What's a Planet?

Well, I’m sure everyone remembers the discovery of a new Kuiper Belt object that is probably larger than Pluto, thus either qualifying as planet number 10 or forcing the dequalification of Pluto. There’s a new story in today’s New York Times about the subject.

I’m thinking, maybe it’s time to get rid of the term “planet” as such. What if we use terms that are strictly based on size and characteristics. I’m thinking of something like the following, although my “terms” could probably use improving.

Gas Giant: Anything smaller than a brown dwarf that doesn’t have a discernable surface.
• Jupiter
• Saturn
• Uranus
• Neptune
Terrestrial: Anything with a significant atmosphere and definite surface.
• Earth
• Venus
• Mars
• Titan
Round Rocks: Any rocky (or icy) object that is a spheroid.
• Ganymede
• Mercury
• “Luna” (Earth’s Moon)
• Pluto
• Sedna
• Ceres
• Assorted other moons, Kuiper Belt Objects
Asteroids: Remaining rocks
• Too many to mention

Any thoughts on the subject?

My idea was based on the Hertzsprung-Russell Diagram for stars. Each object would get an initial letter. G for gas, R for rocky, I for icy.

Then second, a number for size. Define Earth as log 1 (in other words zero) and set things a logarithmic so the numbers don’t get too large.

So Earth could be R0.0 and Pluto might be I-2.7 and Jupiter G2.5. You could talk about R bodies an I bodies. You could probably even sneak stars in there in G bodies. The Sun would be G5.5.

I suppose you could toss in a second letter for surface temperature so you could graph like like the H-R diagram. That’s a little harder to define when it comes to the gas bodies.

Leave planets to the masses. I don’t think Astronomers really use the term planet in a scienctific manner anyway though. Much like the term continent. It has no scientific definition but rather certain objects are defined to be it.

Is Pluto a planet, or just a Kuiper Belt object? I think that it’s a little late to reclassify it. It has been a planet, and I think it should remain a planet. Pluto has enough mass to be round, and it is round. If we are going to arbitrarily decide on a particular size for a ‘planet’, why not arbitrarily say, ‘Pluto is a planet. Any object smaller than Pluto is not a planet. If an object is as large as or larger than Pluto, orbits the Sun, and does not orbit another body that is orbiting the Sun, then it is a planet.’

Personally, I don’t think there’s any shame in Pluto losing planet status. That doesn’t change the fact that it’s the closest, best-studied, and second-largest KBO and a fine, upstanding member of the Solar System community. :slight_smile:

To me, the important question is: What useful information about an object is conveyed by saying, “X is a planet”?

  1. It orbits a star.
  2. It is not, itself, a star or a brown dwarf. (And a brown dwarf is of course defined precisely as . . . as . . . crickets)
  3. It’s a fairly largish body, as these things go.

So really, there’s not a lot of information in calling something “a planet,” which is why astronomers, in general, don’t get too worked up about it. That said, I don’t think Pluto or 2003 UB313 should be called planets for several reasons:

  1. There’s a big gap in sizes between Mercury and 2003 UB313. Good place to draw a boundary, IMHO.

  2. As has already been pointed out, the four terrestrial planets share a set of distinguishing characteristics. The four jovian planets share a set of distinguishing characteristics. And Pluto and 2003 UB313 don’t fit in to the terrestrial or jovian classifications. There’s no inherent problem in creating a new category to fit a new set of planets except . . .

  3. Where’s the line between a KBO and a planet? If we get a better idea of the size distribution of KBOs, and there is a gap in that size distribution similar to the gap between Mercury and the asteroids, or there turns out to be something particular about Pluto and 2003 UB313 that distinguish them from smaller KBOs, then it might be reasonable to create a category of plutonian planets.

In the meantime, it gives me a headache to think of the people squabbling over each newly-discovered large KBO, arguing about which one is the 10th planet, and the 11th planet, and the Nth planet.

The best argument I’ve heard for keeping Pluto as a planet is that we’ve been calling Pluto a planet for a long time, so it’s no longer a scientific label so much as a cultural one. I don’t necessarily disagree, but I don’t think that gives us any guidance about what to do with 2003 UB313 or other KBOs, so the argument is of limited utility in my mind.

ISTM that the logical way to define a planet so that it’s different from moons and stars is that a planet is a non-solar body massive enough to attract and hold a moon of X mass (whatever X turns out to be) for a significant chunk of geologic time (10 million years or 100 million – not sure on this either). That is, a planet is defined in terms of its ability to hold moons in place. And moons being defined as bodies of X mass in orbits around planets for a significant chunk of geologic time.

