How are planets formed?

[shortylickens]

Do they need a star to build around or can those perfect little spheres be formed out in the middle of nowhere?

 

[Paul98]

Yeah, if it doesn't have enough mass to be classified as a star and more than a small object. You would call it a planet.

 

[TechZuru]

Gravity is your answer, followed by dark matter.

 

[AluminumStudios]

When interstellar clouds of gas and dust start to knot up or are compressed by a shockwave from a nearby supernova, they start to gravitationally collapse. The higher density draws in more material and eventually a star starts to form in the center with a disk of dust and gas encircling it which forms planets.

It would seem logical that a cloud with only enough material for a few planets wouldn't be dense enough to undergo gravitational collapse. It would be like expecting fog at ground level to form big rain droplets, it just won't get dense enough to do that. Likewise if it is dense enough to form something like a planet, it would keep drawing in material and form a star.

Also planets are formed when material is brought together into a dense accretion disk around a forming star. Without a forming star's gravity to draw material together into a dense disk to feed a forming planet, I don't suspect a planet could form. A dense region of material is needed to give the dust grains a chance to collide and form clumps, then form asteroid and comet-like bodies which keep colliding and coalescing to dwarf planet size when they can start to gravitationally accrete matter on their own and form larger planets.

So, my answer is no, a stellar environment is needed to form planets. The smallest bodies that can probably form on their own are brown dwarfs (stars that run out of material to draw in before they get big enough to start fusion.) These bodies would be many times the size of Jupiter and not really classified as "planets."

There are rogue, starless planets in space though, but they were unlucky ones which were ejected from their parent solar systems through unfortunate gravitational interactions and now wander between the stars.

-William Milberry

 

[colonelciller]

When interstellar clouds of gas and dust start to knot up or are compressed by a shockwave from a nearby supernova, they start to gravitationally collapse. The higher density draws in more material and eventually a star starts to form in the center with a disk of dust and gas encircling it which forms planets.

It would seem logical that a cloud with only enough material for a few planets wouldn't be dense enough to undergo gravitational collapse. It would be like expecting fog at ground level to form big rain droplets, it just won't get dense enough to do that. Likewise if it is dense enough to form something like a planet, it would keep drawing in material and form a star.

Also planets are formed when material is brought together into a dense accretion disk around a forming star. Without a forming star's gravity to draw material together into a dense disk to feed a forming planet, I don't suspect a planet could form. A dense region of material is needed to give the dust grains a chance to collide and form clumps, then form asteroid and comet-like bodies which keep colliding and coalescing to dwarf planet size when they can start to gravitationally accrete matter on their own and form larger planets.

So, my answer is no, a stellar environment is needed to form planets. The smallest bodies that can probably form on their own are brown dwarfs (stars that run out of material to draw in before they get big enough to start fusion.) These bodies would be many times the size of Jupiter and not really classified as "planets."

There are rogue, starless planets in space though, but they were unlucky ones which were ejected from their parent solar systems through unfortunate gravitational interactions and now wander between the stars.

-William Milberry

this ^

 

[pandemonium]

The other vastly important part of the equation is time. Lots, and lots of time.

 

[Biftheunderstudy]

There are a couple of competing models, and the real answer is likely a combination of the 2.

One, is the process described above where the accretion disk builds up small bodies through collisions which then accrete more material.

The other, is called fragmentation where the accretion disk, through a hydrodynamic fluid instability, more or less condenses a large chunk out which collapses and becomes stable.

In both cases once the object is big enough, it clears a path along its orbit as it gains mass.

Finally, this process happens a lot faster than than you would expect. On the order millions of years...which is a blink of an eye in terms of stellar evolution. (This figure comes from observations of young stellar disks which dissipate on that time scale, suggesting planet formation has to be finished before the disk disappears)