Astronomy

http://curious.astro.cornell.edu/question.php?number=205

http://www.timthrelfall.com/originofsol.html



[edited by - quant on July 13, 2003 3:02:33 PM]

So how often do cases like Pluto come up and how often do entire non-coplanar systems arise?

We have no idea because we haven''t discovered all that many extrasolar planets yet.

For goodness sake, a high school physics course should explain to anyone why planets are much more likely to orbit in the same plane!!!!!

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Making cross-platform software actually cross-platform (meaning compiles on OS X)

Please enlighten us then...

I prefer the simpler explanation... after millions of years of particles colliding, forming bodies crashing into each other, etc. this disk is what we are left with. Had it been anything other than a disk, things might still be crashing.

It would be interesting to see a computerized model of a nebular cloud whose particles influence each other with gravity, allowing bodies to collide, and naturally form a disk to establish equilibrium in the system.

The theory I grew up with as to why there is a narrow band of asteroids is this...

When the sun ignited and became a star, the energy discharge stripped the inner four planets of their softer outer surfaces, resulting in smaller, harder planetary bodies. The blast stopped between Mars and Jupiter, and the leftover material from the blast formed the asteroids. That's why Jupiter and the rest of the outer planets (not including Pluto) are larger and much more liquid and gas based. I don't know what state the formation of the solar system was when this occurred, or if the removed material from the inner planets was enough to account for the density of the asteroid field.

EDIT: There is also a theory that the asteroid field was a planet that was not allowed to develop because Jupiter kept tearing it apart with its gravity as it passed by. What happened to the blast theory I grew up with, I dunno.


[edited by - Waverider on July 14, 2003 4:25:04 PM]

EDIT: There is also a theory that the asteroid field was a planet that was not allowed to develop because Jupiter kept tearing it apart with its gravity as it passed by.
This is the generally accepted theory.

What happened to the blast theory I grew up with, I dunno.
I've never heard of it (even in a graduate astrophysics course). Ejecta from starbirth tends to be in the direction of the axis of rotation, not in the plane of the disc. Also, what do you imagine "stopped" the ejecta like that. I can't think of anything.

EDIT: I spel good

[edited by - sQuid on July 14, 2003 5:25:28 PM]

I figured if supernovas make nebulae, then there is a definable end to the blast. The material, I presume, is being acted upon by gravity from the source it was ejected from (the strongest source around), and failed to reach the escape velocity.

I believe I read this in an encyclopedia. But, it was a long time ago. Also, it doesn''t explain why the material just didn''t fall right back into the sun again. It doesn''t seem there would be enough left between being fllung out into space and dropping back into the sun to leave a fairly regular band of asteroids. Still, it sounded cool.

I still don''t really understand why the planets orbit in one plane. How much do the planets actually influence each others paths? It really seems like there should be stable systems with planets in multiple planes

I''m taking another guess here, but I think the gas cloud had to stablize into a disk before the planets could even form. Otherwise, there would be too much collision to allow them to stay together as they gradually took on more mass. Imagine all the little clumps that were trying to form and exerting gravity on each other. They collide, mix, disperse, etc. Only in a disc could everything form without crossing paths so much that formation isn''t possible.

maybe the plane thing is just coriolis on a galactic scale - when the gases or particules in the area started converging to the center under their own gravity, the rotation around the galaxy center caused those that are planar with the galaxy rotation to gain an angular velocity and thus to enter orbits around the sun; those who werent' just accelerated towards the center and colapsed either in the sun or the forming planets.

(edit: spelling, Dot)
"I woke up sweating and clutching my pillow. Thankfully the powerful and reassuring visage of Optimus Prime staring back at me from my pillow case served to ease my frayed nerves. Like the giant robotic father I never had he always knows just what to say" - Gabe, Penny-Arcade

Alexandre Moura

[edited by - alexmoura on July 14, 2003 9:41:56 PM]

From http://www.pha.jhu.edu/~akir/Seminar/seminar.html

Additionally, the cloud will have some net angular momentum due to galactic rotation. As the cloud shrinks, its moment of inertia decreases and its angular velocity must increase in order for angular momentum to be conserved. So, the cloud will begin to rotate about some axis.

Material above and below the plane of rotation may collapse toward the plane while conserving angular momentum. But for motion about the central axis, the centrifugal force may balance the gravitational force, giving a stable orbit. Shrinkage to a smaller radius is precluded unless the centrifugal force is reduced, which may only happen if angular momentum is ejected from the system. This can happen, as the inner parts of the cloud pass angular momentum to the outer parts through viscosity in the gas, but it is much slower than the collapse of material to the plane of rotation. Thus, the initial spherical nebula collapses into a flat, rotation disk within a few million years.

quote:

---

Original post by Waverider
When the sun ignited and became a star, the energy discharge stripped the inner four planets of their softer outer surfaces, resulting in smaller, harder planetary bodies. The blast stopped between Mars and Jupiter, and the leftover material from the blast formed the asteroids. That''s why Jupiter and the rest of the outer planets (not including Pluto) are larger and much more liquid and gas based.

