Meet the rocks that killed a planet

Back in 2006, the world of astronomy was torn asunder by the controversial decision to demote Pluto. Overnight, the planet was stripped of its status, becoming just a minor planet. Many astronomers, who felt the decision had been usurped by a minority, were furious (as were countless students who laboured to memorise the planetary mnemonic: ‘My Very Educated Mother Just Served Us Nine Pizzas’).

The trouble started back in 2005, when astronomers discovered another world, hiding in the darkness of the Kuiper belt, that seemed to be bigger than Pluto. The world was named Eris – after the Greek goddess of chaos and strife – and it has been living up to its name ever since.

Its discovery sparked a debate about the definition of a planet, which could have seen the number of ‘planets’ in our solar system swell to 14 or more, instead, in 2006, it saw Pluto being chopped off the end of the planetary roll call. The arguments have been raging ever since.

Earlier this month, astronomers had a rare opportunity to measure Eris’ girth with greater accuracy and, yet again, it is causing trouble. Eris, as it turns out, is actually significantly smaller than was first estimated – small enough to pass the Kuiper Belt crown back to Pluto.

The difference is small, a mere 4km (2.5 miles) narrower than Pluto – but enough for the ‘promote Pluto’ trumpets to start sounding once again. Unfortunately, the planet’s demotion wasn’t based on its Kuiper-ranking alone and all the other reasons for its fall from grace still stand – its wacky orbit and its tiny size, relative to ‘real’ planets.

Also, given the margin of error that comes with measuring a tiny, dark object 39 times farther from the Sun than Earth, its size ranking could still change. Intriguingly, the new measurements have shown Eris to be 25 per cent more massive (the term ‘weight’ doesn’t have much bearing in space) than Pluto – implying that the world contains more rock than icy Pluto.

So, until something changes, the new mnemonic, in Pluto’s absence is: ‘Mean, Very Evil Men Just Shortened Up Nature.’

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