alt Hacker NewsAbout that artist's impression with the blue spiral: wouldn't any planet or other object do one of these things:
- approach and hit directly, without spiraling
- approach, make a close fly-by and fly off into deep space again
- be in a steady, eliptical orbit
My knowledge is basically that of a Kerbal.
There are obviously many things KSP's patched conics approximation can't account for, like lagrange points or how orbits often get twisted around over time, or how the small effects from all the other solar bodies add up to be significant over time.
The simplest thing that comes to mind here is that KSP simulates a mature solar system, where space is mostly empty. This event happens in an early solar system, where there's lots of stuff everywhere. Running into stuff can steal some of the planet's momentum, causing it to fall deeper, causing it to run into more stuff
But planetary migration can be a lot more complicated than that. [1] has an overview. You might also enjoy the Grand Tack hypothesis [2] which involves early Jupiter spiraling into the inner solar system until around the orbit of current-day Mars, before eventually reversing course and getting to its current orbit further out
1: https://en.wikipedia.org/wiki/Planetary_migration
2: https://en.wikipedia.org/wiki/Grand_tack_hypothesis