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  • It's proven to be one of the most divisive issues in modern science.

  • in one stroke, thousands of textbooks were out of date.

  • In my role as Public Astronomer here in Greenwich, I'm always being asked about Pluto.

  • Why was it reclassified?

  • And if it's not a planet anymore, then what on earth can it be?

  • Right from its discovery, Pluto seemed different from the other planets.

  • Here at the Royal Observatory, we have records going back for hundreds of years,

  • and somewhere, we should have a record of the discovery of Pluto.

  • 1917, that's too early... 1987, much too late.

  • Clyde Tombaugh had been set the specific task of finding a planet,

  • so when he saw his moving point of light in 1930, what else was he going to think he'd found?

  • Here we have the President, Council, and Fellows at the Royal Astronomical Society sending the Lowell Observatory

  • their heartiest congratulations on the great discovery of the Trans-Neptunian planet.

  • But trouble was brewing from the word go.

  • Within weeks, Pluto's status was being called into question.

  • Its eccentric orbit and small mass set it apart from the other planets.

  • More importantly, some scientists argued that Pluto might not be alone.

  • Over the decades, our ideas about the formation of the Solar System developed.

  • It was suggested that the edge of the dust and gas cloud from which our planets formed

  • would have been too spread out to condense into planets.

  • Instead, they could be something very different.

  • Scientists reasoned that there could be hundreds of thousands of icy objects on the edge of the solar system

  • that had failed to be incorporated into one of the major planets.

  • They called this region the Kuiper Belt,

  • and Pluto's status as a planet started to be called into question.

  • On the fifth of January, 2005, came the fatal blow.

  • Scientists in California discovered another small moving point of light.

  • This was Eris, a world they believed was bigger than Pluto.

  • This was huge news.

  • The likelihood was, there could be many more large objects out there.

  • Scientists were faced with a choice: either open the doors to potentially hundreds of new planets,

  • or Pluto was for the chop.

  • In 2006, the International Astronomical Union put it to the vote,

  • and it wasn't good news for Pluto.

  • The IAU decided that a planet had to be an object in orbit around the sun,

  • that was massive enough for gravity to squeeze it into a spherical shape,

  • but also it had to be gravitationally dominant,

  • and that meant it must have cleared its surrounding region of other similarly sized objects,

  • but with everything else out there in the Kuiper Belt, this was Pluto's downfall.

  • So where did this leave Pluto?

  • The IAU decided to classify it as a dwarf planet.

  • To many, this would seem as a demotion, but I'm not so sure.

  • As a planet, it was the last gasp on the edge of the Solar System,

  • but now it's an exciting example of a brand new class of objects.

  • Whatever we learn from it will change the way we think about our corner of the galaxy.

It's proven to be one of the most divisive issues in modern science.

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