Are alien probes hiding in our backyard? A new study says we've barely looked

Even at this early stage in our spacefaring age, humanity has already begun sending probes that will eventually reach other solar systems, even if that was not their original intention.

Five robotic explorers—Pioneer 10 and 11, Voyager 1 and 2, and New Horizons—are all on escape trajectories out of the solar system and might someday enter another one.

They will no longer be operational at that point, but they serve as proof of concept that spacefaring civilizations do indeed build interstellar probes. ...