Hi Misty and Danielle,

Scientists didn't exactly kick Pluto out of the solar system. Pluto is still out there, orbiting around the Sun, the way it has done for a very, very long time.

Many scientists have argued that Pluto and the other large bodies, like "Xena," 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.

Pluto actually may have many things in common with 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 seems 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, like "Xena," you may have heard about in recent news stories are also thought to be similar to 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.

Because of new knowledge about the objects at the far outer reaches of the solar system, the scientific community decided to come up with a new definition of "planet" and re-classified Pluto as a minor planet or dwarf planet. This new definition is still controversial and there are many people who think we should continue to call Pluto a planet. Many of the arguments in favor of calling Pluto a planet are based upon historical reasons and nostalgia, not scientific ideas. During the entire lifetimes of many people alive today, Pluto has been the ninth planet in the solar system. People want to hold on to the ideas about the solar system they learned in school when they were kids and continue calling Pluto the ninth planet. However, this is not the first time that scientists have changed the number of planets in the solar system. For a very long time, before telescopes were invented and Neptune and Uranus were recognized as planets in the late 1700s and early 1800s, people thought the only planets were Mercury, Venus, Mars, Earth, Jupiter, and Saturn. Pluto was discovered only very recently, around 1930. The number of solar system objects that we call planets has been changed before, due to advances in scientific understanding. The re-classification of Pluto is not really any different than other shifts in solar system knowledge that have occurred in the past.

Kris