Now lets hypothesise ...
EON's ago the sun was a much smaller star and the heat radiating from it meant that a planet much closer to the sun was lush and warm and this encouraged life to form and it was a good life.
After many millennia life ended up as intelligent, even being able to travel through space. This ability to travel was fortunate for the inhabitants since the sun was starting to cool and started to expand, this caused the temperatures to rise over centuries on the planet and finally a few explorers set off on a journey.
This journey led them to a planet within the same solar system which was similar to their own planet excepting that this new planet was far cooler, indeed similar to how their own planet had been millennia ago, it also had beings which were very primitive and it was decided that the new planet could be colonised and that their race could continue on the new planet.
Over time the knowledge of how they managed to colonise and expand on the planet was lost as they integrated with the inhabitants. Over further millennia it was shown that a similar problem with regard to temperature and habitability was starting to occur on their new planet (but they had forgotten their past history and thought that they were causing the warming).
Fortunately for the inhabitants they were just starting to be able to explore the solar system and ...
PS Just in case its MY copyright!