Pluto saved – why not the Moon?

The new solar system. Graphic: IAU
Top astronomers are saving Pluto’s pride by allowing it to remain a planet.

They will demote the distant world by reclassifying it as a new object called a pluton. But they have also changed the definition of what a planet is so that it includes the plutons too.

Basically, the International Astronomical Union’s new definition requires a planet to be something big enough to be round – or roundish – and orbiting a star. As long as it is not another star.

That means the sun keeps Pluto as a planet and has three new planets besides, bringing the total number up to 12. Two are also plutons, including 2003 UB313, nicknamed Xena. And a third is freshly promoted asteroid Ceres.

The other of the new plutons, Charon, is a moon of Pluto. It is less than half the diameter of our own Moon. So does that mean our Moon is a planet too? No.

Giant planets Jupiter, Saturn and Neptune all have moons bigger than Charon. Two are bigger then the planet Mercury. But they are not planets either.

Why not? The bizarre explanation is that Pluto and Charon do their celestial dance around a point that lies between them in space.

The Earth and the Moon orbit each other in a similar way. But the point around which they revolve lies beneath the Earth’s surface. If it had been in space, the Moon would have been promoted. The same goes for the moons of the giants.

Dr Mike Brown, associate professor at the California Institute of Technology and discoverer of 2003 UB313, told me: “You’ve hit on just one of the many crazy parts of this definition.”

The third in the IAU’s new list of planets, Ceres, is the largest of the asteroids. Its 950km diameter is a fraction of the Moon’s 3,476km. The IAU say Ceres may be referred to as a “dwarf planet” but stress that that will not be an official term.

To add to the confusion, the IAU is doing away with their former official term “minor planets” for the asteroids. It will be replaced with the phrase “small solar system bodies”.

For the record, the IAU’s proposed new list of planets, shown in the picture above, is: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and, er, 2003 UB313. Now try to make a mnemonic out of that!

Paul Sutherland

Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.
Paul Sutherland

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Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.

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