"All known gas giants, like Jupiter and Saturn, have solid cores. These cores are either rocky or metallic, and aren’t completely solid throughout, with some of the core being comprised of molten metal and rock."
I would say, anything whose spacetime geodesic (orbital/freefall path) intersects the spheroid defined by the surface of the Earth. Though by this definition, a comet on a 100-year collision course is already "on Earth", so I'm not sure if that's reasonable.