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POLL: Sedna, planet or debris?

Cogman

Cogman

Lifer
simple question, heres a link to where I read about it at. My next question, btw, how come we dont see any planets rotating aroud the top of the sun (every planet we have discovered have be on about the same orbital plane as earth)?
 
I would guess that the planets orbit in generally the same plane due to the fact that the gas/debris cloud from which they formed was rotating in that direction ( fundamentally conservation of angular momentum)
 
Yeah, the rotating disc that condensed into the solar system kept everything in roughly the same plane of orbits. Anything orbiting the sun out of this plane is believed to have been disturbed by some external impact/gravitational influence or be from outside the solar system itself (many comets are do not orbit within the plane of the solar system). As for the initial question, the definitions of "planet" that I am familiar with are very vague in terms of what differentiates a planet from an asteroid or planetoid.

1) nonluminous body that orbits a star - sure, but there's a lot of these that we don't call planets
2) larger than an asteroid - fairly ambiguous in defining size, considering asteroids are defined as being "smaller than planets"
3) revolves around the sun in an independent orbit - appears to meet this criteria


I don't see a clear resolution to this since the only real points of contention is the size and orbital eccentricity of the object, and the definitions supplied for 'planet' give no real specifics on what the requirements are.
 
Originally posted by: Cogman
simple question, heres a link to where I read about it at. My next question, btw, how come we dont see any planets rotating aroud the top of the sun (every planet we have discovered have be on about the same orbital plane as earth)?
Click to expand...

you know...i have always wondered that myself, but never asked anyone who knew the answer. that is very interesting and i hope someone here can shed some light on it.
 
Geez, you see that thing's orbit? Just sliiiightly elliptical.

8 billion miles away right now; ranges up to 84 billion miles. And it takes 10,500 years to complete one orbit. Sounds more like a big comet that got knocked toward the inner solar system.
 
Horrid orbit, but it does distinguish itself from the Kuiper objects because of that. I vote planet, regardless if it was an original member of this solar system, it is substantially larger than other objects they are finding in the Kuiper belt and other known asteriods. Where is Hubble on this one?
 
I voted No. All the reports I've read have called it a "planetoid".

It's big enough not to be "debris", but too small to be a "planet", so it's a "planetoid".

Link to the FOXNews story.
 
ya, "planetoid" is another way to say "I don't like either, so I'm going to be difficult and not choose a side, I'd hate to be wrong"

and linking to FOXNews...couldn't you find another site that held that AP story? jeez
 
The solar system could start looking like Saturn soon - that thing's moon count seems to keep increasing; no doubt it'll skyrocket once the Cassini-Huygens probe enters Saturn orbit. If we ever send a probe out to check out Pluto, it'll probably be in range to find all sorts of other large objects out in that area. We might have to redefine "planet" completely at that point in time, or else we could just have a solar system with hundreds of planets. I doubt that Sedna is the only one of its kind out there.
 
Re: the planar orbit thing:

If you have a diffuse cloud of dust with particles moving randomly, they will have a group centre of mass. If you add up all the motions of the particles, there will be a net angular momentum vector which can describe the total angular momentum of the system. As the cloud collapses, it spins up and flattens out on a plane orthogonal to the angular momentum vector.
 
Personally, I lean more towards the downgrade of Pluto. I just think it really disturbs the whole niceness of the planetary system... four inner planets and four outer planets all moving in roughly circular orbits on roughly the same plane.

The other thing is that Pluto doesn't actually orbit the sun directly... It and Charon orbit about each other, and this system orbits about the sun. Doesn't this violate ZeroNine8's third criterion? [Edit] Actually, I'm not quite sure on this point, different sites seem to have different views. Anybody know for sure? Some places say that Pluto-Charon is a binary system, others that Charon orbits Pluto like a regular moon would.

Anyway, there's no doubt at all about the other 8, so why not just make those 8 the planets and let everything else be something else. They could even make up a new name for them... Objects that are composed kind of like planets and are bigger than most asteroids, but are in really strange orbits or are otherwise not really planet material.
 
Originally posted by: Brucmack
The other thing is that Pluto doesn't actually orbit the sun directly... It and Charon orbit about each other, and this system orbits about the sun. Doesn't this violate ZeroNine8's third criterion?
Click to expand...

Earth + Moon is a binary system orbiting the sun.

 
Originally posted by: glugglug
Originally posted by: Brucmack
The other thing is that Pluto doesn't actually orbit the sun directly... It and Charon orbit about each other, and this system orbits about the sun. Doesn't this violate ZeroNine8's third criterion?
Click to expand...

Earth + Moon is a binary system orbiting the sun.
Click to expand...

No. When talking astronomy, a binary system means that two bodies orbit around a common point. There are many binary star systems, where two stars orbit about a point that I think is usually the centre of mass of the pair.

The earth orbits directly about the sun, and the moon orbits about the earth, so therefore they aren't a binary system.
 
