Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

This question already has an answer here:

Pluto is no longer a member of the planets in the solar system.

  • Why was it disqualified as a planet?
  • What is the criteria for classifying an Astronomical Object as a Planet?
  • Does this criteria apply to all Star Systems or just our Solar System?
share|improve this question

marked as duplicate by UV-D, ChrisF, TildalWave, Timtech, called2voyage Oct 2 '13 at 12:46

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Wiki: Pluto, Classification – TildalWave Oct 2 '13 at 8:26
The question about the classification is a duplicate, but the question about other planetary system seems relevant to me. – MBR Oct 2 '13 at 8:46
@MBR the information in the answers on the linked thread can be applied to other planetary systems - it includes an answer with the IAU definitions. – user8 Oct 2 '13 at 9:18
This question appears to be off-topic because it is easy to answer from wikipedia (which is adequately sourced and informed on the subject). – called2voyage Oct 2 '13 at 12:46


It was disqualified as a planet because orbital dominance was not achieved in the case of Pluto. Orbital dominance means that a the planet candidate should have remove all the small bodies from its orbit, by impact, capture of gravitational disturbance.


According to the IAU in its resolution B5 (IAU is the International Astronomical Union; in particular it is in charge of naming celestrial bodies), a planet is a celestial body:

  1. in orbit around the Sun
  2. with a sufficient mass so that self-gravity overcome rigid body forces (that assumes a hydrostatic equilibrium)
  3. has cleared its neighbourhood.


As you see in the resolution B5, this definition is specified for "Planet in the Solar System". It is mainly because it does not really make sense now that it is already challenging to observe planets, so the distinction between a planet and a dwarf planet is not yet crucial for exoplanets. So the official definition of an exoplanet by IAU makes mainly a difference between a planet and a brown dwarf.

However, I guess the IAU definition of a planet should be kept for the other planetary systems if we are able to detect objects such as dwarf planets.

share|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.