Any earth sized planet in the habitable zone that was too cold for our liking. Mars is a bit too small. Earth is on the inside edge of the habitable zone so warming is bad. We should be terraforming by pulling GHG out and stop adding more. The Sun will eventually get too hot for any amount of GHG levels to make Earth habitable and we would have to actually block a lot solar radiation which would be impractical. Then we would have to find another planet. This scenario has been going on in the universe for a very long time. We are speeding up the process.