THE RISE AND FALL OF PLUTO

 On 18th February 1930, Tombaugh compared an image taken on 23rd January 1930 with one taken on 29th January 1930. In them, he noticed an object which appeared in both images but in different places. Believing that he may have found Lowell's Planet X, he looked at another image from 21st January and again noticed the same object but in another place. He now knew that the object he had found was definitely not a star and shortly afterwards, the existence of the Solar System's ninth planet was confirmed. The new planet received the name Pluto, a name suggested by 11 year old schoolgirl Venetia Burney from England.

The two images that gave Tombaugh his first sightings of Pluto.

Initial estimates put Pluto at a similar size and mass as Earth, estimations that have since been reduced greatly. It has to be remembered that the search for Pluto began because scientists didn't believe that Neptune alone could have the gravitational effect on Uranus that it appeared to have. A planet the size of Pluto, even if it was as big as Earth, simply wouldn't have much, if any, of an effect on Uranus' orbit. This led to the possibility that Pluto wasn't the "Planet X" that Lowell had been looking for and that it remained undiscovered. However, it later turned out that Uranus's orbit could actually be explained by Neptune's presence alone. Scientists had actually slightly miscalculated Neptune's mass and by correcting their calculations, Uranus' orbit suddenly made sense. It was actually purely coincidental that Pluto happened to be located near to where the non-existent Planet X was expected to be. It's also quite fortunate too or else Clyde Tombaugh may have had to continue flicking between images for many more years searching for something that didn't exist! It is interesting to consider that, had Neptune's mass been calculated correctly at first, the search for Pluto would never have started. Who knows when, or even if, it would have been found? 

In 1978, Pluto was found to be accompanied by a moon. Charon, not much smaller than Pluto itself, travels around Pluto closely. Technically, Charon doesn't actually orbit Pluto. The two objects spin around a point between each other - a bit like an invisible axis and the two bodies together are sometimes known as a binary planet. The discovery of Charon allowed astronomers to do some fancy maths and work out a more accurate size and mass of Pluto, revealing that it was actually smaller than all of the other planets in the Solar System and also smaller than seven moons. This once again confirmed that it definitely isn't able to have much of an effect on Uranus' orbit. 

At the beginning of 2006, NASA launched New Horizons, a spacecraft which would be the first to visit Pluto and Charon, therefore meaning that, once it reached them in 2015, each planet in the Solar System will have been visited by spacecrafts. However, in the middle of 2006, Pluto received a huge blow. It lost its status as a planet!

THE PROBLEM PLANET

Pluto has always been a problem planet. Although it had been recognised as a planet since its discovery, it had always been the odd one out; the dark sheep in the Sun's family. Although every planet is different from the other, they are quite often split into two groups: the Inner Planets and the Outer Planets. Quite logically, the Inner Planets are the planets closest to the Sun and the Outer Planets are the furthest. The Inner Planets are Mercury, Venus, Earth and Mars. All four are small planets with solid surfaces. The Outer Planets are Jupiter, Saturn, Uranus and Neptune, four huge planets made up mostly of gas. Pluto is small and has a solid surface, so is similar to the Inner Planets, but because it orbits beyond Neptune, was classed as an Outer Planet despite not sharing any similarities at all to them. Another of Pluto's problems is its orbit. All of the planets in the Solar System orbit the Sun on the same plane as each other. Imagine that they are travelling on a circular racetrack with the Sun positioned in the middle of it. The racetrack they travel on is flat meaning that they all travel fairly level with each other. Scientists like to call this flat level or plane the "ecliptic". Pluto, being a bit odd, doesn't orbit on the same plane. Instead, it has an inclined orbit meaning that it goes above and below the rest of the planets. The diagram below shows what the Solar System would look like if you were able to see it from its side. 

As well as having an inclined orbit, Pluto also has an elliptical orbit (not to be confused with the word "ecliptic" mentioned above!). This means that its orbit is more oval-shaped than circular. No planet orbits in an exact circle but there is usually only a small difference between the closest distance a planet is away from the Sun (its "perihelion") and its furthest distance (its "aphelion"). For example, during its year-long journey around the Sun, Earth can get as close as 147,098,074 kilometres from the Sun and as far away as 152,097,701 kilometres from it. This may be a difference of five million kilometres but, when you're talking about space, that's not much at all! Pluto however, gets as close as 4,436,824,613 kilometres from the Sun and as far away as 7,375,927,931 kilometres. This means Pluto's furthest distance is almost double its closest distance, and when you consider that Pluto is already incredibly far away from the Sun at its closest, it's difficult to imagine how far away it actually gets. Also unusual is that fact that when it is at its closest point to the Sun, Pluto is actually closer to the Sun than Neptune! The diagram on the right shows Pluto's orbit compared to the orbits of the other planets and a few comets.

