OT: The solar system has 12 planets

[capnq]

Suicide Junkie said:
If Pluto AND Charon are planets, then Luna should surely be a planet too.

Not by the proposed standard, which involves where the objects' barycenter is. If the point the objects orbit around is inside one of them (Earth-Luna), the other is a moon. If the barycenter is outside (Pluto-Charon), both are planets (if they're both round).

One of the objections to this new definition is that the number of planets in our solar system could be as high as 53. I'm not sure why that upsets some people, though.

[bearclaw]

I think one of the criteria of a planet should be it's gravitational effect on the rest of the solar system. I had a solar system simulator at one point that showed that if you took any one of the 9 'classical' planets out of the solar system, then the other 8 planets went all squirrely. I have no idea what the magic cutoff point would be, but how many of the larger Kuiper Belt objects or even large asteroids (like Ceres) have that sort of effect?

Of course, according to Alf, the 10th planet in our system is named Alvin, not UB313

[Renegade 13]

bearclaw said:
Of course, according to Alf, the 10th planet in our system is named Alvin, not UB313

With the proposed addition of Ceres, Pluto or Charon would be #10

[henk brouwer]

capnq said:

Not by the proposed standard, which involves where the objects' barycenter is. If the point the objects orbit around is inside one of them (Earth-Luna), the other is a moon. If the barycenter is outside (Pluto-Charon), both are planets (if they're both round).

That's quite a messy definition... If I recall correctly the barycenter is influenced by the distance between the two objects, not just their mass. By this definition Charon wouldn't have been a planet if it had been in a lower orbit. I can agree with the rest of the proposal but the Charon thing is a bit weird.

edit: just found this link:
http://oklo.org/?p=120#comments
Apperently our moon will magically be promoted into planethood 30 billion years from now if this definition is accepted. (if the solar system is not destroyed before)

[Renegade 13]

henk brouwer said:
http://oklo.org/?p=120#comments
Apperently our moon will magically be promoted into planethood 30 billion years from now if this definition is accepted. (if the solar system is not destroyed before)

Within 30 billion years, the Sun will long since have gone into its red giant phase, with it's diameter increasing so much as to possibly engluf the Earth and Moon in its outer atmosphere. We won't care where the Earth/Moon barycenter is at that point All we'd have left of our Sun is the remnants of a really pretty planetary nebula.

[henk brouwer]

Very true
It's still an akward definition for a planet though, what if Charon had been in an eleptical orbit, so the barycenter would sometimes be inside, and sometimes outside of pluto, would charon change planet-moon status during it's orbit?

The whole double planet thing seems unnecessary, just name the heavier of the two objects the planet and the lighter one the moon, you're unlikely to find two bodies orbiting each other with exactly the same mass. If you do, just dump a couple of tons of material on one of them and your problem is solved.

[capnq]

henk brouwer said: what if Charon had been in an eleptical orbit, so the barycenter would sometimes be inside, and sometimes outside of pluto, would charon change planet-moon status during it's orbit?

If I'm understanding the definition of barycenter correctly, a barycenter does not move relative to the objects that define it.

[henk brouwer]

Hmm the barycentre is indeed defined by the *average* distance between the two objects. Still, if they are in eleptical orbits the distance to the barycentre can change, if I understand correctly, actually it would have to because the objects move closer to each other, so one or both of them have to get closer to the barycentre. the wiki page on 'center of mass'( http://en.wikipedia.org/wiki/Center_of_mass )has an animation of two objects orbiting around their barycentre in an eleptical orbit when you scroll down a bit. in this animation the distance between the objects and the barycentre changes. If the radius of one of them had been a bit over twice as large the barycentre would disapear in it for part of each orbit. (I really should do the math, but that scares me a bit actually, I just hope I'm right )

[Cipher7071]

It would seem that the barycenter for two perfectly spherical bodies would never move. But, if either or both were oddly shaped and rotating, then the barycenter could shift somewhat.

I find the definition of a pluton that depends on the object's origin compelling. Otherwise, how to decide according to size or orbit seems abitrary to me. They shouldn't spend too much time arguing over it, as much of the argument is semantic.

[Warshed]

This is where I say who the hell cares. Its a semantic debate based on the word "planet." You want more planets in the solar system, then you widen the definition, if you want less then you curtail the scope.

If they ended up defining Pluto as something other than a planet (or they added those other bodies), then all astronomy books produced in America (and possibly in the world) would have to be rewritten. Do you think that will happen? No. So guess what? While the astronomers go around saying there are X planets (and continue to debate it ad nausium), most people in the world and proboably most publishers would proboably continue to say there are only 9 planets, at least for awhile.

In the end who cares. Rather than wasting countless man hours on the debate of how to define the word planet, lets find out once and for all why the moon is made out of cheese and what type of cheese it is, or better yet astronomers should look for the location of heaven so we can send a space shuttle there and visit God.