Pluto celebrates it's 80th birthday

The amazing story of how ‘Planet X’ was found (then lost once more)

For millennia, man looked up to the heavens certain in the knowledge that there were only six planets. Beyond Saturn there was nothing but empty space and the twinkling stellar wallpaper that surrounded it. Then, in 1781, a British chap called William Herschel had the cheek to discover a brand new planet. It was the first to be discovered since Christ’s birth, it doubled the size of the known solar system and it was called, after some debate, Uranus, to the delight of sniggering schoolboys ever since.

The discovery of Uranus soon opened a Pandora’s box of astronomical chaos. It became clear to observers attempting to plot the orbit of this new planet – which should, by obeying Newton’s Laws of Gravity, be moving in a very particular orbit – that it wasn’t travelling through the heavens as it should. Something was amiss.

It wasn’t until 1824 that a German mathematician, Freidrich Bessel, pointed out that the ‘incorrectness’ of Uranus’s orbit could be explained by adding another, as yet undiscovered, planet to the mix – one that was even farther from the Sun and whose gravity was affecting Uranus’s orbit. The quest was on to find this mysterious new body.

The problem fell to more mathematicians who, after looking at the errors in Uranus’s motion and subtracting the gravitational effects of Saturn and Jupiter, determined where the disturbance should be coming from. But scientists in Britain and France, stuck in an ‘astronomy is a science of observation’ mindset, disregarded the calculations and continued squinting away. The calculations were refined in 1846 by a French mathematician called Urbain Leverrier who predicted the mysterious planet’s orbit, mass and even its position. So, in a triumph of mathematical logic, he told astronomers exactly where to look. The stubborn Brits, still ignoring Leverrier’s prediction, were then trumped by the Germans who, on September 23, 1846, pointed a telescope to the predicated area and, within an hour, found the missing planet. Neptune had been discovered.

Rather annoyingly, though, Neptune only accounted for most of Uranus’s orbital discrepancies and it soon became clear that another body must be the culprit. The hunt was on for planet number nine.

However, it was deemed to be too far away and too hard to find so the hunt was abandoned. It would be almost half a century before the trail was taken up once again.

It was re-started in earnest in 1905 by an American aristocrat called Percival Lowell, who used his impressively stocked wallet to build himself ever larger telescopes in an obsessive search for the body he dubbed ‘Planet X’. Unfortunately, ten years later he was still empty handed and, in 1916, his quest ended when he was killed by a massive stroke at his observatory. Ironically, Lowell actually did photograph ‘Planet X’ on two plates but, as it was dimmer than he expected, it was overlooked. He did, however, leave a million dollars in his will to his observatory.

In 1929 a new astronomer arrived at the Lowell Observatory just in time for the completion of its new telescope. The 22-year-old Clyde Tombaugh was a self-taught astronomer raised on an Illinois farm. He spent ten months photographing the sky, scrutinising the images and looking for that tell-tale ‘moving dot’ that could be the missing planet. Then, on February 18, 1930, he noticed that a faint star had shifted position. It was dimmer than expected but Planet X had finally been found.

The discovery was front-page news and, after disregarding several names including a suggestion by Lowell’s widow, Constance, that it should be called, er, ‘Constance’, the ninth planet was officially named Pluto.

Unfortunately, it soon became clear that Pluto was not the planet astronomers had been hoping for. Lowell had predicted that its mass would be more than six times that of Earth but, as more observations were made, it was obvious that it was many times smaller. Not much happened for 40 years until a new generation of telescopes were able to see the planet as a globe for the first time and its size was knocked down to a far less impressive 5,900km in diameter – less than half that of Earth. To make matters worse, it became clear that Pluto could not have been the mysterious Planet X at all. As it was too small to actually affect the orbits of Uranus and Neptune, scientists would have to look for another culprit. They are still arguing about it today.

The years passed and, with each observation, Pluto became smaller and smaller until its diameter was finally revealed as a paltry 2,900km, even tinier than our Moon.

It was a slippery slope from there for Planet X and, in 2005, it reached the bottom of the hill when another, larger body was discovered. Eris, measuring in at 2,500km, arguably had more right than Pluto to be called a planet. More discoveries threatened to push the number of planets to 13, 14 and even more. In 2006 astronomy’s governing body, the International Astronomical Union, decided enough was enough and took away Pluto’s planetary status, relegated it to a minor planet, and returned the number of bodies that orbit our Sun back to eight.

However, the story may not be over. A considerable number of astronomers reject the IAU’s decision and believe Pluto’s place in the heavenly Pantheon should be reinstated.

Anyway – whether planet, minor planet or plutoid – happy birthday, Planet X.

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