So, that's Pluto. A world which possesses the characteristics of an Inner Planet but orbits so far away it is classed as an Outer Planet despite being nothing like them; a world which orbits on a different plane to the rest and has a massive difference between its closest and furthest points from the Sun; a world which crosses the path of Neptune every couple of hundred years - although the two objects will never collide. Astronomers in the past sometimes considered Pluto to be an escaped moon of Neptune, but couldn't justify this due to its eccentric orbit. It was also considered to be a comet, which also have elliptical orbits, but as most comets are a few kilometres in diameter, Pluto would be too large to be one. Pluto remained a planet simply because it didn't fit into any other category and there was nothing else like it in the Solar System. Or was there? 

INTRODUCING ERIS

Pluto orbits in an area called the Kuiper Belt. For some time, the area has been known to contain small asteroid-like objects. These objects are known as Kuiper Belt objects (KBOs) or, because they orbit beyond Neptune, Trans-Neptunian Objects (TNOs). In 2004, astronomers discovered a new world orbiting beyond Pluto in the Kuiper Belt. They nicknamed the new discovery "Xena" and found out that this world was actually larger than Pluto and, like Pluto, had an inclined and highly elliptical orbit. Since Pluto was then classed as a planet, this would mean that the newly-discovered world should also be a planet. But, astronomers were not happy for this to be the case. "Xena" was not the only potential planet out there. There were others. These other Kuiper Belt Objects were smaller than Pluto and Xena, but again possessed inclined and elliptical orbits. So, technically, they should also be planets. But if we were to start calling all of these small worlds planets, the Solar System would soon have tens, maybe hundreds, of planets in it. Not only does this take something away from the status of planet held by the more well-known planets, it would also be a nightmare to have to add a new section to this website for every new planet that gets announced! Astronomers decided that they needed to get the Solar System sorted out before it got too confusing. So in August 2006, a bunch of important ones met in Prague in the Czech Republic to discuss the state of the Universe and how to simplify it. One of the things they wanted to do first was define exactly what a planet is. Now, this wasn't the first time that the definition of the word "planet" had been up for discussion. In fact, it had happened twice before. So before we continue, we need to go back in time.

If we fast-forward ourselves to the August 2006 meeting of the International Astronomical Union (IAU) in the Czech Republic, we find that astronomers there now face a similar problem to the one they had in the middle of the Nineteenth Century. Imagine that the planets are members of an exclusive club and that the astronomers are responsible for approving memberships. In 2006, they discovered that they had a large number of objects that one day may want to join the Planet Club just because Pluto is a member. The solution was to draw up new rules for what it takes to obtain membership of this most ancient and esteemed group. And here they are.

SO YOU WANT TO BE A PLANET? HERE'S HOW!

1) You must orbit the Sun. That's the big hot bright thing in the middle. If you orbit something else, turn away now.

2) You must be massive enough to be round (or nearly round) in shape. Irregular-looking objects need not apply.

3) You must have cleared the neighbourhood of your orbit. This means that you will either have absorbed any bits and pieces floating your orbit into yourself, usually during your formation, or you have acquired them as moons, or if you are large enough, rings. So, if there are other bits of junk and trash in the area you orbit, but you fit the first two criteria, you may want to look into becoming a new dwarf planet.  

These new rules had an immediate affect on Pluto. Because Pluto orbits in the Kuiper Belt, it orbits in a region shared with other objects, meaning that it hasn't cleared its neighbourhood and can no longer be classed as a planet. Instead, it would be known as a dwarf planet, a round object which orbits the Sun but hasn't cleared its neighbourhood. This also meant that the new object "Xena" that had been discovered in 2005 also met the criteria to be a dwarf planet. Then again, its discovery was the main reason for the creation of the new definition so it would have been a bit strange if it didn't become one! "Xena" was renamed Eris and became classified as a dwarf planet from the introduction of the term. The new definition also affected a much older world. Ceres, orbiting peacefully in the Asteroid Belt and keeping itself to itself, also fitted in with the dwarf planet definition. Like Pluto and Eris, it was a round object orbiting the Sun in an area that it hadn't cleared. It too became known as a dwarf planet bringing the total number of dwarf planets in the Solar System to three. Since the introduction of the term, a further two worlds have been classified as dwarf planets: Haumea and Makemake. These are worlds smaller than Pluto and Eris, but larger than Ceres, that orbit beyond Neptune in elliptical and inclined orbits. 

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