Well the verdict is in, and Pluto is out. In a vote last Tuesday, the International Astronomical Union voted on a new definition of "planet" that would exclude Pluto and other icy objects in the outer parts of the solar system. Ironically, the new definition stipulates that a planet has "cleared the neighborhood around its orbit," which would exclude the gas giant Neptune as well (Pluto’s orbit overlaps the orbit of Neptune). However, Neptune was officially reinstated as a planet by special footnote. Pluto (and other newly discovered bodies, such as Xena and Sedna) is now a "dwarf planet."
Unlike textbooks, ExploreLearning was able to respond to the change quickly by adding a few notes to our Solar System Explorer Gizmo. We left Pluto in because it has such an interesting orbit, but made sure we referred to it as a "dwarf".
On the topic of planets, check out our brand-new Orbital Motion – Kepler’s Laws Gizmo. In this spectacular new Gizmo, you can change the position, mass, and velocity of a planet, then observe its motion around a central star. It was a great surprise to me that almost any starting configuration results in a stable orbit – things don’t gradually spiral in to the star as you might expect. Just as Kepler did in the 17th century, you will discover that the planetary orbits conform to some very interesting laws.