OT: The solar system has 12 planets

[Kamog]

Charon being included as a planet was a bit surprising. Well, we'll know in a week or so whether this new redefinition is approved by the vote. If more planets are discovered, we could end up with more than 12 in the future.

I can imagine that many years from now, I'll show my future kids my old astronomy book that shows only 9 planets, and I'll say, see, there were only 9 planets back then!

[Suicide Junkie]

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

Won't somebody think of the children?!

[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

[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)

[PvK]

From the first article linked in the initial post:
"Under the plan, a planet would be any object that orbits a star, is neither a star itself nor the moon of another planet, and contains enough matter that gravity forces it into a nearly round shape."

Er... how much loose dust does it take for it to gravitate into a ball that isn't scattered by the solar wind? Seems like, especially if one goes out to the Kuiper Belt and beyond, there might be many many many of these, and they wouldn't fit an intuitive definition of what a planet is.

[geoschmo]

Randallw said:
I assume you disagree with Xena rather than just UB313. I hope they choose something better otherwise in 1000 years a little girlboy asks her 4th clone father what does Xena mean.

If that's the name they settle on, then most likely in 1000 years when she asks what Xena means the answer will be "It's a planet." Just like these days there aren't too many people that know the mythology behind the other planet names.

[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

[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.