The First Doctor and Susan once arrived on a planet which orbited twin suns. Susan described the "lush rainforests and endless waterfalls" in the area of the planet she and the Doctor landed. Scarlet rainbows were formed by refraction of the light from the planet's suns through a think mist of ice crystals.
During his visit, the Doctor stumbled and caught his hand on some serrated-leaved plant which oozed a sticky sap. The cut caused a illness for the Doctor, following their immediate departure from the planet, which lasted until Susan was able to find healing equipment on Rua. (AUDIO: The Sleeping Blood)