In 2006 Pluto was officially demoted from planet status to "dwarf planet." Now our solar system has only 8 planets. Why?
Well, in 2006 the International Astronomical Union adopted these standards for classification as a planet:
- is in orbit around the Sun,
- has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
- has "cleared the neighbourhood" around its orbit
Pluto meets the first two of these criteria, but not the third. Part of the change came because astronomers have discovered a number of dwarf planets around the same size as Pluto, and they didn't want all of them to be planets. Pluto has a pretty wacky orbit and is only about 1/5 the size of Earth's moon. Originally, when Pluto was discovered in 1930, astronomers thought Pluto was bigger than Mercury. Then in 1978, the discovery of Charon, Pluto's moon, allowed astronomers to recalculate Pluto's mass, and it was much smaller than originally thought. Basically, if we keep Pluto as a planet, a whole bunch of other dwarf planets have to be planets, too. Some people think Pluto should be kept a planet for historical reasons.
Hopefully, in the next decades, we will be able to learn more about planets in other solar systems and many of them will be much more interesting than Pluto. Actually, we already know of more interesting stuff here in our solar system. Jupiter's moon Europa is made of a rock core covered in ice, and it is thought that the surface of Europa might obscure a liquid water ocean
http://www.solarviews.com/eng/europa.htm
due to the heat produced from shape changing caused by Jupiter's gravitational pull. Time will tell. Hopefully someday we'll get a probe on there to drill for water!
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