Nasa / Space

Geek Trivia: What is the total number of solar system objects that have been called a planet?

TechRepublic's Trivia Geek tells you the total number of local solar system objects that have been called planets.

Today we recognize eight full-fledged planets -- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. If every local celestial object that was ever called a planet was grandfathered in -- as some Pluto enthusiasts want to do for that recently demoted dwarf planet -- our solar system would sport a whopping 36 planets.

Besides the eight planets we recognize today, objects previously considered local planets include:

  • One star: The sun.
  • Three dwarf planets: Ceres, Eris, and Pluto.
  • 10 moons: Jupiter's moons Callisto, Europa, Ganymede, and Io; Saturn's moons Dione, Iapetus, Rhea, Tethys, and Titan; and Earth's moon.
  • 14 asteroids: Astrea, Egeria, Flora, Eumonia, Hebe, Hygeia, Irene, Iris, Juno, Metis, Pallas, Parthenope, Vesta, and Victoria.

Now, no one would seriously argue that the sun should be considered a planet. It's clearly a star. Most wouldn't argue that moons should be considered planets, as they orbit other planets more directly than they orbit the sun. Where the distinction gets fuzzy is the border between asteroids and planets. You can't go by size alone; eight non-planetary objects orbiting our sun are larger than Pluto, and two of them -- Neptune's moon Triton and the trans-Neptunian object Xena -- have never been classified as planets.

That's why the IAU had to define a planet with objective criteria, which to date requires 1) that the object directly orbit the sun, 2) that the object be massive enough to have collapsed into a generally spheroid shape, and 3) that the object has "cleared the neighborhood" by absorbing and deflecting any other major objects in its orbital path.

Pluto fails point #3, but meets the other two. Thus, it's a dwarf planet, and no amount of historical precedent can make it otherwise.

That's more than just an otherworldly orbital reorganization; it's an empirically interplanetary instance of Geek Trivia.

About

Jay Garmon has a vast and terrifying knowledge of all things obscure, obtuse, and irrelevant. One day, he hopes to write science fiction, but for now he'll settle for something stranger -- amusing and abusing IT pros. Read his full profile. You can a...

9 comments
RipVan
RipVan

Did the astrologers just ignore the whole issue?

Ndiaz.fuentes
Ndiaz.fuentes

Does anybody else love Jay's closing sentence in every edition of geek trivia? Seriously, how long does it take you to build those things?

schmidtd
schmidtd

If you restrict the list for candidate planets to objects larger than Pluto and are clearly not a moon (in orbit around a planet) you get a MUCH shorter list... The IAU's definition of "clearing the it's orbital area" DQs both Pluto AND Neptune since Pluto didn't clear Neptune and Neptune didn't clear Pluto! I agree with the IAU that our defintion of planet is incomplete, but the IAU defenition is flawed.

Astronut
Astronut

Hi Jay You say Pluto is smaller than Ceres when actually it's diameter is more than twice the size of Ceres i.e. 2306 Km vs 975 Km. ;-) .

john.a.wills
john.a.wills

I did once, a long time ago, notice an article on the importance of Pluto in a magazine for astrologers. I remember that its effects were said to be slow-changing, because of the slow orbit. But usually, I think, astrologers consider only the 7 classical planets (one for each day of the week). In either The Space Merchants or The Merchants' War, I forget which, there is an astrologer based on Earth's Moon. I suppose he would take into account Earth's apparent position among the fixed stars, but we weren't told that.

Jay Garmon
Jay Garmon

...so the 5-10 minutes I spend on the closing is usually my favorite part of the column. Glad someone else appreciates the effort.

Elezar
Elezar

Without having a firm definition of what that means, it makes defining a planet rather difficult. However, it's quite clear that they don't literally mean that the entire orbit is 100% clear, or it would eliminate Earth, Mars, Jupiter, and Neptune, since they all have asteroids in their orbits, in addition to Pluto being in Neptune's orbit. Several years before the IAU came up with this definition of a planet, Alan Stern and Harold Levison proposed a definition for planets that included sub-classifications of uber- (heh, this forum erases the umlaut-u for some reason) and unterplanets, that also used the "cleared the neighborhood" language. Here's how they defined what they meant by this phrase: "Nearby small bodies are on unstable, transient orbits, or are locked in mean motion resonances or in satellite orbits." This definition basically means that an object that is the DOMINANT object in its orbit is considered to have cleared its neighborhood, which would exclude Pluto (because it's locked into resonance by Neptune), but not Neptune, Mars, or Jupiter (because they all lock their neighbors in resonance, not the other way around), or Earth (because the NEOs are transient). It seems reasonable that the IAU is using the phrase in the same way that Stern and Levison did in their proposal, and is consistent with the IAU specifically listing out the eight planets.

Jay Garmon
Jay Garmon

...but then I think the notion of a planet as a discrete object is flawed. Celestial objects exist on a spectrum, starting (arguably) with stars and passing through brown dwarfs to gas giants to rocky planets down to major asteroids and then minor objects like comets. Some formed from the original accretion disk of the solar system and some are the product of later collisions or dissolutions. Some orbit the gravitational center of the solar system and some belong to gravitational subsystems formed around high-mass objects. There are lots of objects that inhabit the fuzzy borders between these categories, and trying to create a distinct line between them is like trying to argue whether purple is bluish-red or reddish-blue.

Elezar
Elezar

If we didn't have definitions on anything, there would still be broad categories that everyone would understand. It's the edge cases that require firm definitions. Thats why we don't define purple as EITHER bluish-red OR reddish-blue, and instead define it in scientific terms, so there IS a distinct line between colors. Now, coming up with definitions that work isn't always easy, which is why the planet definition debate lasted for so long and is still discussed today (although most of the complaints seem to be a variation on "they shouldn't change it from how I knew it growing up", which is ridiculous). This is why the IAU applied the definition only to planets in our Solar System. This way, they don't have to worry about the line between planet and brown dwarf, or about "rogue" planets. If you make some (reasonable) assumptions about what "clearing the neighborhood" means, the IAU definition draws clear distinctions between asteroids/comets and dwarf planets; between dwarf planets and planets; and between planets and the Sun. Really, the one fuzzy border they left was between planets and satellites. I'm honestly not sure what in their definition causes Pluto to be a dwarf planet, but not Charon. And, surprisingly, this very rarely comes up in discussions about the IAU planet definition???