Exploring the Planets

What is a Planet?

Solar System

Recent discoveries of large objects orbiting the Sun beyond Neptune and Pluto have raised the question, "What is a planet?" The International Astronomical Union (IAU) has reexamined the way planetary bodies are classified, and in August 2006 it passed a resolution redefining the criteria for planetary status.

A planet is defined by three properties:

  • It is a celestial body that orbits the Sun.
  • It is massive enough that its own gravity causes it to form in a spherical shape.
  • It has cleared the neighborhood around its orbit.

Under this definition our solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Because Pluto resides in an area of space populated by numerous other objects, it is no longer considered a planet. Pluto is now designated a dwarf planet, and it is the prototype of a class of objects located beyond the orbit of Neptune. Other dwarf planets are Ceres which is the largest asteroid found in the Asteroid Belt located between the orbits of Mars and Jupiter, and 2003 UB 313 which is an icy body that is larger and farther away than Pluto. The IAU will establish a process to determine what other bodies fit in this category.