Gravity on Isen VI was at least 9 gee.
According to the Twelfth Doctor, before the terraforming Isen VI was home to the coldest, crispest and snowiest snow in the universe. Its icy winds could strip the flesh from the body and freeze the blood in the veins.
The Doctor had also picked the planet because the TARDIS had detected a Time Lord beacon coming from there. This was because a Hyperion hiding away from Rassilon's Alliance of Races in hibernation beneath the planet had been released by the terraforming. The Doctor was able to freeze it before it could leave Isen VI, and restored the planet to its original icy state. (COMIC: Terrorformer)