Science and Society

Makes sense except for Charon (does it not orbit a planet rather than a star or am I misinformed).

Quietman, Pluto and Charon are sometimes decribed as a twin system, as each orbit a center of gravity which is an average between the two (in laymans terms they orbit each other) and both orbit the sun

By the definition you propose, with Charon classified as a 'planet', then you have to classify all of the other "moons" of planets that "are spherical and orbit a star" as planets as well - which would make the solar system have over 100 planets. The MOON would be considered a planet.

To me, the solar system consists of the planets which share a more or less flat orbital plane. From a physics point of view, these are certainly part of the original accretion disk that formed the original sun and planets. Those would be the bodies that formed the 'original solar system'.

That means any spherical object with an orbital plane within 7% of inclination to the sun's equator with the sun's gravity being the average center of its primary orbit and doesn't intersect the orbit of another such body are planets. Anything spherical with a more eccentric orbit is probably captured (or knocked out of orbit) and while it COULD be planet-like, or even an original planet, it's still unlikely to have been part of the original formation of the solar system's planets.

What to call them, though, is problematic. By definition, they are planetoids - an astronomical body resembling a planet, but not defined as such due to eccentricities in orbit. This means a solar system CAN be made up of planetoids since size has nothing to do with it - only shape and orbit.

Why muck things up with a nod to the Pluto fans by calling them a misleading and astronomically incorrect "Plutoids"? Pluto always was a strange duck compared to the 8 other planets. It's a planetoid called Pluto. The Solar System has 8 planets. There are probably far more planetoids than planets, but there comes a point at which we have to stop adding planets to our solar system just based on shape or size and look at what makes it worthy of being called a 'planet' - orbital behavior. Pluto and the others don't measure up.

M.Alto - Don't the earth and moon technically orbit one another then? We sway with the moon after all. I thought the designation of "moon" just belonged to a weaker orbital body that orbits a "planet", rather than a star. Or is there some set cutoff point in the size of the body that makes it a "binary planetoid" system or something of the sort (though that once again throws us into the arena of arbitrarily assigning objects controversial definitions).

Note from Ned--

I suppose, by having this discussion, we're proving the IAU's point that there's been no decent definition of "planet," even though we've known of them since ancient times.

My understanding is that Charon escapes its past status as a moon, as M.Alto suggests, because the center of gravity between it and Pluto is actually in the empty space between them. The Earth's Moon does tug at us, but the center of gravity is still within the body of the Earth. The same is true of the solar system's other known moons.

Dr. Sykes and his allies would argue that Pluto and Charon deserve double-planet status; no more of this 'Plutoid" stuff.

grr - You make a good point that, as you say, is relevant to the whole discussion. If binary systems are both "planets," then it is arbitrary where we draw the line as to when the second body is "too small," or the focal point between them is within the larger body, or whatever.

This guy thinks by suggesting a new definition he has solved the problem. What we really need to recognize is that the "problem" is unsolvable. We are constantly trying to categorize everything in a world that is often not "categorical." Many objects present a continuum of variation from one extreme to another, and any categorical "boundaries" we place are going to be arbitrary. We also get frustrated when something new just doesn't seem to "fit" our previous categories.

Pluto is what it is regardless of what we call it. There is no need for it to "fit" anywhere, unless we are making broad statements about the solar system, and then we use whatever definition works best for the question at hand.

Scientists come across this sort of thing ALL the time, and we learn not to worry about it so much. In fact, such diversity is usually of interest in itself.

grr---- Building off of what ned and I have said, our moon is approx. 1/4 our size while pluto and Charon (I believe) have only about 1/10 a difference in mass, this is why they orbit the average distance between each other. Yet as you said some have proposed that the Earth/Moon be considered a "binary planet" However, the main orbit of both those bodies around the sun is centered within the mass of the Earth.

The History Channel did a wonderful piece on the origins of these conflicting definitions. As I understod it the word "Planet" was used to identify the original seven heavenly bodies the Ancient Greeks identified as "Irregular stars"-(Those who moved contrary to themotions of all the true stars)
Therefore like the current planets "Jupiter", "Venus" and "Mars" our moon had a name "Luna." This name was later interpreted in English as "Moon." So in the begining a moon had no definition b/c "Moon" was truely a name.

I hope I haven't just confused everyone more.

So much for Americans not being science savvy

The point that "science" should NOT being presented as learning a list of facts was well presented, but will probably be missed by most people.

M.Alto
Thank you, I had been misinformed.

John Right
You mean like Skinner's children?

Ned
Saw your comment after making my reply, thank you as well for the clarification. Charon was not known or at least not taught when I was in school. So I'd heard it called Pluto's moon and assumed that it orbited Pluto.
A dual planet may be unique but totally understandable. The greek "planet" means wanderer so their original term had a very wide definition.

M.Alto
When Luna was named it too was called a planet. :)

We already use modifiers to describe planets. Gas giants, rocky, etc. but they are still planets. Why can't pluto be a planet and be further described as a 'dwarf planet'.

Whatever happened to Sedna?

There's a certain amount of sense to making a distinction between bodies that dominate and clear out the regions of their orbits and those that have not, but presumably this is potentially a gradual variation. If we wanted to classify all these objects we might use some numerical scoring system, like P1S0 (definitely a planet, definitely not a star).

Pluto was denied planetary status because it didn't fit the criteria of clearing out the region of its orbit, though it clearly fit the other two. I found the IAU denial very arbitrary and going against the spirit of the scientific discovery process. I hope the criteria can be lifted so that the annual Pluto Day in New Mexico can be legitimatized and my own creative layperson thinking about science can be validated!

I've accepted the new definition. I didn't like it at first, because it's what I grew up knowing. But I also know that science has a tendancy to change at any time. Our percieved understanding of one thing changes as soon as we get more information on said object. I could go for giving Pluto and Charon a Double PLanet tag. Plus any other objects that are similar. As for the 13 planets, I'm not sure. With Pluto and Charon being D.P., that eliminates 2 from the list, leaving 11. I'm not sure about Sedna, it's orbit is highly ecliptic, and takes it quite far from Sol. That may be why it wasn't classified as a planet in this man's definition.

I also think it's sad that the rest of the country doesn't see it this way. It points to a fundamental flaw in our education system. I hope it can be corrected, but I doubt it will anytime soon.

After reading all the posts here, I wonder just how far out from Sol does one makes an object fit the criterias for a 'planet'. Does Seda orbit Sol or is it too far out to be an actual 'orbit'?

If say, for example, we find an object beyond Pluto that is the size of Mercury and does a very long orbit around Sol, does this object qualify for planetary status or is it too far out to be consider a planet?

BTW, I find this whole debate IMO silly. I grew up with the notion of Pluto as a planet. It first shock me to read the IAU changed Pluto's status. I was okay with 'dwarf planet', but 'Plutoids' just sounds too stupid. Least the first title able Pluto to keep the word 'planet' with it.

kat

There were other reason why Pluto did might not be considered a "true" planet. It is not round; it is smaller than several moons, and it probably doesn't have any stratiphied geographic "structure" (related to not being round.) It is essentially a big lump of rock and ice - and apparently more ice than rock.

I don't care if it is called a planet or not, but it IS very different from the others.

Thanks to Ned and others for clearing up the Charon issue, that makes sense. I wonder how or if a removed orbital center would effect the weight of objects on the face of pluto tidally locked towards charon, vs the opposing face...Would walking from the side facing Charon, to the side facing away feel like an uphill climb?