Why may Pluto not be a planet

A. It all depends on which definition of 'planet' is being used.

Q. Is there more than one

A. So it appears, or rather different degrees of the same basic idea. A planet can be described as a natural object in orbit around a star, and in this case Pluto qualifies.

However if astronomers choose to narrow this down to natural objects that are similar to our Earth or to the outer planets, Uranus and Neptune, then Pluto no longer conforms. Pluto is much more similar to a group of small icy bodies which obit the Sun in a zone called the Kuiper Belt, which Pluto also sits in.

But for the moment Pluto is safe, astronomy's governing body, the International Astronomical Union (IAU) has said that tiny Pluto will not be officially demoted from its planetary status.

Q. Has this happened before

A. Yes, Ceres was declared a planet in 1801 but became demoted to an asteroid a year later.

Q. When was Pluto discovered

A. Pluto has enjoyed its planetary status for 70 years after being discovered in 1930 by Clyde Tombaugh.

Q. If Pluto isn't a planet, what is it

A. Pluto, named after the ancient Greek god of the underworld, isn't a rocky world like Earth, Mars, Mercury or Venus, or a gas giant like Jupiter, Saturn, Uranus or Neptune.

Pluto is small, smaller than our moon, and seems to be more closely related to other lumps of rock and ice that keep it company in the Kuiper Belt and maybe just a comet. One other theory is that it once a moon to Neptune and at some point, somehow escaped its gravitational pull and went into its own solar orbit.

Q. Will we ever know the truth

A. Pluto is the only planet in our solar so far not to have been visited by a space probe. But this is about to change as NASA is intending to launch a craft sometime in the next few years. However given Pluto's huge distance from Earth information on what it really is made of still wouldn't be available until 2020 at the earliest.

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by Lisa Cardy