Why did Pluto get kicked out of the planet club?

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1. And then there were eight

At school, most of us were taught there were nine planets in our solar system. But something changed and now there's only eight. When Nasa’s New Horizons mission was launched at the start of 2006, Pluto was a planet, but it's not any more.

Pluto is only 1,430 miles (2,302km) in diameter and that makes it smaller than our Moon. Pluto is 3.7 billion miles (5.9 billion km) from the Sun – that’s 40 times the distance the Earth is from it. Consequently, it was the last 'planet' discovered. It took until 1930, because it was so small and too far away to be seen without the aid of a powerful telescope.

It ended up being a case of ‘last in, first out’ as Pluto was kicked out of the planet club in 2006.

2. What makes a planet a planet?

In 2006, the International Astronomical Union put forward a new definition for what makes a planet. Which of these criteria did Pluto fail?

Orbits the Sun

Image: NASA/GSFC/SDO/S Wiessinger

You selected

Orbits the Sun

Wrong

Pluto does indeed orbit the Sun. Nothing strange here.

Enough mass to be round-ish

Image: D van Ravenswaay/ SPL

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Enough mass to be round-ish

Wrong

Pluto has enough mass for its own gravity to shape it into a rigid, nearly round object and even has its own moons.

Warmer than -220C

Image: NASA/SwRI/A Parker

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Warmer than -220C

Wrong

Pluto’s surface temperature is a chilly -233C. The coldest planet is Neptune at a relatively balmy -214C. But this isn't one of the criteria for being a planet.

Cleared orbit

Image: D van Ravenswaay/ SPL

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Cleared orbit

Correct

Planets have cleared their orbital path by pulling nearby objects in, or pushing them away. Pluto's orbit is still strewn with small objects.

3. Bye, bye Pluto

Because Pluto does not have enough ‘gravitational influence’ to clear its neighbouring orbit it was decided it did not make the cut.

Instead it was rebranded as a ‘dwarf planet’.

Dwarf planets are not officially full-fledged planets, so Pluto was stripped of the status it had held since its discovery in 1930.

4. Meet the dwarf planets

Poor Pluto is, however, in good company. There are four more officially classified dwarf planets in our solar system. Take a look at them in the image gallery below.

1/4: Eris (depicted above) is the largest dwarf planet and the furthest from us. Aptly named after the Greek goddess of conflict, its size provoked a debate about whether it should be the 10th planet and ignited discussion over a planet's criteria.

Image: ESO/Calçada/Risinger

2/4: Ceres (pictured above) is the closest dwarf planet to the Sun. It's also the smallest of the known dwarf planets at 590 mile (950km) in diameter. Ceres, along with Pluto and Eris, was classified as a dwarf planet in 2006.

Image: NASA/JPL-Caltech/UCLA

3/4: Haumea (depicted above) is roughly the same size as Pluto but looks very different. Haumea spins on its axis every four hours. This rapid rotation has given Haumea its elongated egg-like shape, making it the least round of the dwarf planets.

Image: NASA

4/4: Makemake (depicted above) has no moons and is about two thirds of the size of Pluto. It takes 310 Earth years for Makemake to orbit the Sun. It is thought to have an extremely low temperature of about -240°C.

Image: ESO/Calçada/Risinger