Schopenhauer and Berkeley
Schopenhauer is an idealist. That is he believes the world is in a sense as it appears to the observer. However, Schopenhauer is not a Berkelean or a subjective idealist; he posits a thing-in-itself. For Berkeley, the world is contained in the mind and space is relative to the perceiver. This makes the stars in the sky specks in the sky. That is, they exist as they appear to the observer. If you can get closer pictures of the stars, they are simply big specks in the sky. This is further solidified by Berkeley’s instrumentalism, which says the abstraction of the sciences only has value as far as it wields practically results. That is, it is a fake abstraction that happens to work.
The nature of planets
However, Schopenhauer posits a thing-in-itself. Thus, the world in a sense exists independent of the perceiver. There is a world out there independent of the mind but it simply exists as a compactification point, i.e. something that exists as a unity without diversity. Thus, the planets really exist for Schopenhauer and you could theoretically travel to them. However, Schopenhauer still accepts the phenomenal world as appearance. This in essence cuts out anything that would potentially lead to alien life. You can go to the moon but the microbes on the planet don’t constitute life since you don’t see them under ordinary circumstances. When you look at them under the microscope they are simply specks in your vision. Organic compound doesn’t necessarily produce life and things are that way simply because they are that way on earth. Thus, the planets are simply balls of mass that exist in a spacial void. Another place where Schopenhauer deviates from materialism is that cosmologically earth is at the center of the universe. Humans are no more than animals but are distinct from animals as they are the highest grades of the wills manifestation.
Idealism Studies 