Why (if anything) is a planet clearing its neighborhood important?

[Peelee]

It's the biggest planet. It's very light for a star, in fact too light to be a star.

I wasn't initially sure what you meant about the rest fitting inside Jupiter, thinking about it, you probably mean their combined volumes are less than Jupiter's. Which is probably true, but it seems it's well within an order of magnitude.

My first thought was that if you put spheres with the radius of Saturn (58,232), Neptune (24,622) and Uranus (25,362) touching, they would not fit within a sphere with the radius of Jupiter (69,911) (using Wikipedia's mean radii).

All I'm really saying is you can't have a stellar system without at least one star.

Yeah, I meant just volume-wise, not practically putting them in planetary shoulder to shoulder.

Also, we're less than 600 years away from renaming it Urectum. I'll probably set up a countdown clock as we get closer.