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The answer to this question is simple: "it depends on your definition of a planet." In 1930, after decades of searching, astronomer Percival Lowell discovered Pluto nearly four billion miles from the Sun. Apparently about the size of the Earth and much larger than any asteroid or comet; Pluto was quickly declared the ninth planet. Since that time, astronomers have been scouring the heavens looking for another possible planet, deemed Planet X, on the edge of the solar system.

Finally on July 29, 2005, two US observatories announced the discovery of a new object, named 2003UB313 that was larger than Pluto and nearly 9 billion miles from the sun. This new body, with the provisional name of "Xena", has been hailed by the media as the tenth planet. The same day, incidentally, a potential eleventh planet, 2003EL61 , was announced as well. In addition, at least two other large Pluto-like bodies have also been observed in the outer depths of the solar system. So, depending on your point of view, we could say that the Sun has 10, 11, or 13 planets now; given the advanced telescopes opening in the next decade, it seems almost certain that at least a dozen or more of these new "planets" will be discovered.

Some astronomers question whether all these new objects should be considered planets. Take Pluto for instance: as telescopes improved over the 75 years since its discovery, astronomers learned that Pluto was much smaller than originally thought. In fact Pluto is now known to have a diameter of only 1400 miles, only 60% the size of our moon. In recent years astronomers have identified a whole class of ice bodies beyond Neptune in the so-called Kuiper Belt (analogous to the asteroid belt in the inner solar system). Most astronomers agree that Pluto, its moon Charon, Xena, and hundreds of other trans-Neptunian bodies are just Kuiper objects.

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