What is considered the real breakthrough needed to gain confidence in an extraterrestrial discovery?

Answer

Finding a signature that has no known terrestrial analogue, even among resilient contaminants.

The high bar for proof demands finding a biosignature, such as a specific isotopic ratio, that cannot be matched by any known terrestrial contaminant, including stress-adapted hitchhikers found in clean rooms.

What is considered the real breakthrough needed to gain confidence in an extraterrestrial discovery?
discoverylifealienbacterium