What, exactly, is a "planet"?

The International Astronomical Union thought it had settled the matter when its members voted, two years ago, for a first-ever scientific definition of the term. Boy, were they wrong!

Taken in Prague during the final day of the group's triennial General Assembly, the vote was deemed necessary because an object larger than Pluto (now called Eris) had been discovered in the distant Kuiper Belt. Was it a planet, a giant comet, an asteroid? If Pluto is a planet, then isn't Eris one too? Or if Eris was just the newly crowned "King of the Kuiper Belt," then where did that leave Pluto? Naming rights were at stake!

A "planet," the IAU decided, must circle the Sun (it's not a satellite), has enough mass for gravity to have drawn it into a round shape, and has enough mass to have cleared out everything else in its orbital neighborhood through impact or scattering. This left Pluto and Eris, literally and formally, out in the cold.

Great Planet Debate

The Great Planet Debate drew 150 scientists, educators, students, and others for a three-day discussion of what it takes to be planet.


Few planetary scientists like the IAU's definition, which is confusing and vague. Within days of the IAU's vote, some of them started a recall petition.

"Clearing" is dynamicist-speak for a gravitationally dominant object, such as Jupiter. Except that Jupiter has thousands of asteroids, called Trojans, that share its orbit — so maybe Jupiter and, likewise, Saturn aren't planets. Earth still has to fend off stray asteroids that pass its way, so is Earth a planet? And if Earth were out at the orbit of Neptune, it wouldn't have the gravitational chops to dominate much of anything. No clearing, no planethood.

In its haste to get something on the books, the IAU failed to define what the maximum size for a planet should be, or to deal with the growing count of planets known to circle other stars.

Oh, and did I mention that the IAU also decided to make Pluto, Eris, and Ceres charter members of a new class of "dwarf planets" that aren't technically planets? To add to the confusion, two months ago the IAU made good on a action item left over from Prague that Pluto and bodies like it would henceforth be called Plutoids.

To try to bring some sanity to this mess, Pluto petitioners Alan Stern (Southwest Research Institute) and Mark Sykes (Planetary Science Institute) felt that it would be in science's best interest to hold a meeting to explore what "planet" really means. That "Great Planet Debate" is under way at Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland.

Although it's been billed as a scientific meeting, the mix of 150 attendees is skewed toward educators, students, and reporters interested in the outcome. Still, the few dozen scientists here have had plenty of give and take. Some attendees, like Hal Levison (Southwest Research Institute) maintain that the IAU basically got it right, that gravitational dominance is the single best truth-test for planethood.

But others (and probably most others, I sense), think "roundness" is a better metric. As Sykes points out, an object becomes round once its own gravity wins out over the material strength of whatever it's made of. Geology happens. But how round is "round"? Where do you draw the line between "round enough" and "a little too bumpy"?

Neil deGrasse Tyson (left) and Mark Sykes were all smiles moments before airing their disparate views at the Great Planet Debate.

S&T: J. Kelly Beatty

Capping off the first day's activities was a debate of sorts between Sykes and Neil deGrasse Tyson, who directs the Hayden Planetarium in New York City. Tyson's planetary preferences became clear when the Rose Center opened in 2000. Pluto was conspicuously missing from its "Walk of the Planets."

Ira Flatow, host of National Public Radio's "Talk of the Nation," gamely tried to moderate the discussion. But he could do little to contain the flamboyant and sometimes animated verbal sparring that ensued.

Sykes, a "roundness" devotee, argued that the IAU should have been more, not less, inclusive in its definition. By his count, the solar system now boasts 13 bodies that qualify as planets: the usual eight plus Ceres, Eris, Pluto, Charon (which he claims isn't really Pluto's satellite because their combined center of mass lies between them), and the recently discovered Makemake.

But Tyson countered that "The word planet has lost all scientific value." Instead, he said, come up with a lexicon that does a better job of characterizing the objects — define it however you want, he stressed, but make it useful.

At least they agree that science shouldn't be legislated by a vote. It's an often-messy enterprise whose outcome often falls outside neat categorizations. And that's part of what makes science so exciting.


Image of Bill


August 15, 2008 at 7:14 am

It's probably always going to be a messy situation, but what's wrong with the following criteria?

o Circles a star
o Has enough mass for gravity to have drawn it into a round shape
o Is not a star, brown dwarf, or member of any well-defined sub-planetary group (asteroids, comets, Kuiper Belt Objects, etc.)

