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The Story of Pluto


If you ask most adults, or look in books more than about ten years old, they will tell you that there are nine planets in the solar system, rather than the eight we talk about today. But about ten years ago, one planet got dropped. The story of the missing object – Pluto – is one of discovery, debate and a momentous decision that explains how we found a whole new class of objects: the dwarf planets, explains Dr Elizabeth Stanway.


When we look at our night sky with the naked eye, as well as the Moon, you can generally see the five classical planets: Mercury, Venus, Mars, Jupiter and Saturn, each further from the Sun than the one before. When astronomers started using telescopes they found another - Uranus, identified by Sir William Herschel in 1781. But Uranus seemed to be pulled in a direction no one quite understood – something big had to be lurking out there, and by figuring out where and looking hard, astronomers Adams and Le Verrier found Neptune in 1846. So, including Earth in third place, we had the eight planets we know today.

Astronomers studying Neptune saw that it too wobbled in a surprising way – so they predicted, and eventually found a ninth object. This cold and distant rock was seen by Clyde Tombaugh in 1930 and eventually named as the new planet Pluto – after the god who presided over Ancient Greece’s cold, dark underworld.


So Pluto had been discovered, but fairly soon its surprising properties caused debate. The planets we already knew about in the outer Solar System were much larger than the Earth and made of cold gas or frozen ice. Pluto, on the other hand, was smaller than Earth (we now know it’s got a mass only two thousands that of Earth) and largely made of rock. In fact, it’s not that much larger than some of the big asteroids like Ceres and Pallas that orbit between Mars and Jupiter. Pluto was also in a weird orbit, stretched into an ellipse and tilted compared to all the other planets. Even so, it seemed to be the biggest thing out there, so the planet label stuck for about 70 years, simply because it was planet-like and what else could it be?

But around the year 2000, scientists started looking hard at the scattered rocks which lie even further from the Sun than Pluto – a region known as the Kuiper belt. One team studied the sky for faint, distant objects than seemed to be moving too slowly to be the asteroids we knew about and some of them looked uncomfortably similar to Pluto. So were these big space rocks – Quaoar, Sedna, Orcus, Makemake and probably many others – also planets? In 2005 a new object was found which really challenged our ideas: this rock wasn’t just similar to Pluto, it actually seemed to be bigger! If Pluto was a planet, then this new rock (eventually named Eris after the Goddess of Discord), definitely had to be.


So did the Solar system have just eight planets? Did it have ten? Did it have dozens, or even hundreds? Where did we draw the line?

In August 2006, the International Astrophysical Union met to discuss this question and it wasn’t easy to find an answer. Generations of people had grown up thinking Pluto was a planet and they didn’t want to let it go but the evidence was overwhelming: whatever the IAU decided would change our picture of the Solar System forever.

In the end, they realised that all the major planets had an important thing in common: there was nothing else even close to their own size close to them. Anything that started out there had either collided with the planet, been collected up as a moon, or been thrown out in space: they had cleared their orbits. But Pluto had all these companions, close to its own size, and nearby in spatial terms. The IAU decided that Pluto had failed an important test. It couldn’t be a planet any more.

Dwarf Planets

Pluto, as well as Eris, Ceres and several others, is now known as a dwarf planet. These are big enough that they’ve become rounded into a ball by gravity, but not big enough to get rid of their neighbours.

So the story of Pluto is one of how we found a planet, how we lost it but then found out about a whole new type of object that shares the solar system we inhabit.


29 May 2020



Images - credit NASA:

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