How do lone planets get captured?

[Idrryl]

So yeah, how do they get captured? It's not like they can aerobrake near the star. And having an escape velocity from a whole solar system, it's speed must be massive.

[Dkmdlb]

They usually don't. But if they do, it's in the exact opposite way in which they are flung out of their parent system in the first place: gravitational interaction with a star and the things orbiting the star.

Their escape velocity from their parent system might be big, but that doesn't mean that they still have all that velocity after drifting for a few million years through interstellar space.

Edited August 21, 2014 by Dkmdlb

[Skyler4856]

It's not like they can aerobrake near the star

There is a principle called Radiation Pressure, it's based off of the fact that light has mass-energy, and thus may transfer force. You could think of the principle as being analogous to aerobreaking...

[Idobox]

If the star is young enough, they might "aerobrake" by interacting with protoplanetary disk.

If the planet gets very close to the star, tidal effects might slow it down, or not, I'm too lazy to properly think this through.

[Skyler4856]

If the star is young enough, they might "aerobrake" by interacting with protoplanetary disk.

Are protoplanetary disks dense enough?

[Guest]

For a planet, yes, they are. Also, KSP's gravity simulation is very primitive. Actual n-body systems can, though somewhat convoluted gravitational interaction, actually cause an object to end up in orbit around a body.

An object moving from infinity (or "far away) towards the Sun will have at least the escape velocity at any point of it's trajectory, so it can't be that it's going "slow enough" to suddenly get captured. However, gravitational influence of other planets, as well as other objects can slow it down. This can happen in a manner similar to gravity assist, but working in reverse, or in other, subtler ways that patched conics can't account for.

[Jovus]

Also, it depends on injection angle. Even assuming a capture star of exactly the same mass and no loss of speed, a planet can still be captured by another star if it cuts across the system in the right fashion. Viz. gravity is a lot more coplicated than KSP gives it credit for.

[WestAir]

A lot of star systems have more than one, some as many as three stars. Planets passing through these systems might be handed off between stars until a capture is completed.

Additionally, some star systems may have large Jupiter-esque planets that can loop a passing planet back in for a second or third pass of the star, lowering its speed below escape velocities.

Finally, most (all?) planets are born from the protoplanetary disk. Planets that escape probably rarely get captured by a new system.

[SelectHalfling0]

They also may kick out a nearby planet. When the forces cancel each other out the rogue planet will have a new family and the other planet will be forcibly flung out of the system.

[Krosulhah]

With a net.

Sorreh..... That's all i've got to add to this one...