There is no longer a ninth planet from the sun. Little Pluto is ejected from the planetary family.
The IAU decided in a close vote at its annual meeting in Prague on Thursday to demote Pluto. Discovered in 1930 by Clyde Tombaugh, Pluto has always been the odd duck in the planetary family. The four inner rocky planets and the four outer gas giants have put up with the intrusion of this little interloper for more than 70 years. Pluto, being the smallest object out of the nine, has a highly eccentric orbit, meaning that sometimes it was closer to the sun than Neptune and sometimes it was farther away. It was also not alone in its part of the solar system. It orbits the sun among many other Kuiper Belt objects that are both smaller and larger than it, with and without moons. Pluto is just one of a whole collection of objects that would someday have to be considered as planets if it were to retain its title.
The original definition proposed to the IAU, retaining Pluto's title and adding other planets to the solar system, was disputed from the beginning. About half the astronomers at the meeting wanted Pluto to keep its planetary status, while the other half were looking for a way to organize the system without nostalgia playing a role. Before the vote today, other definitions were proposed that did not receive as much press as the original, broad planet definition. The final definition that won the vote today orders solar system bodies in the following way:
Planets are the eight classical worlds Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
A new category called Dwarf Planets includes Pluto and any other round object that, due to its small size, has not cleared the orbit in its neighborhood. This would include objects that lie within a belt, such as the other Kuiper Belt objects recently found including Sedna, Eris (Xena), and Quaoar. Dwarf planets do not include satellites.
The third main category established by the IAU today is called Small Solar System Bodies and includes anything else orbiting the sun that does not fit the first two categories.
This new organization of the solar system by a small group of humans on planet Earth was an attempt to help people understand their universe a bit better. It won't make everyone happy, but it was the most logical classification system proposed. And no matter what humans call them, the solar system objects themselves have not changed at all. As Galileo whispered after being forced to declare publicly that the sun was not the center of the solar system, "Eppur si muove." ("And yet it does move.") His sentiment being that it does not matter how we describe it, the universe is what it is, and our words cannot change that.
