Why are there (at most) eight planets in the Solar System? (Synopsis)

“I have announced this star as a comet, but since it is not accompanied by any nebulosity and, further, since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet. But I have been careful not to advance this supposition to the public.” -Giuseppe Piazzi

I know there are many of you out there who miss Pluto "officially" being a planet, and I know that there are a large fraction of you who still count it as one in your hearts. After all, our Solar System is an amazing place, and the discovery of other large objects beyond Neptune doesn't negate what Pluto actually is.

Image credit: NASA’s The Space Place, via http://spaceplace.nasa.gov/ice-dwarf/. Image credit: NASA’s The Space Place, via http://spaceplace.nasa.gov/ice-dwarf/.

But there are some very special properties that the inner, rocky worlds of our Solar System share with the Gas Giants, and none of the other candidates have them. As we are now learning the details about thousands of solar systems in our galaxy, we need to better understand just what makes something a planet and why that's important.

Image credit: Alien Robot Zombies by Bryan Magnum, at http://www.alienrobotzombies.com/. Image credit: Alien Robot Zombies by Bryan Magnum, at http://www.alienrobotzombies.com/.

Go and read the whole thing at Starts With A Bang on Medium.

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No, Pluto did NOT have it coming, and no, this is not about "feelings."

Ceres is a planet, and our solar system does not have only 8 planets, at least according to the equally legitimate geophysical planet definition. 19th century astronomers did not know that Ceres is in hydrostatic equilibrium and therefore a complex world that doesn’t belong in the category of asteroids and comets. It makes little sense to lump objects that structurally and compositionally are much more akin to the larger planets than to asteroids and comets with the latter category. It also makes little sense to require an object to “clear its orbit” to be a planet. We cannot look only at where an item is; we have to look at the individual item and what it is. These small spherical worlds are planets that have most of the same features the rocky planets have; the only difference is they are smaller.

The notion that we have to limit the number of planets in the solar system because we cannot have too many makes absolutely no sense. Memorization is not important to learning. Kids can be taught the different subclasses of planets and the characteristic of each subclass. After all, we don’t require kids to memorize the names of all rivers or mountains on Earth or the names of all Jupiter’s 67 moons.

The “IAU criteria” are not some gospel truth that should be taken as THE standard an object has to meet to be considered a planet. They represent one point of view in an ongoing debate. And they could result in the same object being considered a planet in one location and not a planet in another, something that makes absolutely no sense.

Why not instead recognize that some planets clear their orbits and some don’t? Similarly, some planets have no orbits to clear as they don’t orbit any stars (rogue planets). Yet structurally, they are planets.

Exoplanet systems clearly show that our solar system is not necessarily typical in its formation or layout. Every time an anomalous system is discovered, astronomers are sent back to the drawing board in determining how that system formed. There may very well be more than one way to make planetary systems. Otherwise, how do you explain hot Jupiters in close orbits around their stars? How do you classify giant planets in extremely elliptical orbits that travel through asteroid belts in those orbits? How do you determine what it means for an orbit to be “clear?”

Dwarf planets are special in the same way the larger planets are and are NOT special in the way asteroids, comets, and Small Solar System bodies are. The latter are frequently loose rubble piles shaped only by their chemical bonds. They are tiny and don’t have the complex structure that dwarf planets, which are small planets, have.

This isn’t about Pluto. It is about a dynamical versus a geophysical definition of planet and about whether the term planet should be kept narrow or broad. Given that Dawn has shown Vesta, which is not completely in hydrostatic equilibrium, to be more planet-like than asteroid-like, why not wait for the data from New Horizons and Dawn to show us just how unique and complex Pluto and Ceres are?

By Laurel Kornfeld (not verified) on 01 Apr 2014 #permalink

The solar system has nine planets:

Mercury, Venus, Earth,
Mars, Jupiter, Saturn,
Uranus, Neptune & Pluto.

Why did the asteroid belt form at the frost line, rather than another frozen planet/s? Is there something special about the asteroids straddling this line, that caused an inability for planet formation?

By Colin Burgess (not verified) on 01 Apr 2014 #permalink

Pluto and Eris are notable for being the biggest chunks of crap that haven't collided with each other.


Jupiter's gravity is responsible for the asteroid belt. I'm not knowledgeable about all the details, but roughly, Jupiter's gravity has a tendency to speed up asteroids in certain orbits and to eject asteroids from orbits. (This has to do with the types of orbital resonances the asteroids have with Jupiter.) So the planetoids that would have formed a planet were thrown out of the orbit entirely or were going so fast that they shattered each other upon impact rather than consolidating.

Of all the opinions about Pluto's planet-hood, Hans of Lummerland is by far the most aggravating to me. We might use any number of definitions, like clearing your orbit, able to form a sphere, or any other clear set of rules. But following those rules is just as important as making them. If Pluto is a planet, then so is Eris, and any other number of similar objects. Keeping our planet list set at 9 just 'because that's how it always has been" is so blatantly ignorant of why we teach science that it really grinds my gears.

