With the recent discovery of several large objects orbiting the Sun, located beyond the nine planets in the distant Kuiper belt and similar to Pluto in properties, astronomers are revisiting the classic definition of what qualifies an object to be a planet. Here are five competing definitions of what it means to be a planet.

• Planets are Round
  Any solar system object larger than a certain size, no matter its density or composition, will be molded by self-gravity into a spherical shape. The limiting size for this behavior is a radius of roughly 400 kilometers. This would increase the number of recognized planets from nine to at least fourteen, by adding the recently discovered Quaor, Eris (UB 313), EL61, and Sedna Kuiper belt objects, and by including the largest asteroid Ceres (the most massive object in the asteroid belt, between Mars and Jupiter).

  This definition sets a clear limit, and is based on physical principles. It also makes sense intuitively (ask almost anyone to draw a picture of a planet, and they will start with a circle).

• Planets are Large
  Some believe that anything with a radius larger than 500 kilometers should automatically qualify as a planet. However, it can be difficult to accurately estimate the sizes of small, distant bodies in the outer solar system. This would keep the nine planets as such, and add Eris to the mix.

  This definition again sets a clear limit, and is based on physical principles (though also somewhat arbitrary). It is also easy to explain, like the roundness requirement.

• Planets are Isolated
  One can argue that planets are objects that dominate their region of the solar system, orbiting alone (ignoring dependent satellites) in their orbital zones. This definition would give us eight planets, excluding Pluto, the recently discovered large Kuiper belt objects, and the horde of asteroids which orbit together within the asteroid belt between Mars and Jupiter.

  This definition is also motivated by physics.

• Planets are Historical Artifacts
  The more romantic among us hold that there are eight planets based on physical properties, but that Pluto occupies a special place in history and should be grandfathered in for this reason, having been treated as a planet for the last 75 years.

• Planets? What planets?
  One could also strike the word planet as an unadorned term, replacing it with four types of planets (and increasing the total number immensely). This would leave us with four terrestrial (rocky) planets, four gas giant planets, a large number of minor planets (asteroids), and an uncountable number of trans-Neptunian (ice-dwarf) planets.