Pluto and Changing Astronomy

Five years ago, the Milky Way galaxy held nine planets.  Then, the comparison of dwarf planet Eris to previous title holder of ‘Ninth Planet’, Pluto, forced a change that would cause textbooks worldwide to be rewritten and transform the way we defined planets forever.  In 2006, astronomists from all over Earth came together to determine a universal rule by which planets could be identified.

The decided the following:

• A planet orbits a sun
• A planet has enough gravity to pull itself into a spherical shape
• A planet must be the dominant gravitational body in its orbit

Pluto passes the first two tests, but within its far-flung orbit, known now as the Kuiper Belt, there exist many other objects of similar size or mass, including Eris, a round icy object larger and more massive than the previous ninth planet.  With these definitions in place, it became clear that Pluto no longer made the cut.  It, Eris, and several other bodies in the Kuiper Belt are now known as dwarf planets.

Every book on astronomy which endeavored to remain current had to be rewritten, which was no simple task.  Our own Astronomy For Beginners was no exception, and shortly after Pluto’s reclassification, a new version was published.  It just goes to show that nothing lasts forever.  Sorry, Pluto!