Stephen Hawking believes aliens are real, and humans should tread carefully when first meeting them. Mathematically, he believes there is a challenge in figuring out what their characteristics may be. He also says that primitive life in space is common, but intelligent life is a rarity.
Stephen Hawking further notes that interacting with alien beings could be risky for Earth. He likens the encounter to Christopher Columbus setting foot on America, which did not favor the Native Americans, according to Hawking. He speculates that aliens may have advanced technologies that make them nomads throughout the galaxy, and they could end up dominating or conquering earth in pursuit of natural resources. However, the aliens could pillage Earth for its natural resources and leave in search of another planet. Aliens could be in massive ships in constant search for other planets to raid, since the natural resources on their own planet are dwindling.
The capabilities of aliens would be based on how much power they have and would maintain. Humans should be careful, according to Hawking, since these beings could have far more power than ever realized. He also speculates that advanced beings may have the technologies to harness power from stars. He further theorizes that aliens may have the potential to create nuclear weapons.