Hi Sierra. That is a very interesting question these days!

Astronomers have discovered another object going around the Sun which is even bigger than Pluto. It even has a moon. It does not have a name yet. Here is an article about it:
http://science.nasa.gov/h...05/29jul_planetx.htm

Astronomers think that there are probably lots of objects about the size of Pluto out in the far reaches of the solar system. They are small and dark, so they are hard to see and we are just now beginning to discover them. If this is true then there may be hundreds of planets!

Of course, that all depends on what you want to call a planet. Maybe people will decide that these should not be called planets (similar to the way asteroids are not considered official planets). If that happens then maybe we should not consider Pluto a planet, which would get us down to 8 planets.

Here is an essay about the whole problem. It was written before the 10th planet was found, but after the discovery of another object named Sedna which was not quite as big as Pluto:
http://www.spacedaily.com...uterplanets-04b.html

Terry

Hi Sierra,

This is actually a quite controversial topic. There are some scientists who argue that Pluto and the other large bodies recently discovered in our solar system are not planets. The reason why some people argue that these objects are not planets is that scientists have generally grouped the planets into two categories: terrestrial planets and Jovian planets. This division was made based upon the overall sizes and masses, densities, and compositions of the planets in our solar system. The terrestrial planets are relatively small, rocky bodies that orbit close to the Sun. The terrestrial planets are Mercury, Venus, Earth, and Mars. The Jovian planets are enormous, gaseous bodies composed of mainly hydrogen and helium that orbit far away from the Sun. The Jovian planets are Jupiter, Saturn, Uranus, and Neptune. We don't know a whole lot about Pluto, but most scientists will agree that it is definitely not a Jovian planet. Unfortunately, Pluto does not exactly fit into the same category as the terrestrial planets either.

There is now evidence that Pluto may actually belong to a class of solar system bodies known as Kuiper Belt objects. The Kuiper Belt is a region just beyond the orbit of Neptune, extending from about 30 to 50 AU from the Sun, that is filled with many icy objects. At distances even further away from the Sun, there is another region of icy objects called the Oort Cloud. Comets may come from both the Kuiper Belt and the Oort Cloud. In many ways, Pluto is more like the objects in the Kuiper Belt than it is like any of the other planets, which has caused some people to argue that Pluto is not really a planet. The objects you may have heard about in recent news stories about the discovery of "new planets" in our solar system most likely fall into the categories of Kuiper Belt and Oort Cloud objects. The confusion and controversy over what should be called a planet has arisen because some of these newly discovered objects may be even bigger than Pluto. Scientists still don't really agree on how to classify them, but we are making progress.

Kris