Is {pluto|sedna} A Planet?

Dr. Zowie writes "NASA's announcement last week of Sedna's discovery reignited the debate over whether Pluto is a planet. Dr. Alan Stern a noted planetary scientist and leader of the New Horizons mission to Pluto, pours on some gasoline with this article in which he skewers the various arguments against Pluto-as-planet. Choice quotes include 'You wouldn't deny a chihauhau a place among dogs because it is too small,' and 'if your brain was so completely full of names of people that it just couldn't take any more, would anyone new who you met after that, therefore not be a person?'"

Comments

Re:Asteroids?

[korielgraculus]

Th proposal in the article is that every body that is rounded by it's own gravity (apparently this happens at a few hundred kilometres) should be considered a planet. Actually sounds a reasonable definition to me.

You say Brontosaurus, I say Apatosaurus

[TrentL]

The Atlantic Monthly had an article about the Pluto situation years ago. The problem, though, is that "kids love Pluto." Scientists have tried to change names before (such as the dinosaur example). It'll be interesting to see what the public says about Pluto's demotion (if it occurs).

Re:You say Brontosaurus, I say Apatosaurus

[pavon]

Again, kids are very flexible about learning new things. They latched onto that Aptosaurus like nothing. Actually I think they kind of enjoyed being able to correct all the adults that still called it brontosaurus.

If you were to tell them that we have learned that Charon is not really a moon of Pluto, but that they are close to the same size so they revolve around each other like people dancing, they would think that is really cool. If you further went on to tell them that we have found out that there are a whole bunch of icy subplanets like Pluto and Charon but smaller, and maybe one day we will find one that is bigger, and maybe they could be the one to find it, they will get even more excited about astronomy.

Honestly, it is the adults that are stubborn about keeping the status quo, not the kids.

Re:You say Brontosaurus, I say Apatosaurus

[MammaMia]

Exactly. Adults in general need to put things in categories; kids in general categorize by how interesting things are. I think so many kids are fascinated by dinosaurs because there is SOOO much information out there - weird names to learn, incredible variety of sizes, shapes, habitats, behaviors, how fossils were formed, how they are discovered and studied, etc. Same goes for astronomy, if kids are given the opportunity to learn more than just the names of the major planets, they can become fascinated by all the differences between them, the different sizes and colors and surface features and moons and composition... and that's just the planets, never mind all the other interesting stuff out there.

Whatever the scientific community ends up agreeing on in this case, there are some people that will always insist there are nine planets because that's what they were taught as kids and that's that. So what. Those of us who know better will raise a generation with sharper critical thinking skills, who can understand not only the concept of evolution but also that science itself evolves as we continue to integrate newfound knowledge.

Re:Asteroids?

[Xzzy]

Honestly, who cares.

It's not like there's some legal reason to have the definition of a planet rigidly defined, it doesn't effect the money anyone gets, it doesn't influence political boundaries, and it won't get anyone out of jail.. so who cares? :P

If it's big, it's a planet. If it's not big, it's an asteroid. If it's not big and made of ice, it's a comet.

Might as well debate which text editor is bettor or whether we should be putting GNU in front of Linux.. it's such a silly thing to discuss it baffles me this shows up in the news so often.

Inconsistency

[Michael Woodhams]

He wants a Boolean criterion (yes/no) for planethood, but then criticizes a 'minimum mass' limit as being arbitrary. It is not possible to impose a Boolean criterion onto parameters that vary continuously without there being an arbitrary boundary somewhere.

Other than that, a pretty good discussion. His suggestion will still require an arbitrary boundary (how round is round?) but it is not totally arbitrary.

His rule has a problem that it turns into planets objects that we had previously decided were not planets. It has the advantage of being less arbitrary than the alternatives. Whether the advantage outweighs the disadvantage is a matter of taste.

Approach to understanding the solar system.

[pavon]

I think part of the problem is the fact that memorizing the 9 planets are all most people really know about our solar system, and so they tend to be fairly sentimental about it. I think a much more accurate and interesting approach to teaching kids would be to start of by brainstorming all the different types of objects in space - galaxies, solar systems, stars, moons, astroids, comets, nebulas. Then instead of memorizing just the planets memorize all the different regions of our solar system and what makes them special. Start with the sun, then you get to the inner planets, then astroid belt made mostly of rock, then giant gass planets, then the Kupier Belt full of icy objects and finally the Oort cloud. Then lastly you describe the interesting features of each area, including the planets and what makes them unique.

This journey approach would be far more interesting to the kids and by the time you got to the point of describing pluto and charon, they would have an understanding of how diverse (for lack of non PC word) matter in space is and would be less concerned about sticking a specific catagory on it, and just be excited that it was yet another unique and interesting thing.

It's the difference between decribing the cool terrain, people and features in country as opposed to just memorizing the state capitals. The former is far more interesting, and informative, and kids will eat it up.

Re:Requirements? Look to gravity!

[Chris Burke]

How about the simple argument that planets are gravitationally strong enough to pull themselves into nearly spherical objects, whereas asteroids are not.

