Bob the Alien's Tour of the Solar System
Bob the Alien's Tour of the Solar System

The Rise and Fall of Pluto


Before the invention of the telescope in the early part of the Seventeenth century, it was thought that Saturn was the most distant planet in the Solar System. This changed when Uranus was found in 1781 by English astronomer William Herschel, its discovery doubling the size of the known Solar System. Without the aid of a telescope, it would have been virtually impossible to find Uranus since it is barely visible with the naked eye.

Observations of Uranus revealed something strange about its orbit. It was as if there was something beyond it which was having a gravitational influence on it. It was believed that there was likely to be another planet after Uranus. Mathematicians did some sums and worked out where they expected this planet to be. Using these calculations, German astronomer Johann Gotfried Galle found the planet in 1846. It became known as Neptune and took the total number of planets to eight (although, as you'll find out later, it was thought to be the Solar System's twenty-third planet at the time!) and once again the size of the known Solar System had doubled. But, based on early calculations of Neptune's mass, astronomers didn't believe that the gravitational influence of that planet alone could cause the effect that it was apparently having on Uranus' orbit. This implied that there was another planet beyond Neptune!

Lowell, Tombaugh and Planet X

Percival Lowell

American astronomer Percival Lowell (pictured right) had founded the Lowell Observatory in Flagstaff, Arizona in 1894. From 1906, he began looking for a ninth planet in the Solar System, a planet which he termed "Planet X". Like the method used for discovering Neptune, Lowell used mathematical calculations to locate his planet. Expecting to find a large world of gas similar to Uranus and Neptune, Lowell instead found nothing and died in 1916 having failed to discover his Planet X. The search for a new planet ended and wasn't resumed until 1929.

In 1929, Clyde Tombaugh was a young astronomer working at Lowell's Flagstaff Observatory. He got the job there by sending the observatory some drawings he had made of his observations of Mars and Jupiter through his home-made telescope. At Flagstaff, he was given the task of taking images of space and comparing the images with others taken of the same region on a different day. Because planets can be identified by their movement when compared to the movement of stars (stars always appear to move together), any object which appears in different places in the images being compared is likely not to be a star. Images were transferred to photographic plates which were put into a blink comparator, a machine which rapidly alternates the two images.

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.

Pluto discovery plates
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.

Pluto orbit from side

Orbits of outer planets

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. 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.

What is a planet?

Diagram of Earth-centred universe

Hundreds of years ago, people believed that Earth was at the centre of the Universe. Apart from the stars, anything that orbited Earth was a planet. For a while, this included the Sun, the Moon and the five other known planets: Mercury, Venus, Mars, Jupiter and Saturn. The diagram on the right shows how people used the think the Universe looked. Astronomers like Nicholas Copernicus and Galileo Galilei put forward the idea that the Sun was actually at the centre of everything and not Earth, but people more important (but not more intelligent) told them they were wrong. In 1610, Galileo discovered four objects around Jupiter - objects that we now know as Io, Europa, Ganymede and Callisto. Since they travelled with Jupiter as they orbited Earth, they were considered to be planets. Galileo himself actually called them the Medicean Planets. In addition, five moons discovered to orbit Saturn (Titan, Iapetus, Rhea, Tethys and Dione) were considered to be planets too. However, as the evidence piled up that the Sun was actually at the centre of the Solar System, and people finally accepted this as being correct, the objects in it were redefined. Basically, any object that orbits the Sun directly would be a planet, and any object that orbits a planet would be a moon. So the Moon became a, er, moon, and the newly-discovered objects around Jupiter and Saturn also became moons. Simple! As mentioned earlier, when Uranus was discovered to orbit the Sun, it became classified as a planet, and objects found to orbit Uranus became its moons. The same happened when Neptune was discovered. But, in between the discoveries of Uranus and Neptune, astronomers discovered some other objects slightly closer to Earth.


In 1801, Italian astronomer Giuseppe Piazzi discovered a small-ish round object orbiting the Sun between the orbits of Mars and Jupiter. This object became known as Ceres and because it orbited the Sun, it fitted in with the criteria of what it takes to be a planet and became recognised as one. Over the next few years, other objects were also found between the orbits of Mars and Jupiter. These were Pallas, Juna and Vesta. Again, they were classified as planets. In 1845, Astrea was found, again orbiting between Mars and Jupiter. Neptune was found the following year, followed by a further ten objects again between Mars and Jupiter. This means that after their discoveries, Neptune was actually considered to be the twenty-third planet in the Solar System! The problem though was that many of the objects discovered between Mars and Jupiter were small, irregularly-shaped and quite unremarkable, nothing like the more well-known planets. And it was expected that there would later be discovered many more similar objects in the same region. So, in 1854, it was decided that only large objects orbiting the Sun should be known as planets. Small objects in orbit of the Sun would be called minor planets, or asteroids (rather strangely, the word asteroid is Greek for "star-like"). The asteroid-populated region between Mars and Jupiter became known as The Asteroid Belt. This meant that Ceres, despite probably being large enough to be classed as a planet, still became recognised as an asteroid just because it was situated in the Asteroid Belt. 

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. 

New Horizons

Pluto's reclassification means that it sadly is no longer recognised as a planet. The Solar System is now seen as containing eight planets and five dwarf planets and loads of moons, asteroids, comets and meteoroids. Some people, including astronomers, still refuse to accept that Pluto is not a planet. Despite its small size, throughout most of the Twentieth Century, Pluto has been seen as an important member of the Solar System, possibly due to its oddities. Its reclassification of status to dwarf planet is seen by many as demoting its importance. Probably as a way to give Pluto back some of its former glory, the word Plutoid has been created to refer to any dwarf planet which orbits beyond Neptune. So Pluto, Huamea, Makemake and Eris are now all Plutoids as well as being dwarf planets! Pluto isn't the largest of the Plutoids, but since the term is derived from its name, it means that Pluto is the template for what it takes to be one and, in a way, ensures that it remains, at least for now, the most important of the Plutoids. Also worth remembering is that in 2015, the New Horizons space craft is expected to reach Pluto and its moons. This will make Pluto the most distant object ever to have been visited by a space craft and will undoubtedly bring more attention to the tiny world. By discovering more about Pluto and accepting that it is a template for other Plutoids, it will help us to understand more about these small icy worlds that orbit so far away from the Sun. Pluto may not officially hold the status it once did, but for many people, it remains as important as it has always been.

Bob the Alien's Tour of the Solar System
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