"The fact that terrestrial microbes are the Earth's best colonizers means we can never completely discount terrestrial contamination."

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

Last year, researchers excitedly announced that they had found two organic compounds essential for living organisms in samples retrieved from an asteroid called Ryugu. The Japanese Space Agency's Hayabusa2 spacecraft obtained the samples from the space rock in 2019.

A chunk of rock collected from the asteroid Ryugu contains bacteria—but, unfortunately, it's not evidence of alien life.