I just read this article. I don't know how credible it is, since I couldn't find any link to the original paper. But it says that pluto will be considered a planet again, along with more than 100 other celestial objects including our moon. So, if the definition given here is applied, then would there be anything as dwarf planet or satellite? I think even comets and asteriouds would come under this defintion. I am talking about this:

The paper, published in Lunar and Planetary Science, declares a planet as a round body that's never undergone fusion, and has less mass than a star. That's it. This means, Pluto would be planet again-- But it would also bring along Ceres, Eris, Makemake and Haumea... as well as 97 other objects.

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    $\begingroup$ Including Charon? It's really a toss up as to who is orbiting who. $\endgroup$ Commented Mar 5, 2017 at 4:17
  • $\begingroup$ @HowardMiller yeah, that's why I asked it here... $\endgroup$ Commented Mar 5, 2017 at 6:10
  • $\begingroup$ This is a proposal made at a smallish conference, a desperate attempt to somehow get Pluto promoted back to planethood It is not a done deal. The proposed change has little meaning outside the smallish circle of terrestrial body geophysicists, and only if that small group look cross-eyed at objects. This proposal has very little discriminating power. Objects just below the cutoff of "roundishness" look a whole lot like objects just above it. $\endgroup$ Commented Mar 5, 2017 at 12:08
  • $\begingroup$ Compare with the concept of "clearing the neighborhood," which has a well-defined mathematical basis. There's a five to six order of magnitude difference between Mars (the planet with the smallest discriminant) and Pluto and Ceres. $\endgroup$ Commented Mar 5, 2017 at 12:12
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    $\begingroup$ I think that objects of the same order of magnitude as Pluto should be referred to as 'Plutoids'. $\endgroup$ Commented Mar 5, 2017 at 12:31

3 Answers 3


This is really a question of Language rather than Astronomy. The meaning of words is defined by their use. The IAU has proposed one way of using the word "planet", this group of scientists have proposed another.

Anyone can propose a definition of any word. This group proposes that a "planet" is any astronomical body which 1) Does not have, and never has had sustained nuclear fusion in its core, and 2) Is large enough for gravity to pull it into an ellipsoidal shape.

The intention of this definition is to make a planet defined entirely by "what" a body is, not by "where" it is. Under this definition the classic 8 planets, Ceres, Pluto, Charon, the Moon, the larger moons of Jupiter, Saturn Uranus and Neptune, Eris, Sedna, and the larger Kuiper belt objects would all be "planets" - about 100 known in total. The exoplanets would also be "planets"

Asteroids (except Ceres), comets and other small solar system bodies are too small, and so would not be planets.

The advantage of this definition is that the usually where something is located is not part of the definition of what it is. However, it contradicts several hundred years of use by defining the larger moons as planets.

Unlike the 8 planet definition, this does not have the support of a large international body of astronomers (the IAU). If you wish to use this definition you are free to. I doubt that without the support of the IAU this definition is ever going to be widely used. Most people will either use the 8 planet definition or the 8+Pluto definition.

  • $\begingroup$ Maybe we can just make the division between spherical and non spherical objects. Luna is a planet because it's spherical. Phobos is not a planet because it's not spherical. I think spherical planets of Pluto's size an smaller out to be called 'Plutoids'. $\endgroup$ Commented Mar 5, 2017 at 4:19

Small objects like comets and asteroids are generaly not round. The roundness of bigger bodies is a consequence of the body own gravity.

So, to answer your question, no, not all celestial bodies would become a planet with this definition as small objects like comets and asteroids doesn't have a sufficent mass to become round.


I doubt if the definition given above will actually be applied ever, but if it is applied then all the current natural satellites (moons) will not be considered as a planet. So is the case with dwarf planets, comets, asteroids, etc.

Asteroids and comets are not round in shape (even if some are, they are not round as a result of their own gravity), so they will not be considered as planets. Most of the moons in the solar system are not round in shape. For example, moons of Mars, Phobos and Demos, are not in round shape. They will continue to be considered as moons or natural satellites.

So, in short, all the natural satellites, dwarf planets, etc., will not become planets.

  • $\begingroup$ You may be technically correct that most of the moons in the solar system are not round, but only because most of the moons in the solar system are small. All the large moons in the solar system are round as a result of their own gravity; this includes all moons known before the Voyager missions except Phobos and Deimos. $\endgroup$
    – TonyK
    Commented Mar 4, 2017 at 20:48
  • $\begingroup$ @TonyK where is the disagreement. I am saying the same thing. I am saying all the moons will not be considered as a planet. This means some (large moons, like ours, Titan, Ganymede, Triton, etc.) moons will be considered as planets. I already explained why asteroids and comets are not round (due to their small gravity), I thought explaining the same for moons in just next line will be redundant. I thought it would be clear why most moons are not round. So, why this downvote. $\endgroup$
    – utsav_deep
    Commented Mar 4, 2017 at 22:08

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