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What Is a Planet?

  • Teacher Resource
  • Posted 04.19.07
  • NOVA scienceNOW

In this video segment adapted from NOVA scienceNOW, learn about the debate over the definition of a planet. Historically, there has been no scientific definition for a planet, leaving astronomers with the difficult task of properly classifying new discoveries in our solar system, such as Ceres and Eris. However, in August 2006, members of the International Astronomical Union (IAU) passed a resolution that defined a planet. Under the new definition, Pluto is not classified as a planet, but rather as a dwarf planet along with Ceres and Eris.

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NOVA scienceNOW What Is a Planet?
  • Media Type: Video
  • Running Time: 3m 04s
  • Size: 10.0 MB
  • Level: Grades 3-12

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Source: NOVA scienceNOW: "10th Planet"

This media asset was adapted from NOVA scienceNOW "10th Planet"


Astronomers have attempted to develop a uniform standard of classification for the variety of astronomical objects that have been, and continue to be, discovered. The International Astronomical Union (IAU), founded in 1919 and composed of professional astronomers from around the world, serves as the authority for naming celestial bodies and the surface features found on them.

One of the most hotly debated issues for the IAU to resolve was the scientific definition of a planet. For centuries, the common understanding was that a planet was a large object orbiting a star. However, with the continual advancement of technology and astronomy, new objects were being discovered that called upon the need for an official definition. This debate was fully ignited in 2005 with the discovery of a new object in our solar system larger than Pluto. Originally known as 2003 UB313, this object was eventually named Eris in 2006.

In August 2006, members of the IAU passed a resolution that defined a planet as a celestial body that

  1. is in orbit around the Sun;

  2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape (i.e., it assumes a nearly round shape due to its own gravity); and

  3. has cleared the neighborhood around its orbit (i.e., it is the dominant mass in its orbit).

According to this definition, our solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The IAU determined that Pluto has not cleared its neighborhood because it orbits among the objects of the Kuiper Belt. As such, Pluto is no longer classified as a planet, but rather as a dwarf planet. Eris has also been designated as a dwarf planet. It is expected that the list of dwarf planets will increase while the number of planets will remain at eight.

Significant controversy surrounds this definition of a planet. For the general public, it was difficult to unlearn what they had been taught about the number of planets in the solar system and to lose Pluto, often a sentimental favorite. However, in addition to the media frenzy over the demotion of Pluto, there was also protest within the scientific community.

Among astronomers, the objection was not over the loss of Pluto as a planet but over the wording of the definition, which is ambiguous. For example, what defines a "cleared neighborhood," and how round is "nearly round"? In addition, the definition applies only to our solar system, so there is no universal definition for a planet. Within one week of the resolution's passage, more than 300 scientists signed a petition stating that they did not agree with the IAU's definition of a planet and that a better definition was needed. As of 2006, the debate is not over. The definition put in place by the members of the IAU who voted (only about 5 percent of the world's astronomers) may yet be redefined.

To learn more about the planets, check out All Planet Sizes.

To learn more about extrasolar planets, check out The Search for Another Earth and A Strange New Planet.

To learn more about discovering astronomical objects, check out Galileo: Discovering Jupiter's Moons.

To learn more about how astronomy has evolved, check out Galileo: Sun-Centered System.

Find viewing ideas on NOVA Teachers for this video segment.

Questions for Discussion

    • According to this video segment, what are the criteria that must be met in order for a celestial body to be considered a planet?
    • Which of these criteria does Pluto meet? Which does Pluto not meet?
    • Why do you think scientists disagree over whether Pluto is a planet?
    • Will any other planets in our solar system join Pluto in the dwarf planet category? Why or why not?

Resource Produced by:

					WGBH Educational Foundation

Collection Developed by:

						WGBH Educational Foundation

Collection Credits

Collection Funded by:

						The William and Flora Hewlett Foundation

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