Aliens may be using free-floating rogue planets as spacecrafts in universe, says study
There may also be a substantial number of such rogue planets in the universe. "This increases the chance that some advanced extraterrestrial civilizations if they exist, might hitch a ride on free-floating planets. Which is why I call such hypothetical civilizations Cosmic Hitchhikers," Romanovskaya said.
These nomad worlds may be habitable even as they don’t draw energy from a star. It can happen if they have subsurface oceans, atmospheres with hydrogen to preserve it and residual core heat, which can provide energy for some period of time.
As per Romanovskaya's hypothesis, rogue planets have advantages over spaceships as they have own resources and habitats and don’t need artificial gravity. They could also prove to be an apt escape for the inhabitants of an aging system.
So, it's just a matter of time when such a rogue planet comes close to a system with a habitable world, like Earth, and then aliens use a regular spacecraft to jump over to our planet. Romanovskaya said, "There are no traffic lights in the galaxy. If the solar system happens to be in the way of some free-floating planet, the planet will not stop at the red light. It will fly right through the solar system."