Dwarf Planets
Thedwarf planetsof our solar system are exciting proof of how much we are learning
about our solar system. With the discovery of many new objects in our solar system, in
2006, astronomers refined the definition of a planet. According to the new definition, a
planet must:
- orbit a star
- be big enough that its own gravity causes it to be shaped like a sphere
- be small enough that it isn’t a star itself
- have cleared the area of its orbit of smaller objects
At the same time, astronomers defined a new type of object: dwarf planets. A dwarf planet
is an object that meets numbers 1–3 above, but not number 4. There are four dwarf planets
in our solar system: Ceres, Pluto, Makemake and Eris.
Figure25.36shows Ceres, a rocky, spherical body that is by far the largest object in the
asteroid belt. Before 2006, Ceres was considered the largest of the asteroids. Ceres has
enough mass that its gravity causes it to be shaped like a sphere. Still, Ceres only has about
1.3% of the mass of the Earth’s Moon. Ceres orbits the Sun, is round and is not a star but
the area of its orbit is full of other smaller bodies, so Ceres fails the fourth criterion for being
a planet, and is now considered a dwarf planet.
Figure 25.36: This composite image compares the size of the dwarf planet Ceres to Earth
and the Moon. ( 31 )