"Course I could be wrong about this, as I haven’t “done the math.”

IMHO, this is gonna be another situation where it’s best to just let popular and scientific use of terms diverge, a la brontosaurus/apatosaurus and schizophrenic, and accept that lay usage will be what it will be. Ordinary people are not required to use words in the same way that they would if they were professionals using those words as part of a professional vocabulary.

Wherever scientists decide to draw the line between planets and lesser objects, Pluto will be regarded as a planet by most laypersons simply because it’s been called a planet since its discovery 75 years back. If the astronomical community decides Pluto’s not a planet, then first there’ll be a wait while people see if they change their minds. If they keep to the script for a couple of decades, it’ll start to gradually work its way into the school textbooks that Pluto’s not a planet. And when most kids are taught from those textbooks - when they grow up, it’ll be generally accepted that the Solar System has eight planets.

But it’ll take that long.

Why do the scientific community want to decide Pluto isn’t a planet? Is it based on its size alone? If so, then where do they draw the line?

What you’re basically talking about is defining the size of a planet by its Hill Sphere. The problem is, the Hill Sphere depends on the planet’s mass but also on its distance from the Sun, because whether a moon is in a stable orbit depends on whether the planet’s gravity is strong enough to hold it in place, or whether small perturbations will let the moon wander out of orbit and just orbit the Sun instead. (The size of the moon doesn’t matter, BTW.)

Mercury and Venus could have moons, but don’t. Some asteroids have moons, as do many KBOs. I don’t think that having, or having the potential to have, moons is a useful criterion for planets.

Er, that first sentence got scrambled. “What you’re basically talking about is defining the status of a planet by the size of its Hill Sphere.”

Johnny, The problem is that there is no firm definition of what a planet is, or how large it has to be. The only planets discovered in the modern era were Uranus, Neptune, and Pluto; the rest have been known since ancient times. When Pluto was first discovered in 1930, it was believed to be another gas giant, the size of Uranus or Neptune, or larger. However, over decades (!) its true size was finally understood. Now, if in 1930 they had known that they were looking at an object that was only 2000 km across, it is possible that they might never have started calling it a planet in the first place.

Whether Pluto should really be classified among the other planets has been debated about for some time, especially once we had the technology to observe KBOs and determine their sizes. The discovery of KBOs larger than Pluto really forces the issue. If Pluto is considered to be the minimum size for a planet, we might be in the position of inaugurating a whole bunch of new planets. Is that really a useful way to classify those objects? If we decide we don’t want to name new planets, then would we be justified in allowing Pluto to keep its planet status?

There are asteroids that aren’t even big enough to be spherical, yet they have moons.

And I don’t think there’s a rigorous definition for “moon”, either- that just shifts the problem from “what is a planet” to “what is a moon”.

Here’s a song about the debate over whether Pluto is a planet.

Well, that’s certainly true. People could call Pluto a yummy scoop of Chocolate Chip Cookie Dough Ice Cream, and that isn’t going to change what it is.

Yes. I see Podkayne has further addressed this in relation to KBOs, and that basically sounds good to me.

See, although RTFirefly is quite right as regards popular usage, the discovery of a rock out there that is LARGER than Pluto makes this an issue, even in the popular mind. If we just arbitrarily “grandfather” Pluto into the planet category, people are going to be continuously asking, “If Pluto is a planet, why isn’t Blabla* a planet?” That’s why I think we oughta just de-classify Pluto right now, just to simplify things as new KBOs are discovered.

    • IFAIK, the new rock hasn’t been named yet.

<lizardo> WHERE ARE WE GOING! </l>

Aw, I was going to post that link.

Larry Esposito; boo, hiss!

At the moment, AIUI, the Kupier Belt is even more of a misnomer than Asteroid Belt. It’s simply a region too close to the sun for cometary bodies to exist, and the bodies are too large, and too dispersed, to have actually coalesced into full sized planets.

As Podkayne suggests, with the large gap in size between Mercury and even this new KBO, it seems a logical place to draw the line.

Pluto will remain a planet but only because of the accidents of history that lead to its discovery and the misconceptions about its mass and effect on the rest of the Solar System. In reality I think that thinking of it as the ‘King of the KBOs’ is what is most logical.