Planetary Puzzle

Important note: Pluto is no longer a planet!
In August 2006, the International Astronomical Union (IAU), at a meeting in Prague (Czechoslovakia), established for the first time a definition of what constitutes a planet. The definition, adopted by the astronomers assembled in plenary, is based on three criteria:

• A planet is a nonluminous object in orbit around a star (the Sun, in this case);
• A planet is massive enough to be a sphere by its own gravitational force (it is considered that bodies whose diameter is greater than approximately 800 km have this property);
• A planet dominates the region of space of its orbit (in other words, it absorbed or expelled smaller objects cluttering its orbit).

This last criterion excludes from the list of planets Ceres, a spherical object (about 1,000 km in diameter) in the middle of the asteroid belt, as well as Pluto, in the Kuiper belt (a vast reservoir of comets). In addition to comets, a host of objects similar to Pluto are found in this remote region of the solar system, which also rotate around the Sun in orbits similar to Pluto. Recently, an object named Eris was discovered, whose diameter is greater than that of Pluto!

In Prague, astronomers also agreed to consolidate under the term “dwarf planets” objects large enough to be be spheres, but which have not “cleared the neighbourhood” around their orbits. Ceres, Pluto, Eris and many other similar objects are therefore dwarf planets. As for the smaller (non-spherical) asteroids, comets and interplanetary dust, they have been grouped under the term “small Solar System bodies.”

In this activity, Pluto is treated as a planet due to historical and mythological tradition, but students should keep in mind that it is indeed a dwarf planet, according to the new definition of the IAU.