Yes, they are planets most likely, because they probably formed around a star and then got kicked out dynamically. This is expected generically in models of how solar systems form and evolve (in particular we think it happened multiple times in our own Solar System).

Free-floating planets are generically expected: Essentially all models for how solar systems like ours (and the others we now know) involve dynamical interactions that would kick out planets at high velocity, leaving them unbound. Astronomers have expected to find these for decades, but have been unable to do so because a planet not warmed by a nearby star gets cold fast (hundreds of thousands to millions of years) and therefore invisible even in the infrared. This result is very important if correct, because gravitational lensing is an emission-insensitive way to find the planets. And yes, IAAA! (ps As for whether they are "spacecraft": I love that idea, but the "people" onboard probably wouldn't give the planet an impulse themselves (way, way, espensivo), they would make use of a free-floater passing by and hitch the ride.)

Damn that game was fun and it ran on an Apple II loaded from a floppy!