Originally posted by: Brucmack
Originally posted by: glugglug
Originally posted by: Brucmack
The other thing is that Pluto doesn't actually orbit the sun directly... It and Charon orbit about each other, and this system orbits about the sun. Doesn't this violate ZeroNine8's third criterion?
Click to expand...

Earth + Moon is a binary system orbiting the sun.
Click to expand...

No. When talking astronomy, a binary system means that two bodies orbit around a common point. There are many binary star systems, where two stars orbit about a point that I think is usually the centre of mass of the pair.

The earth orbits directly about the sun, and the moon orbits about the earth, so therefore they aren't a binary system.
Click to expand...

The moon and the earth are a binary system, they both orbit about a point that is not the center of mass of either. The same is true for for the Earth/sun. Just because the difference in size results in one object rotating 'approximately' about the center of mass of another doesn't mean it is technically correct to say that the moon orbits the Earth but the Earth doesn't orbit the moon.

And those definitions that I posted were just some of the first ones I found, they are by no means complete or 100% correct.
 
The center of mass of the Earth + moon system is approximately 11,000 miles above the surface of the Earth. Don't know if you would even consider this " 'approximately' about the center of mass" of the Earth.
 
Originally posted by: Brucmack
Originally posted by: glugglug
Originally posted by: Brucmack
The other thing is that Pluto doesn't actually orbit the sun directly... It and Charon orbit about each other, and this system orbits about the sun. Doesn't this violate ZeroNine8's third criterion?
Click to expand...

Earth + Moon is a binary system orbiting the sun.
Click to expand...

No. When talking astronomy, a binary system means that two bodies orbit around a common point. There are many binary star systems, where two stars orbit about a point that I think is usually the centre of mass of the pair.

The earth orbits directly about the sun, and the moon orbits about the earth, so therefore they aren't a binary system.
Click to expand...

Any two bodies in orbit about each other will orbit the same point. A black hole and a single atom orbiting each other orbit around a common central point. In this extreme case it's essentially the black hole itself, but still... The earth and moon orbit a common point, same as the earth and sun. Of course this is all approximate as all bodies in the solar system tug on each other and the sun with different amounts, so we usually just approximate their motions (and to a very good degree as well).

When you drop an apple the earth actually "falls up" to meet it too...
 
Originally posted by: glugglug
The center of mass of the Earth + moon system is approximately 11,000 miles above the surface of the Earth. Don't know if you would even consider this " 'approximately' about the center of mass" of the Earth.
Click to expand...

That's not the data that I have found... hmm, it seems like there is a lot of conflicting data about, heh.

I read that the centre of mass is about 1000 km below the earth's surface. So closer to the earth's surface than to the core, but still inside the earth.
 
Originally posted by: Brucmack
Originally posted by: glugglug
The center of mass of the Earth + moon system is approximately 11,000 miles above the surface of the Earth. Don't know if you would even consider this " 'approximately' about the center of mass" of the Earth.
Click to expand...

That's not the data that I have found... hmm, it seems like there is a lot of conflicting data about, heh.

I read that the centre of mass is about 1000 km below the earth's surface. So closer to the earth's surface than to the core, but still inside the earth.
Click to expand...

The moon is 0.0123 the earth's mass. So their centre of mass will be 1/1.0123 of the distance from the moon's centre to the earth's (avg say 370 000 km)

Multiply that out to get ~365 000 km away from the moon, or ~5000 km from the earth's core. The earth is 6380 km in radius, so it's about 1000 km below the earth's surface 🙂
 
First: Rotation = spinning on its axis.
The moon does not rotate around the earth. The moon revolves around the earth.
The earth rotates on it's axis. It revolves around the sun.

Second: Unless Newton made a huge mistake, the earth exerts a gravitational force on the moon, and the moon exerts an exactly equal force on the Earth. (people have trouble with this fact. They can "repeat for every force there's an equal and opposite force" ad nauseum, but they don't believe it) Since this results in a net force on the earth, it would be wrong to say that the earth continues in a path, unaffected by the moon, while the moon simply revolves around the earth.
 
Second: Unless Newton made a huge mistake, the earth exerts a gravitational force on the moon, and the moon exerts an exactly equal force on the Earth. (people have trouble with this fact. They can "repeat for every force there's an equal and opposite force" ad nauseum, but they don't believe it) Since this results in a net force on the earth, it would be wrong to say that the earth continues in a path, unaffected by the moon, while the moon simply revolves around the earth.
Click to expand...

Yes, this is correct, and that's why we get tides. But that doesn't make the earth-moon pair a binary system. It's a simple matter of the weight ratio... The earth is so much more massive than the moon, the centre of mass of the system is actually inside the earth. So the earth can't orbit about that point. Sure, it gets pulled by the moon, but it's insignificant compared to the effect the earth has on the moon.

With a system like Pluto-Charon, they are much closer to being the same mass (I think Pluto's only about twice as massive?) This means that the centre of mass is between the two bodies, and they orbit about this point, making it a binary system. As such, Charon isn't a moon in the traditional sense, only because Pluto is considered a planet and Charon is a smaller object in the vicinity.
 
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