Pluto used to be the ninth planet of our Solar System, but its not anymore. We only have 8 planets now.
Pluto was discovered in 1930, and from this time until 2006 it was the ninth planet of our Solar System, spending most of its time beyond the orbit of Neptune. However, in August 2006 Pluto was demoted to a Dwarf Planet, and a lot of people, both scientists and non-scientists, were pretty angry.
It was the right choice though. Why? Read on…
When I was in school we had nine planets; Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. But it wasn’t always like this; at one time we had far fewer planets, and at one time we even had more.
The word planet comes from the ancient Greek word ‘planette’, which means ‘wandering star’. Thousands of years ago when the Greeks looked up at the heavens, they realised that some points of light stayed in one place (stars) and that others moved (actually it wasn’t just the Greeks who realised this, many other cultures did too, from the Maya’s to the Egyptians). Everything that moved was called a planet, so to the ancient Greeks the Moon and even the Sun were planets as well. However Copernicus showed humanity this wasn’t the case (for which the Church was absolutely furious) and over time early astronomers realised the Sun was a star, and the Moon was, well, a moon.
For hundreds of years astronomers recognized six planets in our Solar System; Mercury, Venus, the Earth, Mars, Jupiter and Saturn. Then in 1781 the German-English astronomer Sir Frederick William Herschel discovered Uranus, and four more planets were discovered in the early 1800’s, called Ceres, Pallus, Juno and Vesta, giving us a total of 11 planets. 11 planets? Yep. That’s more than today, and that doesn’t even include Neptune either. Never heard of Ceres, Pallus, Juno and Vesta? That’s probably because they’re actually asteroids.
In the 1850’s more and more small bodies were found orbiting between Mars and Jupiter, these bodies were found to belong to a belt of hundreds of thousands of objects, which today we call the asteroid belt. Once it was realised that asteroids weren’t planets, they were demoted, and Ceres, Pallus, Juno and Vesta are rightly not considered planets today.
A very similar process has happened with Pluto. In the 1990’s astronomers discovered that Pluto wasn’t a planet orbiting out there all by itself, it was actually one body in a belt of hundreds of thousands, called the Kuiper Belt. Whereas the bodies in the asteroid belt are primarily made of silicate rock, Kuiper Belt Objects (KBOs) are made from a mix of rock and ice. There are a lot of KBOs; scientists think there are 70,000 with diameters greater than 100 km. It took astronomers such a long time to find the Kuiper Belt as its so far away, and because few astronomers were actually looking for it, preferring to spend their time looking for more interesting objects like quasars and black holes. However, once a few had been found the pace of discovery quickened and hundreds were quickly revealed.
Some astronomers began to argue that if Pluto was actually one KBO amongst hundreds of thousands then it didn’t deserve to be called a planet, just as asteroids aren’t called planets, even the really big ones. The case was made stronger in 1995 when a KBO was found that was larger than Pluto called Eris, if Eris wasn’t a planet, then surely Pluto shouldn’t be either. In fact, some astronomers argued that some moons in our Solar System are also larger than Pluto (Ganymede, Titan, Callisto, Io, Europa, Triton and the Moon), further evidence that it shouldn’t be counted as a planet. However, other astronomers argued that Pluto should remain a planet, especially a group of scientists who were working on an expensive NASA project to send a spacecraft to Pluto, they feared that if Pluto was demoted to a non-planet then the mission may have been cancelled (it wasn’t, New Horizons was launched in 2006 and should arrive at Pluto in July 2015).
This prompted an argument about what the definition of a planet should be, as up until this point there was no strict definition. Some argued for a ‘loose’ definition, in which a planet would be defined as any object that orbits a star and has enough mass that it becomes spherical due to its self-compression and rotation. This would mean that moons, no matter how large, couldn’t be counted, as they orbit another planet rather than orbiting the Sun, and smaller asteroids or KBOs wouldn’t count as they would be too small to become spherical. With this definition Pluto would be a planet, but then so would some other bodies, such as the asteroid Ceres, and a number of KBOs including Charon, Eris and Makemake. If Pluto is to be a planet then these bodies should be planets too.
In August 2006 the International Astronomical Union (IAU) agreed on a definition, and Pluto wasn’t included.
To be a planet 3 criteria must be met:
1. A planet must have enough mass to be spherical
2. A planet must orbit the Sun
3. A planet must have cleared its orbital path (it can’t be part of a belt)
With this definition we’re left with 8 planets, as no objects in the asteroid belt or Kuiper Belt are allowed.
This is the right choice. If Pluto is to be a planet then the other large belt objects should also be planets too, either we have 8 planets or close to 30. I don’t think we should have 30, as although there has been no official definition until 2006, the implicit understanding was that planets were objects that existed outside of belts, back in the 1850s the scientific community decided that asteroids were a different class of body as they were part of a belt of similar objects, and were thus distinguished from planets. Now that we know that Pluto is part of a belt, and not even the largest object in that belt, it shouldn’t be thought of as a planet either.
And we shouldn’t be upset that we’ve changed the number of planets in our Solar System, this has happened multiple times in the past, the more we’ve learnt the more we’ve changed our definition, if Pluto should be kept as a planet then why not keep the Sun and the Moon as planets too? And we shouldn’t be afraid to change our understanding of a planet, especially as it’s for a good reason. Science changes, this is what makes it such a powerful tool for understanding the world around us, as we learn more our old concepts and ideas are altered, to keep Pluto as a planet for sentimental reasons would be an insult to science.