Asgard, even though inhabited, is physically different from Earth and other planets. Asgard is not a sphere-like shape, but a piece of land that floats in space. Asgard also doesn't rotate on an axis nor does it rotate around the sun, though there may be an object of some sort that gives it its sunlight. Gravity is similarly like Earth, although heavier. For example, a chair made of wood would be considerably heavier than one on Earth. What keeps the land together is unknown, but it keeps it together. Water doesn't pour from the land, as the gravity keeps it there as well.

Post Civil War

After the destruction of Asgard and the Civil War had concluded Thor returned from the dead. Choosing an appropriate spot in the Oklahoma desert Thor re-created Asgard exactly as it was before Ragnarok. When the farmer that owned the barren land resented Thor putting a mystical land above the land he owned Thor offered to purchase the land, allowing him to enter Asgard's vaults and take whatever price that he deemed the land worth from the treasure stores. Asgard currently hovers above this spot in the Oklahoma desert.