By Bart A Mitchell (not verified) on 02 Apr 2014 #permalink


I agree with you. Classifying astronomical bodies as planets is in some sense an arbitrary task. Some, such as Laurel in post #1, look more at the property of having enough mass to reach hydrostatic equilibrium as the primary factor in classification. Others such as the IAU definition use criteria involved with planetary formation. Either way, though, we must apply the criterion we choose in consistent fashion. If we use the hydrostatic equilibrium criterion, then there are obviously many more than 9 planets; there may be almost 200 under that definition. If we use the IAU criterion, then there are 8. There is no scientifically justifiable definition that includes Pluto, but excluldes other Kuiper belt objects that are similar to Pluto.

While this may be important to planetary scientists and astronomers, it should not really make much difference to those not involved in those fields. Pluto is still Pluto. It hasn't changed because we've changed our classification of it. It is still just as worthy of study as it was before the change.

The whole thing reminds me of what happened with biology. Prior to modern understanding of evolutionary pathways, organisms were classified into kingdoms, phylla, classes, etc. based on shared anatomical and physiological characteristics. Modern classification is based on phylogeny, ie. evolutionary relationships. While the classification based on anatomy and physiology is very close to the phylogenteic one, it's not identical. There were organisms whose classifications were changed. That, of course, changed nothing about the organism itself or whether or not scientists studied it. It just led to a more useful and enlightening classification scheme. Now, I don't know if the Pluto reclassification is an improvement or not; I'm not an expert in the field. If the experts think it is, then I am willing to accept their new classification.

It depends on how planets are made, and the standard theory involving planetesimals has no idea how planetesimals are formed. In my opinion, the ice cores are formed through physical chemistry, specifically melt brazing of ices in collision through adsorbed "ices", thus we have Jupiter (water), Saturn (MeOH/NH3/water), Uranus (methane/argon) Neptune (CO.N2). The Kuiper belt is of the same sort as Neptune. In principle, this permits one more, employing a neon ice. If we extrapolate the required temperatures, the neon ice is difficult to place because there are too many unknowns and variations are a significant fraction of the available temperature range (e.g. neon freezes at 25 degrees K, but what is the starting temperature, what is the required temperature (less than melting) and how much energy loss is there in the outer accretion disk?) The net reuslt is that one further planet is possible, BUT it would be expected to be greater than 100 A.U., and it might be dark, and hence difficult to observe. In principle Sedna and the new similar object might be formed this way, and if their eccentricities are correct, they may have been perturbed by something larger.

By Ian Miller (not verified) on 06 Apr 2014 #permalink

Yes, I agree with you first need to understand planets and you have explained very well in your post.. Thanks for sharing such informative post with us..

"Ceres is a planet, and our solar system does not have only 8 planets, at least according to the equally legitimate geophysical planet definition"

The last one to rant about that was entirely unable to explain what this geophysical definition was, thereby demonstrating that they did not know, but had by rote learned of the phrase as a sovereign specific to make demands that their feelings be given sovereignty over everyone else.

If you happen to be able to define it, you now need to show why it is the legitimate definition of a planet.

You can't just proclaim it.

"Keeping our planet list set at 9 just ‘because that’s how it always has been”"

Especially seeing as it's not always been that way, it makes an EXTREMELY poor definition of what the planets of our solar system is.

It would be as valid to claim they are physical manifestations of Gods, since that was "what they always have been" (up until the loss of the religions that believed it as strongly as some believe their god *really* exists).

Wow, according to the geophysical planet definition, a planet is any non-self-luminous spheroidal body orbiting a star, free floating in space, or even orbiting another planet. If the object in question is large enough and massive enough to be squeezed into a round or nearly round shape by its gravity--as opposed to being shaped solely by its chemical bonds--it is a planet. Different types of planets are distinguished through the use of subcategories.

There clearly is more than one legitimate definition of planet, and that is fine. Those of us who prefer the geophysical planet definition do so because we believe objects should be defined first and foremost by their intrinsic properties and not by their location.

This has nothing to do with feelings.

By Laurel Kornfeld (not verified) on 06 Dec 2014 #permalink

Wow, according to the geophysical planet definition, a planet is any non-self-luminous spheroidal body orbiting a star, free floating in space, or even orbiting another planet.

That is only ONE of the definitions "geophysical definition" profferers like yourself put forward.

However, lets go with yours.

We then have currently over 16 planets by that definition, including our moon, Titan and a few other larger moons, plus likely 200 more candidates currently known to be heavy enough to cause that effect, but not large enough to be imaged to confirm visually it is spherical.

Hence, as I claimed earlier, your definition doesn't make it nine planets. It doubles it, and likely increases them by an order of magnitude.

This is not a useful definition of a planet.

Hence the IAU dropped the geophysical definition as being the requirement.

The reason why you don't want it is definitely solely because of feelings. Don't try lying about it, you're not fooling anyone.

Hopefully it's ok that I add to this after 7 months....

| This is not a useful definition of a planet.

I tend to agree with you that the current definition of 8 planets makes more sense but I'm curious why you say the definition of a planet with makes the number of planets in our solar system 16-200 is not useful.