Matt Jones' Random Acts of Verbiage

Take out a sharpie and start crossing out your text book. Our solar system just got a tad smaller (well, sort of). We have eight planets, not nine. The IAU (International Astronomical Union) has decreed that (among other things) Pluto (and Ceres) are no longer defined as planets, but as dwarf planets.

This means that the Solar System consists of eight “planets” Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called “dwarf planets” was also decided. It was agreed that “planets” and “dwarf planets” are two distinct classes of objects. The first members of the “dwarf planet” category are Ceres, Pluto and 2003 UB313 (temporary name). More “dwarf planets” are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate “dwarf planets” are listed on IAU’s “dwarf planet” watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

What is the universe coming to!?

[Addendum] For loads of more information, please check out the IAU General Assembly Special 2006 at The Jodcast (via the comments at Tom’s Astronomy Blog: Pluto is NOT a Planet).

[Addendum II] I want to make it clear WHY Pluto is not a planet (as I have been getting lots of search referrals here relating to this) - I will be referencing IAU’s resolutions 5 and 6 (PDF): It is because of the nature of Pluto’s orbit (in that it does not “clear its neighborhood” [there are objects in its orbit] and that it crosses with Neptune’s orbit). The following reasons DO NOT have any bearing on the decision: (1) Its size (Resolution 5.1.b and 5.2.b are identical meaning that both a dwarf planet and a planet must have sufficient mass to become round, this does not mean that a dwarf planet must be small in size [although generally they are because they don't have enough mass to comply with resolution 5.1.c, so in that sense, size does play a factor, but only in that sense.]). (2) New scientific information/evidence (there was NO NEW DATA discovered about Pluto to changed how we describe Pluto, it was merely a decision by the astronomical community to change how a planet is defined). (3) A realization that Pluto is not a planet (no, the definition of Planet just changed). (4) Pluto is not part of the Solar System (no, Pluto is part of the Solar System, in the same orbit it has always been in, it is just defined differently). (5) New scientific theory about planets (this isn’t exactly right. It doesn’t have anything to do with scientific theories, it has to do with scientific definitions, the thories about planets and how they work has not changed).

I hope that helps, let me know if you have any questions.

[Addendum III] Also, I wanted to add that the reason this came up was because of the discovery of other objects in out Solar System that didn’t seem to quite be asteroids but also didn’t seem to be planets. In order to keep things consistent, this debate came up. It either meant making the new objects into new planets or creating a new class as a way to define these objects. When that was done, Pluto fit into that new category, the Dwarf Planet. Eris (formerly nicknamed Xena) especially, which is actually larger than Pluto, would have been considered a planet had the decision gone the other way. With the new class, Eris is currently the largest Dwarf Planet.

[Addendum IV] Came across a great article by NASA’s Chief Historian, Steven J. Dick: Pluto, Classification and Exploration. It is a great read.

[Addendum V] As I have said, the reason Pluto is not a planet is because it did not pass the third criterion: a planet must “clear its neighborhood” meaning it must remove other objects in its orbit. While looking at the Eris WikiPedia article, I came across a great image showing all the trans-Neptunian objects. Check it out for a VERY helpful visual as to what “clearing the neighborhood” would mean. All the little red dots are “Plutinos” or objects that share Pluto’s orbit. These objects are what remove Pluto from the planetary lineup. You will also notice that ther other planets also have other objects in their orbital distance, this is where the ambiguity in the language of the new definition comes in. Technically it seems that most planets have not completely cleared their neighborhood. But the visual does a good job of showing the huge difference in number of objects in each planet’s (or dwarf planet’s) path, which I am assuming was a factor.

[Addendum VI] Ok, to be explicit, here are the criteria for being a planet: Resolution 5.1

5.1) A Planet is a celestial body that
5.1.a) is in orbit around the sun,
5.1.b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
5.1.c) has cleared the neighborhood around its orbit.

It is this final criterion that Pluto fails to meet as discussed in Addendum V above.

[UPDATE] Also, Pluto has a new name: 134340 Pluto! Exciting, eh? Just another common asteroid…

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Matthew Jones