Question: is pluto a planet

Derek McKay-Bukowski answered on 10 Jun 2011:

Great question… the best one I’ve had so far. Well, the answer is “not any more”.

You see, astronomers _used_ to regard Pluto as a planet. But that was until Eris was discovered in 2005. Eris is actually bigger than Pluto, so did that make Eris a planet too? And what exactly is a planet? Astronomers considered these questions very carefully and concluded that this is what makes a planet:

— Orbits a star (in our case, the Sun)
— Is not so big that it starts burning (that would then be a double star)
— But big enough to sweep all the rock and dust out of the way.

It is the last of these that Eris, Pluto, Ceres and the other asteroids and various other “dwarf planets” can’t do. Where they are, there is still plenty of debris orbiting about. Only “proper” planets have cleared their own orbits out. That’s Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Thus, in 2006, after much argument, astronomers official declared that they had decided that Pluto is not a planet.

However, some people will still call Pluto a “dwarf planet”, to indicate that although it is not a real planet, it comes close.