"Why are there so many planets?! I thought there were eight!"
There are eight planets according to the International Astronomical
Union, but that is just one valid definition. The geophysical
definition is the same, but removes the requirement that a planet must "clear
the neighborhood around its orbit". This way, planets are defined based on their
intrinsic characteristics and not their surroundings. For example, if Earth and
Pluto traded places, Pluto would be a planet according to the IAU while Earth would
be gravitationally dominated by Neptune, and therefore not be a planet.
If you like categorizing celestial objects based on their dynamical relationships with
other objects, the IAU definition works fine! But if you're more interested in what the
object is like physically, then the geophysical definition may be more useful and intuitive
for you. Fortunately, using one definition doesn't negate the other!
The Table of Planets provides a way of visualizing the Solar System's geophysical planets while
taking cues from the periodic table of chemical elements for presenting a large number of sciencey
things at once. This way, every planet is shown on equal ground, and planets often ignored, such
as the large asteroids or the giant planets' icy moons, are shown as intriguing, unique worlds of their own.
If you'd like to learn more, you can read our paper below! If you'd like to use the Table of Planets
yourself, check out our downloads page!