What is preventing asteroid fields from bunching up and becoming planets?

I'm new to astronomy and have read some articles on the formation of a solar system. I have gathered this: the planets are supposed to be formed from a lot of material that is attracted to each other by gravity, eventually, because of gravitational attraction to each other, bunching up (combining into) and forming a body like a terrestial planet, gas giant or star, depending on how much matter bunches up, and the gravitational influence on that body. Eventually, if the body gets big enough, the pressure in the core is so high that it starts fusion and becomes a star. Please tell me if/where I am wrong in my current understanding of this.

Our solar system has an asteroid belt and I understand that the gravitational pull from the sun and other planets keeps it from forming more planets like that. There is no more room.

But what about asteroid fields like the Oort cloud? Why wouldn't planets form in that (if all the asteroids/comets are attracted to each other and eventually bunch up)? Why is it unlikely? Assuming there isn't enough material for a star to form there.

What prevents large asteroid clouds from forming planets?

 

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Good question. You've got a lot of common sense sg, more than most astronomers. The mainstream answer is that they are too small to start to coalesce. It's the same reason two pieces of rock don't stick together on Earth's surface. The formation of the planets and stars is therefore something of an enigma. What gave the planets the necessary size in the first place? An alternative idea is to think of a highly gravitationally attractive inner innermost core, which is different to normal matter. See here for a hint at this possibility Earth's New Center May Be The Seed Of Our Planet's Formation.

 

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It would make sense to assume that highly magnetic asteroids would coalesce more easily than ordinary rocks. Magnetic iron is assumed to reside at the center of the planet..Magnets on Earth's surface do stick together..

Therefore we should start with magnetism rather than gravity as an explanation perhaps??

 

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You may be surprised to learn that there is a dwarf planet named Ceres in the asteroid belt between Mars and Jupiter. This is off the cuff and you may find better material online, but to my knowledge Jupiter (or any first order Jovian planet in any given solar system) slings material from the inner solar system outward. Not only that, but a lot of comets in the orbital radii of the Oort cloud are made of gases that are frozen. They are much further from one another and have much more eccentric orbits than bodies that are closer to the sun. Therefore, their gravitational attraction to one another is much less, and interaction between each object not as uniform as in a closely circular system. That is the best answer I can give you to your question with my current knowledge, keeping in mind that the Oort cloud is not observed and is only in fact hypothetical/theoretical. I'm sure wikipedia or any astrophysics textbooks will have more in depth answers with appropriate equations that will help answer you question.

 

That is essentially correct.

There is actually several belts. There are areas that have few asteroids and there are areas that have many. The areas that have many asteroids are not like asteroid belts in Starwars, it is more along the lines of what you would consider essentially empty space. The asteroids even in the belts are few and far inbetween.

As far as planet formation this is an ongoing process. The earth and all of the planets are pulling in material and gaining mass as we speak. Planet formation has not stopped it has just slowed down as the amount of free material has been captured.

There probably are some small planets in the oort cloud. But the oort cloud is thousands of AU out from the center of the solar system. That means tha the average distance between individual objects in the 'cloud' are huge.

There is no where near enough material to forma a star in the oort cloud.

The Sun comprises more than 99% of the mass of the solar system. The minimum mass to trigger fusion for star formation is estimated to be about 75 times the mass of Jupiter or 24,000 times the mass of earth. It is thought that the oort cloud probably contains several times the mass of the earth.

 

I don't remember if it's still an accepted theory, but at one time it was thought that having Jupiter nearby disrupted the asteroid belt enough to prevent planet forming. That is, because of Jupiter, the belt remains close to its initial state, and can't form anything larger.

Related to the Roche limit, which is how belts such as Saturn's formed, only it wasn't allowed to get to a formation point.

 

Thank you for all your answers. I have another question: Is it theoretically possible for Jupiter to have an earth-sized moon (is its gravity strong enough to pull one into orbit and keep it there), without it getting captured by the sun's gravity eventually? If yes, why are all of its moons tiny in comparison instead?

 

Yes and no.

Jupiter's gravity is certainly strong enough to hold a body as large as the Earth in orbit. A stable orbit would require more than just it passing by Jupiter though.

The larger moons are the size they are due to the mass that was left over or captured during formation. Others are likely captured asteroids.

 

What'd it take take for Jupiter to capture an Earth sized planet into a stable orbit? Some scenarios?

 

For starters it would take a earth sized planet and that is a lot of mass the jovian moons have a combinend mass that is larger then the planet mercury (1/20 earths mass), so if all that mass was centered in a single body (like Saturns Titan) you would get a pretty large moon.

The most realistic way would be that during the planets migration a earth sized planet simply got pulled into a stable orbital resonance with Jupiter, or that a eficient feeding proces occured at Jupiters lagrange point (earth got a mars sized lagrange moon at one point but these points are only stable to a certain mass, so it crashed into the earth and formed the moon)