Shellworld

A planet could be encased inside an artificial shell, like building a roof over it, so that Earth-like conditions could be maintained. This could provide a shortcut for terraforming, or redesigning a planet’s surface to be more hospitable for human habitation.

The shell could be made of dirt, steel and Kevlar fiber. With air pressure raised to a safe level and enough oxygen in the atmosphere, people could live freely on the surface. A small planet would have low enough gravity to allow for human activities.

Lighting would be artificial since the shell would not be able to admit sunlight. Heavy industry would be located on the outside of the shell (in space) to prevent pollution. Airlocks in the shell would allow spaceships to enter and land.

They could also be used for gas giants, where cities could be built attached to graphene tethers dangling from the interior of their shells.