In the past the 12th Planet was considered part of your Solar System, and in general the Council of Worlds allows 3rd Density species who are local to interact with each other, especially when they are the same species. The hominoids on the 12th Planet were seeded in the same manner as the hominoids on Earth, and during the same time frame, but uneven development occurred. The hominoids on Earth dealt with carnivores and the violent geological changes that accompany pole shifts. If they were not running away they were having to pick up the pieces and start over again. Thus, the hominoids on the 12th Planet reached the point where Homo Sapiens on Earth is now, many millennia ago. They developed rockets, not unlike the manned rockets the US sends aloft regularly to maintain its satellite system.
The goal was not simply to orbit their planet, but to gather resources for their planet, essentially a water world where mining operations were limited. They were driven to develop a Solar System shuttle, and their early astronauts were much at risk and died by the dozens. They could plainly see, via their telescope equivalents, that the Earth and Mars were planets that could sustain them. As with international travelers on Earth, they expected to get infected with new bugs, and that their immune systems would either adapt or they would die. Eventually, during repeated visits of the 12th Planet to your Solar System, they worked the kinks out of their shuttle and settled into colonies on Earth and Mars. Then trouble emerged as, being advanced, the giant hominoids demanded and got subservience from Earthlings, in the same manner that humans living among chimpanzees would behave. In addition to their technological advantage, the giant hominoids from the 12th Planet had size on their side, just as humans do with monkeys. They ruled.
As these types of arrangements become progressively more extreme, the Council of Worlds stepped in and effected a quarantine. The 12th Planet inhabitants do not know they are quarantined, they simply came to the realization that they must leave. This was arranged as most such matters, with the 3rd Density hominoids believing to this day that the decision was theirs. They ran into increasing difficulties - rebellious slaves, bad weather, loss of their stores - and found that their brethren elsewhere were at least masters of their environment. The grass was greener, and they left. They were discouraged by walk-ins sent to accomplish this mission, and the untimely storms that scattered their pack animals and sank their ships were accomplished by air turbulence caused by temperature differences, a minor feat quickly arranged.