Now it works for other star systems, and the third requirement allows for exclusion if an object would fit better somewhere else. Eight planets. Case closed.

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August 15, 2008 at 3:19 pm

Bill's suggestion still comes up short. By previous definition Ceres was also a planet. Why is it now an asteroid, not even a Plutoid? It seems that there is no real cutoff line between planet and asteroid and Kuiper Belt Object. Case still open.

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Bill Dodds

August 15, 2008 at 4:16 pm

Not having the blueprints for the Original solar system, how can one comment at all?
Science tells us the asteroid belt is 'leftover' planet forming material.
But, perhaps it is the remnants of a planet exploded by the Annunaki, with scalar waves from the Giza pyramid trio.
There were interplanetary wars, I understand.
One must determine if a planet is resident or transient.
As the solar system swings in and out and up and down inside the Orion arm, it is possible the Sun collects other planets for fun.
Planet classification should be based on geological content.
How can Pluto be related to the Sun if is is supposedly just ice?
Cab anyone prove that the elements in Pluto originated in the protoplanetary nebula?
Can we prove that Jupiter formed alongside Saturn and Neptune?
If one can realistically prove that the recipe for all solar systems includes Jupiters, moons, etc. Then there should be no arguments.
If the solar system was designed by intelligentr beings with the ability to acquire or move existing planets into optimum orbits, then this precludes solar system classification.
The trick would be to find a reasonably concentric solar system that formed naturally.
I believe our solar system was genetically altered to be predictable.
Deciding what is a planet or not could prove to be fatal.
The ancients knew enough to place the acquired moon into an engineered orbit to minimize axial precession.
It is getting so complex, that I must leave soon.
In short, if something proves crucial to the centrifugal/magnetic/whatever, balancing of a solar system,
it may just be a planet.
A chunk of space rock may be the remnents of an exploded planet.
Now, this is getting exciting!!!

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Poindexter McSmash

August 15, 2008 at 5:36 pm

If it holds an orbit, round, big enough or has at least one satellite (moon), it's a planet...

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John Mahony

August 15, 2008 at 5:37 pm

Kelly wrote:
"'Clearing' is dynamicist-speak for a gravitationally dominant object, such as Jupiter. Except that Jupiter has thousands of asteroids, called Trojans, that share its orbit — so maybe Jupiter and, likewise, Saturn aren't planets. Earth still has to fend off stray asteroids that pass its way, so is Earth a planet?"

Except that Jupiter's trojan asteroids are where they are- at the L4 and L5 Lagrange points 60 degrees ahead and behind Jupiter in its orbit- specifically _because_ of the dominance of Jupiter's gravity.

Yes, there are a few stray asteroids near earth's orbit and near the orbits of most other planets, but these objects are _way_ smaller than the "planet" they're near, typically less than 1/1000 the diameter of the planet. There are several objects in the Kuiper belt with diameters at least half the size of Pluto and Eris.

"Round" is no better as a metric, because none of the planets are perfectly round. Even if you replace "round" with "hydrodynamically stable" to allow for a planet's equatorial bulge, there are still small variations, and these variations are larger with smaller objects. So where is the exact cut-off? And as you point out, the size or weight required for "roundness" will depend on the strength and density of the material an object is made of.


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Donn Mukensnable

August 15, 2008 at 8:00 pm

Bill August makes a good point, but some of the "well-defined" categories he listed (in particular, Asteroids and Comets) are themselves rather ambiguously defined, as evidenced by the number of "asteroids" that form tails when close to the sun and those ancient "comets" that do not.

Trying to divide them by other arbitrary means such as eccentricity of orbit doesn't work conclusively either. There is always going to be a point where a choice is necessary when the object span a continuous measure.

This is true even for "roundness", by the way, since few planets approach a geometric sphere and even non-rotating neutron stars will have some variation at the quantum level.

So, there's no black and white (even in the case of Iapetus) and the question devolves down to picking a shade of gray that most folks can agree with. Clearly the IAU failed in that measure last time; let's hope this latest effort comes up with something better and more universal.

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Arthur Jackson

August 15, 2008 at 10:56 pm

I agree with Poindexter McSmash, We as a Solar System neighborhood should be expanding our base not subtracting from it . The more the merrier. It`s not "those stars with the most planets wins" contest, but really "PLUTOIDS" sounds like a breath mint. So the count stands at what 12 now ? As much as I like Neil Tyson, he needs a better PR guy.

