Planets

This seemingly simple question does not have a simple answer. Everyone knows that Earth, Mars and Jupiter are planets. But both Pluto and Ceres were once considered planets until new discoveries triggered scientific debate about how to best describe them—a vigorous debate that continues to this day. The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things: 1. It must orbit a star (in our cosmic neighborhood, the Sun). 2. It must be big enough to have enough gravity to force it into a spherical shape. 3. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun. Discussion—and debate—will continue as our view of the cosmos continues to expand.

The key difference between a planet and a dwarf planet is the kinds of objects that share its orbit around the Sun. Pluto, for example, has not cleared its orbit of similar objects while Earth or Jupiter have no similarly-sized worlds on the same path around the Sun. Like planets, dwarf planets are generally round (Haumea looks like an overinflated football) and orbit the Sun. There are likely thousands of dwarf planets waiting to be discovered beyond Neptune. The five best-known dwarf planets are Ceres, Pluto, Makemake, Haumea, and Eris. Except for Ceres, which lies in the main asteroid belt, these small worlds are located in the Kuiper Belt. They’re considered dwarfs because they are massive, round, and orbit the Sun, but have not cleared their orbital path.

The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians.