It’s proven to be one of the most divisive issues in modern science. In one stroke, thousands of textbooks were after date. As astronomer, I’m always being asked about Pluto, why was it reclassified? And, if it’s not a planet anymore, than what can it be? — So, why isn’t Pluto a planet anymore?

Right from its discovery, pluto seen different from the other planets. Back in 1930, Clyde W. Tombaugh, who was using the Lowell Observatory in Arizona, had been set the specific task of finding a planet. So, when he saw his moving point of light in 1930, what else he was going to think he’d found. It was a great discovery, the first trans-Neptunian planet. But, trouble was brewing from the word go.

Within weeks, pluto status has been called into question. Its eccentric orbit and small mass set it apart from other planets. More importantly, some scientists argue 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 are planets formed, would have been too spread out to condense into planets. Instead, that could be something very different.

Scientists reasoned that there could be hundreds of thousands of icy object 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 so, Pluto status as a planet started to be called into question.

On the 5th 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 door to potentially hundreds of new planets or Pluto for the chop. In 2006, the International Astronomical Union (IAU), 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 have to be gravitation dominant. 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 does this leave Pluto? The IAU decided to classify it as a dwarf planet. To many, this was seen as a demotion, but I’m not so sure! As a planet, this 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 learned from it, will change the way we think about our corner of the galaxy.



