The picture is obviously about Lem.
Yes, the writer is Stanisław Lem! So you've guessed the first part of the riddle :) 2021 was declared Stanisław Lem Year by the Polish Parliament, because it's been 100 years since his birth.
Is there something in it about Planet 9?
Nope, but being interested in astronomy should help in spotting the third reference that is the most difficult one :)
The ocean from Solaris.
The ocean is from "Solaris", yes, and the whole mural is inspired by this most famous Lem's novel :) But the references I wrote about aren't connected to his books, but to Lem himself. Each of them symbolise something about him (they aren't as big as the ocean though, so you have to look more closely :)).