I have talked with spirits from our earth about this a number of times. We concluded that anyone with a capable mind can see, on the basis of things that are well known, that there must be many planets and they must have people on them. That is, we can determine on rational grounds that bodies as large as the planets—and some of them are significantly larger than our own—are not uninhabited lumps created only to be carried along on a wandering course around the Sun and shed their feeble light for the benefit of just one planet. Their function must be more worthwhile than this. If we believe, as everyone should, that the Divine created the universe for the sole purpose of bringing humankind into being as the source of heaven (because humankind is the seedbed of heaven), then we cannot help but believe that wherever there is a planet there must be people on it.
As for the objects visible to our eyes because they are within our solar system, we can obviously tell that they are planets from the fact that they are bodies of physical matter. They reflect the light of the Sun, and when we look at them through a telescope they do not look like stars, which twinkle because of their fire, but appear earthlike, with darker and lighter patches. There is also the fact that they, like our own planet, travel around the Sun along the path of the zodiac, which must cause years and the seasons of the year called spring, summer, fall, and winter. Similarly, they rotate on their axes as our planet does, which must cause days and the times of day called morning, afternoon, evening, and night. Not only that, some of them have moons called satellites, which have their own periodic orbits around their sphere the way our moon orbits our planet.
The planet Saturn, which is very far from the Sun, has a huge luminous ring around it that gives a great deal of light to that planet, even though it is reflected light. Can any rational individual who knows all this maintain that these bodies are uninhabited?
from Other Planets (New Century Edition), Section 3