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Tony Dethier

August 16, 2008 at 2:39 am

I'm sick and tired of this discussion. It's all semantic. I'm an instructor at a planetarium and people ask me time and again what astronomers are doing, with the exception of having a row about Pluto. I'll end with a provocative thought. Suppose Pluto was discovered by, say a Peruvian. What would be the outcome of this debate?

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Michael C. Emmert

August 18, 2008 at 12:52 pm

I think we can make a broader classification of the two camps in this debate. On one side are those who wish to have fewer classifications, believing that this is simpler for the dimwitted public to understand. Thus, Pluto and anything larger is a "planet" and anything smaller is "debris". The other camp believes that such a binary situation is not only nondescriptive but also makes it harder to work out exactly what the essence of solar systems are. It is important to put objects into the right classification. So a multiplicity, for instance, planet, asteroid, Lagrangoid or Trojanoid, classical KBO (cold and hot), plutino, scattered disc object, all these terms help understand the Solar system, and by extension other solar systesm, more completely. Just today, a new object, 2006 SQ372, was announced that may be the prototype for a new class - we'll see. Meanwhile; S. Alan Stern bringing "sanity" to this discussion? Dr. Stern, people are not plotting in smoke filled rooms to overthrow the Kind of the Kuiper Belt (http://dsc.discovery.com/news/2008/06/11/pluto-plutoids.html). And trying to start an alternative to the IAU is like trying to overthrow the United Nations. 🙂 We do need to have all these different classifications to sort out what's happening in our Solar system and beyond.

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Enrico the Great

August 19, 2008 at 8:15 am

I am going to violate my self imposed rule of civility in this debate. The status of Pluto as Plsnet is not decided by the contents of an already painfully dated exhibit at the Hayden Planetarium, an institution which for years has benn a shadow of its former self. Neil de Grasse Tyson, though I enjoy his splendid books anf articles, seems on this question to have made a totally boneheaded decision to make a statement by excluding Pluto from the Walk of the Planets exhibit. This is the typpe of chessy statement making one expects in political posturing, not sciemce or science education.
Wake up, excluding small speroidal bodies from the definition of "Plsnrt" is just as arbitrary as excluding them. I fell "Planet" is a broad category, if one wants to specify differences in properties of these bodies, then just use ADJECTIVES!!!

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Robert Casey

August 19, 2008 at 11:36 pm

Some theories of solar system and planet formation mention that some planets may get tossed away from the system and end up not owned by any star. These would still be planets, yes? Well, the upper limit for a planet's mass looks to be just below the mass an object needs to be to do fusion in its core. About 80J if I recall correctly. Lower limit? Massive enough to overcome the strength of its materials to make it round. But that's around 500km, a tad small...

Another apparent split in the planet classication looks to be if it's a gas ball (Jupiter, Saturn, Uranus, Neptune) or something with a hard surface you could walk on. (there may be planets intermediate between these out there we don't know about yet).

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Peter Moss

August 20, 2008 at 5:12 am

... that the presence of moons should matter; but this would still leave one "loose end," and that concerns Mercury and Venus.

As for Ceres: Doesn't it differ fundamentally from Pluto and the other Kuiper Belt Objects - as to composition, density, etc.?

No easy answers here.

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Glenn Muller

August 20, 2008 at 9:46 am

Suppose our solar system was disrupted by a passing star and Earth was flung out of the galaxy. Would Earth no longer be a “planet” because it didn’t orbit a sun?

I think the IAU is trying to pigeon-hole without enough cubby holes in it’s rack. There’s as much variety in the make-up of space objects as there are creatures on Earth so perhaps we should adopt something similar to the biological hierarchy – Class, Order, Family, Genus, Species.

Horticulture, for example, gives us trees, shrubs, flowers etc. and a zillion different varieties of each; yet we have identified every one discovered, and given it a distinct classification. Of course, very few of us can actually remember them all, but that's why we have books and computers.

The Universe is a big place - don't be afraid to grow into it.

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Odo Siahaya

August 21, 2008 at 10:51 pm

I'll try the non-scientific approach... The word planet comes from the Greek word "planetes" (sorry, no greek characters available) which means "deceiver" or "cheater." It seems to me that none of the bodies that circle the Sun, including Earth and the gas giants, fulfil the definition of the IAU to the letter, i.e. roundness, cleared its orbit neighborhood, etc. One could say then that all these bodies are living up to their name of being "cheaters," none of them follow the rules to the letter. It follows then that "planet" is indeed the proper description of these heavenly bodies that orbit our Sun (and other stars as well)! It seems that the ancient Greek astronomers had it right to begin with...

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