for National Geographic News
Astronomers proposed a new definition of "planet" today.
The proposal would add at least three more planets to the nine we're familiar with, instantly outdating textbooks.
But it could add as many as 50and that's a problem, some researchers say.
The term "planet" has never had an official definition.
Many scientists have long argued that it was a mistake to call Pluto a planet, since it is much smaller than the other eight generally accepted planets.
Recent discoveries have intensified the debate over Pluto's status and brought the issue to the forefront of the astronomy community.
Now the International Astronomical Union is meeting in Prague, in the Czech Republic, to debate how to best define the term based on physical properties.
Astronomers vote on the proposal next week.
Planets and Plutons
The IAU proposal says that a planet is an object large enough to have become rounded due to the force of its own gravity.
But it's not that simple. What counts as a planet also depends on what it's orbiting around.
A planet has to orbit a star, so rounded objects floating freely through space won't make the cut.
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