Name: Why is Pluto not a planet? - Sky at Night: Pluto Revealed Preview - BBC Four
Uploaded: 2015-07-31T05:00:27.000Z
Duration: 3 min 42 s
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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in one stroke, thousands of textbooks were out of date.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.

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.

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.

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,

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

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.

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.