Lunar and Planetary Institute






Planets and the Solar System
Possible Issues and Questions

Most people do not understand the difference between a solar system and a galaxy, or between our own Solar System and a system of planets around another star. This can lead to confusion when discussing a newly discovered planet (and whether it is a part of our Solar System), the formation of our Solar System (compared to the formation of our Galaxy or even of the Universe), etc.

Students often become confused with rotation and revolution and may mistakenly think that Venus is orbiting the Sun clockwise. In fact, Venus orbits counterclockwise like all the other planets; it spins backwards on its axis, unlike most of the other planets.

Students may also be confused about the differences between planets, asteroids, moons, and stars. Until 2006, there was no formal definition of planet, and the new definition only applies to our own Solar System and not to objects discovered around other stars. By this definition, there are currently 8 planets in our Solar System. Most of those planets have moons orbiting them, with new moons regularly being discovered around the gas giants. There are similarly increasing numbers of known comets and asteroids, with many new objects being discovered out past the orbit of Neptune. These objects are too small to be planets, but are often known as "minor planets" among other terms.

Students may be confused or concerned about Pluto's status. Pluto does still exist (it hasn't been altered or destroyed) but is currently classified as a "dwarf planet" rather than a planet, largely due to its orbit. There are several other "dwarf planets" as well, including one that is larger than Pluto. All dwarf planets orbit our Sun and are part of our Solar System.