I like this definition a lot. While it does leave some wiggle room as to what exactly constitutes "spherical", it is still based on a physical property of the object related to its mass. This makes it better than any arbitrary size/mass requirement (e.g. "Anything as big or bigger than Pluto").

Pluto, BTW, Sedna, and many of the largest moons can all do this.

I'm going to be extremely unhappy with any definition that demotes Pluto. Also, anything that makes Pluto not a planet is going to be close to making Mercury not a planet, and that's just not acceptable. :)

I also think, for the record, that if something as large as Luna, or Titan, or Europa were out floating in space orbiting the sun and not another planet, they would be considered planets too.

Titan is bigger than Mercury, so a Sun-orbiting Titan not being considered a planet is unacceptable. :) But clearly a planet-sized object orbiting another planet is a moon. Again, this definition makes perfect sense.

I'm not an astronomer (but I play one on occasional weekends), but of all the definitions I've heard, "big enough to be spherical and orbiting a star" is the simplest and most logical.

And for the record -- if there was some comet out in the Ort cloud with an incredibly eccentric orbit around the sun that was the size of Titan, that'd be a planet too. IMHO. :)

Re:Enough already

[Keebler71]

so by your reasoning, the baseball on my desk is a planet?

I wrote a cool program in Matlab for a graduate astrodynamics class I took that would plot the planets and their orbits at any time. One thing immediately jumps out at you.... Pluto is not a freakin planet! Any good diagram of the solar system shows to screwed up Pluto is.

For those who hate pictures, here are the orbit elements of the planets in tabular form

First off, note that Pluto has an eccentricity of almost 0.25, that is WAY oblate. Now, someone will probably point out that Mercury is nearly that oblate and we can argue whether Mercury is really a planet also. It probably is, however, it is soooo close to the Sun that it has comparatively zero angular momentum - and remember, that is the job of the planets, to store the bulk of the angular momentum of the solar system as it was formed (you do remember that right?) Anyway, Mercury is so close to the Sun, that its orbit is much more easily perturbed by higher J2 and J3 harmonics of the Sun and you would expect it to have be a little out of plane and eccentric due to multibody effects as well.

Moving on, how about that inclination... 17 degrees. Again, excluding Mercury, the next closest is 3.4 deg and the next closest outer planet is 2.5 deg.

And how bout these data. Check out the rotational period... 153 hrs.. the next closest outer planet is 17 hrs.

Sorry folks, it is a captured Kupiter belt object... move along.

Re:planet, definitely (both Pluto & Sedna)

[Guppy06]

"Both even have satellites of their own."

Ehhh...

To be really picky about Newton's third law, moons don't orbit the planet itself, but instead both tend to revolve around a point between the two centers of mass (ie. the center of mass of the planet-moon system) because of mutual gravitational attraction. For example, the reason we're able to find (disgustingly massive) extrasolar planets is that the planets pull on its parent star enough for the star's motion to be visible from here.

I don't know off the top of my head whether the mass ratio between the earth and the moon is enough to pull the center of mass of the earth-moon system outside of the earth, but I do know the center of mass of Pluto-Charon is well outside of Pluto.

So that might throw a wrench into the works of a "it has a moon so it's a planet" idea.

Re:Gravitational Rounding AND Atmosphere

[egomaniac]

I think there needs to be the addition of an atmosphere to be considered a planet. Really it's just a round rock without one. It pretty much classifies moons as planets without that qualifier.

First -- how much atmosphere? Every sizable rock will be able to hold onto at least a few gas molecules.

Second -- the Sun has stripped Mercury of its atmosphere. However, if Mercury were orbiting at the distance of Mars, it would have been able to retain quite a bit more air. Your definition is biased against close planets.

Third -- our atmosphere came largely from outgassing. A planet with a different composition (say, similar to the moon), or less active tectonics, might have dramatically less of an atmosphere.

So, you have now tied the definition of what is a planet to a complicated interplay between its size, composition, geology, distance from the sun, and who-knows-what-else factors.

Can't we just say "You need to be this big to be called a planet" and leave it at that?

Ceres is round

[erice]

Ceres is round. Vesta is nearly so. Do they get promoted to planet status?

Re:Requirements? Look to gravity!

[maladroit]

> that the center of gravity for our orbit is well off from the center of the earth.

Cool - another point to debate. What is the transition point from 'planet-moon' to 'bi-planetary' ?

Basing it on the center of gravity seems like a good idea, but 'well off from the center' is a little bit fuzzy. We could pick a number - say, 50% of the larger planet's radius - in which case the Earth-Moon system meets the criterion, since the center point is about 75% of the Earth's radius away from the Earth's center (some references).

But now we've done the same thing the original article was complaining about - we picked an arbitrary value, just, well, because.

It's seems like a physical point would work a bit better - say, the surface of the larger planet. Then the definition becomes a bit easier: if the center of gravity is in space, it is a dual-planet system. Otherwise, it's a planet-moon.

How you categorize a center of gravity within an atmosphere is left as an exercise ...