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Is Sedna a real planet, or what? It's time to define some limits ; Astronomers can no longer go on with their loosey-goosey ways.

Posted on: Wednesday, 17 March 2004, 06:00 CST

The solar system's newest member may be a reason to rejoice, because Old Sol's family just went from nine planets to 10. Alternately, it could be a reason to ponder whether the real number of planets in our solar system should have been eight all along.

At issue is the definition of "planet," a question made harder because, in scientific terms, there is no definition.

The ancient Greeks came up with the name, which means "wanderer." That's because, unlike the stars, which never move relative to one another, planets wander about in the sky (though they keep to the path of the sun, called the "ecliptic.")

Astronomers, however, have placed no size limits on planets, although they aren't calling any of the asteroids between Mars and Jupiter by that exalted name - probably because there are thousands of them.

Now comes "Sedna," an object slightly smaller than Pluto, which itself is slightly smaller than Earth's moon.

Pluto was called a planet when it was discovered in 1930, but that's been controversial: Its strange orbit sometimes brings it closer to the sun than Neptune, the gas giant that clearly meets the planet category - whatever it is.

However, Sedna is much farther away than Pluto. If it gets to keep its title, Sedna will continue the process of labeling planets after divinities, as that's the name of the Inuit goddess of the sea. Some astronomers are speculating that it may be the first object to be detected that is in the "Oort Cloud," a group of icy objects on the edge of interstellar space where comets hide at the far end of their paths around the sun.

So, is Sedna a planet, or something smaller? If it isn't a planet, then should we redefine Pluto's status? Most importantly, did the solar system just get bigger, or smaller?

It's time for astronomers to say what a planet is. That will let us move on to important questions, such as, "If people settle on Mars, what will they use for ZIP codes?"

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