Further note that the animals cannot digest plants when they are cold. They can eat them and store them, but the disposal of the solid water and CO2 is too difficult a problem. When they warm up, the water in the plants melts and can be disposed of, and things are simpler.
II
THE FLUORINE PLANET
1. THE STAR AND PLANET
The planet named Niflheim is the fourth planet of Nu Puppis, right angle 6:36, declension -43:09; B8 type star, blue-white and hot, 148 light years distant from Earth, which will require a speed in excess of light to reach it.
Niflheim is 462,000,000 miles from its primary, a little less than the distance of Jupiter from our sun. It thus does not receive too great a total amount of energy, but what it does receive is of high potential, a large fraction of it being in the ultra-violet and higher frequencies. (Watch out for really super-special sunburn, etc., on unwarned personnel.)
The gravity of Niflheim is approximately 1 g, the atmospheric pressure approximately 1 atmosphere, and the average ambient temperature about -60°C; -76°F.
2. ATMOSPHERE
The oxidizer in the atmosphere is free fluorine (F2) in a rather low concentration, about 4 or 5 percent. With it appears a mad collection of gases. There are a few inert diluents, such as N2 (nitrogen), argon, helium, neon, etc., but the major fraction consists of CF4 (carbon tetrafluoride), BF3 (boron trifluoride), SiF4 (silicon tetrafluoride), PF5 (phosphorous pentafluoride), SF6 (sulphur hexafluoride) and probably others. In other words, the fluorides of all the non-metals that can form fluorides. The phosphorous pentafluoride rains out when the weather gets cold. There is also free oxygen, but no chlorine. That would be liquid except in very hot weather. It sometimes appears combined with fluorine in chlorine trifluoride. The atmosphere has a slight yellowish tinge.