I don't know how to understand this sentence ' it seems inconceivable that at least one of these planets does not have a life form on it'

The trick is inconceivable.

The fact is the phrase you, quite rightly, have trouble with is preceded by "inconceivable". This usage negates the phrase following. The phrase without the negative in front is clearly wrong since we on earth have a few forms of life and are not excluded in his enumerations.

A clearer version would be;

"It seems impossible that at least one of these planets does not have a life form on it" some of it intelligent.

The writer failed to say what they meant. No wonder you have trouble understanding the sentence.

The idea the writer states is that because there are so many planets, one would think that at least one must have life on it. (The writer certainly doesn't go so far as to suggest that every one of those planets has life on it.)

As Elliot implied, after the words "It seems inconceivable" there comes the negation of that idea which the writer expects to think about all those planets. The trouble is that the writer hasn't correctly expressed the negation of that idea. The passage would be better phrased:

"... it seems inconceivable that none of these planets has a life form on it ..."