---

from http://physics.colorado.edu/~ml/ASTR3210/Homework/ReviewSession.pdf

(quote)
The temperature of the nebular disk decreases with distance from the Sun. Therefor, the rough compositional gradient of the planets can be explained by the fact that it never got cold enough in the inner Solar System for the volatile elements to condense before being blown away [by solar wind].
(end quote)

To better understand that, and for context, etc, view the link.

The Angular Momentum issue is the main reason the planets are in the same plane. The (current) most likely formation theory is as follows:

1. Start with a region of InterStellar Medium (ISM) that is slightly more dense than the surroundings. This exerts a slight gravitational pull.

2. As dust and gas is pulled in, the central region becomes more and more dense and the pull becomes stronger. As the dust contracts around this centre there''s a change in angular momentum, which forces the dust into a thick disc (thicker further out, though less dense)

3. Central density becomes great enough to cause star formation. Some of the dust near the star gets blown out, but there''s still some left - say about 1% of the mass of the star.

4. Dust collapses, disc becomes thinner and more defined. The shockwave from the star''s explosion has made some very dense regions in the disc. These start to "sweep up" matter. The matter has very irregular orbits in the disc. Many collisions occur.

5. Star''s magnetic field and solar wind starts to push the disc away from the star. Planets start to hit each other, break up, reform new planets. An eccentric orbit causes greater gravitational tension so a planet with a circular orbit is more likely to survive. Outer orbits can be more eccentric since the gravitational influence is weaker.


Other interesting things:

* I''m not sure if the asteroid belt was one a planet or if it''s just remained that way, but it''s almost certainly being "sheparded" by the inner planets and Jupiter, the same way that Saturn''s rings are sheparded by its moons.

* There''s dust and gas in the Lagrange 4,5 points in the Earth-Sun system. It''s believed that this is matter captured soon after the Earth was formed.

* Impact craters on the Moon tell us that there was heavy bombardment early on in its lifetime, but then it suddenly (i.e. over the course of 10,000 years or so) stopped.

* The Earth and the Moon were probably formed from the same body, but split apart after a collision with another large body. (The moon''s rotation syncs with Earth''s because its centre of mass is not central, and the Earth pulls it round)

* The Oort Cloud is a spherical shell of asteroids at the edge of the Solar system (I want to say 200AU, but I can''t remember if that''s right), and we''re not quite sure how it formed.


Incidentally, "Universe" by Kaufmann is a very accessible textbook.

[teamonkey]

cause god doesnt fill in the y value when he translates the planets

glTranslatef(planet.x, 0, planet.z);

'nuff said



[edited by - jenison on July 15, 2003 9:44:36 AM]

[edited by - jenison on July 15, 2003 9:45:22 AM]

As I understand it, out sun is second generation ie we are build out of the remains of the first explosion. I can understand that if the old sun had spin, the layers of gas would settle down into orbit in the direction of spin. So what we have is this

             ----->
        ----->    ----->
     ----->  xxxxxx  ------>
  ----->  xxxxxxxxxxxx  ----->
----->  xxxxxxxxxxxxxxxx  ----->
-----> xxxxxxxxxxxxxxxxxx  ----->
-----> xxxxxxxxxxxxxxxxxx  ----->
----->  xxxxxxxxxxxxxxxx  ----->
  ----->  xxxxxxxxxxxx  ----->
    ----->   xxxxxx  ----->
        ----->    ----->
             ----->
 

Where -----> indicates the orbit of particles on the nearest face.

The particles will be evenly distributed (more or less) so what compells them to fall to the middle?

********


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I have absolutely no idea what you''re trying to convey with that diagram, but read the links I listed. They have full accounts of (I think) generally accepted theories of solar system formation. I also already quoted this:

From http://www.pha.jhu.edu/~akir/Seminar/seminar.html

(quote)
Additionally, the cloud will have some net angular momentum due to galactic rotation. As the cloud shrinks, its moment of inertia decreases and its angular velocity must increase in order for angular momentum to be conserved. So, the cloud will begin to rotate about some axis.
(end quote)

As for what causes the cloud to shrink / fall into the middle: Gravity!

i read the links and i think i''ve just about got it: a disk is energy efficient

quote:

---

Additionally, the cloud will have some net angular momentum due to galactic rotation.

---

not all galaxies rotate. i suppose it''s not out of the question to have a star which doesn''t rotate (except in local turbulent regions) overall. would it be possible for such a star to have planets? if the explosion was sufficiently weak and the star sufficiently isolated, couldn''t all the material eventuall fall back?

********


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To join: Put these lines in your signature and don''t post crap!