According to Osho, certainty comes through enlightened vision: when awareness flowers, one's consciousness is no longer confined and directly perceives the cosmos. From this inner science he asserts that only Earth hosts human-level self-awareness; elsewhere life exists only as vegetable or animal. Do not believe him - verify by becoming aware of awareness; then the claim becomes experiential, not speculative.
We can be sure only by waking up inside and seeing for ourselves; from that inner seeing, he says Earth alone has people who know they are aware.