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Why must spacecraft be sterilized before they search for extraterrestrial life?

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Planet TOI 700e — extraterrestrial life An illustration of the Earth-size planet TOI 700e orbiting within the habitable zone of its star. The habitable zone is the area around a star where liquid water, and thus possibly life, can survive on a planet's surface. Its Earth-size sibling, TOI 700d, can be seen in the distance.

Spacecraft must be rigorously sterilized before searching for extraterrestrial life primarily to prevent the contamination of the target environment with terrestrial microorganisms.12

If a spacecraft carries living organisms from Earth, there is a significant risk that these microbes could be detected by the craft's instruments and erroneously reported as indigenous life on the target planet. This potential for contamination creates ambiguity in scientific results, making it difficult to distinguish between genuine extraterrestrial biological activity and the presence of hitchhiking Earth-based life.2

To maintain the integrity of astrobiology research, robotic spacecraft are subject to strict cleanliness requirements, and components intended to touch the surface of a planet are sterilized to minimize the possibility of interfering with the detection of indigenous life. This practice is a fundamental aspect of planetary protection, ensuring that scientific missions do not compromise the very environments they are designed to study.12