Astronomers are now trying to re-define the concept "planet" based on recent discoveries. But this is not as easy as it looks:

The effort has been ongoing for several years, as new sorts of objects in the solar system and beyond have rendered the traditional idea of planet useless to astronomers and confusing for the public.

The announcement in July of a world larger than Pluto orbiting our Sun out beyond Neptune brought discussions to a head. A 19-member working group within the International Astronomical Union has been scrambling ever since to reach consensus, but to no avail.

The main sticking point: If Pluto is a planet, then so is 2003 UB313, the object discovered in July. But by that logic, there are several other round objects nearly as big as Pluto that should be considered planets, some astronomers say.

The compromise currently being floated by the working group is to add an adjective in front of the term planet for each different type of non-stellar round object.

Pluto and 2003 UB313 could be called Trans-Neptunian objects. Earth would be called either a terrestrial planet or perhaps a "cisjovian" planet, meaning it's inside Jupiter.

Further complicating the matter are extrasolar planets much more massive then Jupiter, planet-like objects orbiting dead stars called pulsars, and possibly even free-floating worlds that don't orbit stars.