Scientists have found microorganisms crawling over a sample retrieved from the 200 million-mile-distant asteroid Ryugu. But they almost certainly came from Earth.

Asteroid Ryugu was altered by water and contains organic molecules older than the Sun. The building blocks of RNA and Vitamin B3 have been found in its samples, together with polycyclic aromatic hydrocarbons (PAHs) such as naphthalene, a molecule used in mothballs. Earthly bacteria don’t seem to care where the molecules originated from.

Excitement turned to surprise when scientists studying asteroid Ryugu samples found elongated microorganisms. Were they alien life? Not quite. Despite rigorous handling, the samples were contaminated with Earth-based bacteria.