George Adamski gave a complete explanation for the habitability on the more distant planets, and the following is taken from his book, FlyingSaucers Farewell.
"One of the most frequent problems encountered when giving a lecture on space is the insistence of scientists that the outer planets are devoid of light and heat. Their objection is that radiation from the sun is so weak at these vast distances that Pluto, for instance, would be at absolute zero or close to it, with a frozen atmosphere, and totally incapable of supporting life forms of any type.
This is the main argument brought against me when doubt is expressed about my meeting human beings from some of these other planets.
The first thing to realize is that the sun does not emit light and heat in the form we observe here on Earth. Radiation from the sun does not manifest itself as light and heat until it penetrates the atmospheres of the planets themselves. Outer space is devoid of light as we know it. The light in outer space is a cold light caused by the phosphorescence of vast clouds of particles and gases responding to the radiation given off by the sun. To a human observer, outer space looks like a dark, vast void, filled with billions upon billions of tiny specks of multicolored light. All of these tiny lights are in a state of continuous motion and activity.
Radiation from the sun is composed of ultra¬violet waves, hard and soft X rays, cosmic and gamma rays. The greater portion of these destructive rays are filtered out by a planet's ionosphere and upper atmosphere.”
Thus, it is not so much a matter of the strength of the Sun’s radiation at a certain distance that determines the heat and light that manifests on a planet’s surface. Rather, it is the density of a planet’s atmosphere that determines the ability of a planet to make use of the radiation that reaches them to produce heat and light.