The new, improved solar system, 2
It was once thought that the main asteroid belt of our solar system had its origins in a mysterious ‘fifth planet’ which orbited between Mars and Jupiter until it was destroyed millions of years ago by a catastrophic explosion, possibly involving primeval forms of Mentos and cola.
Modern astronomy frowns on this theory, depending as it does on a faintly embarrassing piece of numerology known as the Titius-Bode law, and prefers to blame the asteroids on the meddlesome gravitational influence of Jupiter. But just because the fifth planet never formed in the first place, it doesn’t mean that we can’t make one, and the materials are already to hand.
A solar system without asteroids will be a tidier and more well-proportioned place, with no pesky chunks of space rubble dropping out of the sky and causing extinction events.
It’s simply a matter of magnetising a couple of the bigger nickel-iron asteroids and waiting for them to glom onto each other in an ever-growing clump. Non-metallic asteroids can be hauled in with nets or static cling.
As a bonus, we get to make a new planet to spec, which will be much more fun than terraforming Venus or Mars. Renovations are such a bore.
Nerds may object that the total mass of all known asteroids is only 4% of the mass of the Moon, but remember that we’re building this thing from scratch. It’s not like it has to be solid all the way through. And anyway, once we get around to cleaning them all up, there are sure to be many more asteroids than we thought, just like when you move house.