"....A few years after the 2014 incident, a test pilot flying in a nearby area told Graves of an encounter with such an object. According to Graves, a cube-in-a-sphere UFO was “just riding along with him,” about 30 feet from the aircraft, before it “zipped off.”
Square Balloon filled with helium at altitude ( expand into more circular form ) drifting in high wind is more prosaic explanation .
"....The orbits of our planets will someday become perturbed anyway. About 5 billion years from now, our sun—that glowing, life-giving, seemingly immutable orb—will start to die. The star will lose mass until it can no longer hold onto its outermost planets. Neptune and Uranus—and Pluto too—will probably become rogue planets. They will drift away, taking their icy atmospheres with them. Unbothered by the cold of interstellar space, the planets will remain mostly unchanged, relics of a solar system that once huddled close around a warm sun....."
Also an expanding civilization on a planet , will populate all the outer planets very quickly , surviving in closed atmosphere enclosures on surface or more likely underground shelters , using fusion power for everything , including growing food . The speed of Rogue planet can be as high as 13000 km per second . A nomadic civilization can even accidentally come across many Earth- like planets in other solar system . No need to break the light speed , if your space- ship is as big as a planet . Colonizing becomes possible , within a million years of travelling . A planet could also be pushed by fusion Rockets , to higher speed in particular direction . I still think a Rogue planet offers one of the best chances of colonizing the Galaxy !
Also as a Sun ages , it expands and becomes hotter . Swallow it the inner planets or burning them up . While the outer planets heat - up more and become more habitable and forcing migration to outer planets .