, the debate originating with Pluto and its classification from planet to perhaps an asteroid and back to dwarf planet.

Everybody was once sitting pretty thinking that Pluto was just another planet but a long way away and not really that big. This was before the Kuiper belt had been discovered which consisted of loads of larger bodies. In 2003 scientists were scouring the sky and looking at the objects beyond Pluto in the Kuiper belt. They came across a body that was 2500 km in diameter and which was orbiting the Sun. With Pluto’s diameter being 2274 km astronomers were beginning to ask what actually is a planet?

The objects beyond Neptune were starting to be called trans–Neptunian objects as more and more objects larger than Pluto were being discovered. I suppose we could do what the Church tried to do to Kepler and think that the Earth was at the centre of the universe, but nowadays scientists are putting ideas that have no evidence back in the closet until evidence actually arrives. The trouble with Pluto and the bodies in the Kuiper belt was that evidence was now available that Pluto was not alone but had friends further out.

Trans-Neptunian objects

Rather than have 20 or 30 or even more planets and have to write a load of books again and have also a lot of disgruntled schoolchildren with a lot of mnemonics that they would have to learn to remember the new planets it would probably be best, astronomers thought, to have a way of classifying the new objects and also Pluto.

The International astronomical union (IAU) is the organisation responsible for the naming and classification of planetary bodies and their satellites and has been around since the 1900s. It is not an organisation that can enforce anything but it acts as a standard for classification of the planets and other bodies. The main use is for a common frame of reference for scientists and so that they can classify and chat about things without having a debate every time. The IAU also bases everything on scientific fact and has an input from a wide range of fields.

There are inputs to the International astronomical union from professional astronomers, historians, science publishers, planetary scientists, writers and some educators. The International astronomical union formed the planet definition committee and came up with a definition of what a planet actually is and how to classify it. After a lot of shouting, writing and then more writing and shouting three things were to be looked at for the definition of a planet-

• The body should be in orbit around the Sun.
• The body should have sufficient mass so that the gravity of the body creates a nearly round shape.
• The body has cleared its neighbourhood of any junk (I wish there were a few more bodies like that round where we live).

They also came up with two other definitions one for dwarf planets-

• This body must also be in orbit around the Sun.
• The body must also have a nearly round shape.
• The body has not cleared the neighbourhood around its orbit.
• The body is not a satellite, such as the moon.

The last definition was that all other objects, except satellites, were to be defined as small solar system bodies.

This means that there are five objects at the moment that are dwarf planets-

• Ceres
• Pluto
• Eris
• Makemake
• Haumea

so, coming back to the history of Ceres. It was first discovered in 1801 orbiting in the asteroid belt between Mars and Jupiter as the largest asteroid recorded. Lots of other bodies were found in the same region as Ceres and in the 19th century astronomers couldn’t work out the shape and size of this elusive body. These reasons meant that Ceres was referred to as an asteroid or planet for more than a century but had no real definition.

ceres

Ceres is classified now as a dwarf planet because it is large enough to produce a gravitational force that pulls it into a round shape, orbits the Sun in the asteroid belt but because it is in the asteroid belt it is not orbiting in a clear path and many other asteroid’s visit Ceres every now and then.

The classification of planets, dwarf planets and other bodies got really out of hand as there was quite an uproar about poor old Pluto not being a planet any more and having to live its life as a dwarf. Pluto is one of those planets (probably because of the cartoon character)  that has a great name and kids grow up developing a connection with it.

Hopefully, now you can see why it was classified as it is today for purely scientific and classification reasons, which is not a bad idea as you don’t want to spend half an hour trying to explain which planetary body you are talking about if you’re talking about them at all that is.