Defining a Planet
We know that we are the inhabitants of planet Earth, but a celestial body doesn’t have to have a thriving populace to be considered a planet. It does, however have to meet certain criteria. The standard definition of a planet is a celestial body that orbits around a star such as our Sun. The word "planet" is Greek and means wanderer. Our Solar System was previously believed to contain nine planets until the demotion of Pluto, relegating it to the status of dwarf planet. In the astronomical community, new discoveries and the debate over whether Pluto should be considered a planet gave rise to a new set of criteria on which to base the status of a planetary body.
The Planetary Debate
The classification of a celestial body as a planet has always been based on dynamics rather than geophysics. The International Astronomical Union decided that orbit clearing criterion would be used in defining a planet. The new classification is designed to give us a better understanding of the universe.
What is orbit clearing criterion? To put it simply, it means that the planet has cleared its orbit of other bodies similar to itself. Orbit clearing was an important factor in Pluto’s demotion. Unlike the other planets, there is a swarm of bodies similar to Pluto orbiting nearby. Planets eject, accumulate or control all the mass in their immediate vicinity, which is something that Pluto is not able to do.
A Planet According to the IAU
The assembly of the International Astronomical Union voted in favor of a resolution that classifies Solar System objects in three categories known as planets, dwarf planets and small Solar System bodies. The Solar System now contains eight planets with Pluto falling into the class of dwarf planets. Although the term dwarf planets is used, these celestial bodies are not considered planets in any sense. The IAU defines a planet as a celestial body that is in orbit around the sun, has sufficient mass for its gravity to overcome rigid body forces so that it assumes hydrostatic equilibrium (a nearly round) shape and has cleared the area around its orbit.
Defining Putoids and other Celestial Bodies
A dwarf planet is defined as a celestial body that is in orbit around the Sun, has sufficient mass for its body to overcome forces so that it assumes a nearly round shape, has not cleared the area around its orbit and is not a satellite. The name, Plutoids was given by the IAU to celestial bodies that meet the above criteria and are in orbit around the Sun at a semi-major axis greater than that of Neptune. All plutoids are dwarf planets but not all dwarf planets are plutoids. Planets aren’t the only objects in our neighborhood. Our Solar System is home to other celestial bodies such as asteroids. The International Astronomical Union (IAU) has created a class for all other objects orbiting the Sun, except satellites and refers to them collectively as "Small Solar System Bodies."
Cornell University Department of Astronomy: What Makes a Planet?
Windows to the Universe: What is a Planet?
Nasa's New Frontier Mission to Jupiter: Juno: What is a Planet?
Nasa: Solar System Exploration: Dwarf Planets