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Obelisk
2012-Jan-25, 03:58 PM
Hi

I've a question for this forum I hope you'll be able to answer, The 2006 IAU definition of a planet excluded Pluto, Eris and all sorts of objects as they had not become gravitationally dominant in their neighbourhoods. Primarily this is because none of them are big enough.

In fact, no object in the Kuiper belt seems to be much bigger than Pluto and Eris. My question is, why is this? Why isn't there an Earth (or bigger) sized Kuiper belt object out there that could achieve sufficient mass to clear their neighbourhoods and be counted as planets?
And would the reason be unique to our solar system, or could large scale Plutinos form in any Kuiper belts around other stars?

Thanks for your time.

antoniseb
2012-Jan-25, 04:22 PM
As I understand it, the Kuiper belt is made of objects that mostly weren't formed out there, and at this point the planet-forming disk is long gone, so I wouldn't expect any new collecting into heavier planets to occur ... and none of the KBOs have the gravitas to eject the other objects from the space.

Jason Thompson
2012-Jan-25, 04:27 PM
Would the fact that the material in the Kuiper Belt region is spread over a much greater volume have any bearing on this? For objects in small orbits near the Sun wouldn't you expect it to start off very crowded and there to be a high likelihood of further aggregation/collision to form large planets, then the further out the material is the more disperse, so a large number of smaller planetoids would be the most likely outcome?

profloater
2012-Jan-25, 06:21 PM
Maybe the lack of a planet in the asteroid belt is a clue too. For small objects collisions tend to be destructive rather than constructive?

Jeff Root
2012-Jan-26, 09:39 AM
Whether a collision between small bodies leads to a larger
single body depends mainly on the speed of the collision.
When the bodies are in very different orbits, any collision
will be at a higher speed. The asteroids are generally in
quite different orbits, and spaced far apart. So collisions
are rare, and when they do occur, usually result in smaller
asteroids rather than one larger asteroid. The asteroid belt
must be very, very, very slowly disappearing as individual
asteroids are pulverized back to dust. Some of that dust
ends up on the larger asteroids, some on the planets, but
most is eventually vaporized by the Sun's heat and the
resulting vapor blown away by solar wind.

-- Jeff, in Minneapolis

Rhaedas
2012-Jan-26, 03:46 PM
Kuiper objects are very slow moving. I don't think we do yet, but do we know if they are more than basic rubble collections, like a lot of asteroids seem to be that are at much higher orbital speeds.

Xibalba
2012-Jan-29, 08:58 PM
I've read somewhere that it might be possible that there is a bigger object out in the Kuiper Belt (or was it in the Oort Cloud?). Just that the huge distance separating it from the Sun would make the object almost as cold as the background, thus barely noticeable to our instruments. In fact, I think it'd have to occult or disturb the light from a background star to detect it.

antoniseb
2012-Jan-29, 09:11 PM
I've read somewhere that it might be possible that there is a bigger object out in the Kuiper Belt (or was it in the Oort Cloud?). Just that the huge distance separating it from the Sun would make the object almost as cold as the background, thus barely noticeable to our instruments. In fact, I think it'd have to occult or disturb the light from a background star to detect it.

There are limits as to the combination of how large and far away something would NEED to be to escape our notice. Our deep infrared sky surveys have lowered that limit in the last few years by quite a bit. It is true that something the size of Jupiter could be in the outer parts of the Oort cloud, and not be noticed yet, but there is no reason to think that such a thing IS there, and very few processes which could have resulted in its creation and stable membership of our system. It would NOT have been created from the Sun's planet forming disk.

There is a group that have identified a statistical anomaly about the orbital axes of the super-long period comets suggesting something is out there with an orbital period in the tens of thousands of years. It could be Earth-sized or a little smaller. This could be true, but it is so far supported by weak evidence... and nothing refutes it except the inability of any process we know so far to create it.

Xibalba
2012-Jan-29, 10:01 PM
There is a group that have identified a statistical anomaly about the orbital axes of the super-long period comets suggesting something is out there with an orbital period in the tens of thousands of years. It could be Earth-sized or a little smaller. This could be true, but it is so far supported by weak evidence... and nothing refutes it except the inability of any process we know so far to create it.

That's exactly what I was talking about! It is "possible" but yeah, it's just an intuition for now. I also approve it would be made of the Sun's material, unless we're wrong about how the solar system was formed.