The belief that humanity has accidentally contaminated other planets with life by sending rovers and landers to their surface that were not sufficiently decontaminated
Humans have sent rovers and probes to the surface of 4 bodies in our Solar System. These are The Moon, Mars, Venus and Titan. Although before being sent they're thoroughly decontaminated, and are again decontaminated while being exposed to the vacuum of space and solar radiation, it is believed that there is likely still life on the landers when they arrive on the surface of the planets. It is possible that any organism tough enough to survive these condition could reproduce and thrive on their new planets/moons
On earth there are known organisms, often termed Extremophiles, that are capable of surviving conditions that until recently were thought to be completely inhospitable to life. One example of this is the tardigrade. The tardigrade has been shown to be able to survive incredibly unfavourable conditions, including being left in a vacuum, left without water for months as well as left under intense radiation. It has adapted abilities to survive all of these conditions for years.They've also been found all over the world, from hot springs to under layers of solid ice, all not simply surviving, but thriving. In 2013, scientists discovered a new species of bacteria, Tersicoccus Pheonicis within the clean rooms, that they use to decontaminate their aircraft. It is likely that some of this bacteria will have made it onto some of the landers.
Though it is know that there was bacteria in the clean room, there is no evidence yet that any of it made it onto the landers leaving earth. There is little evidence showing that it can survive the harshness of space as well as the extreme heats of re-entry experienced by the landers. These conditions are considered harsher than those experienced in the clean room
We have sent probes to land on other planets We have found organisms that can survive the probe cleaning process and the harshness of outer pace It is possible that these organisms could survive landing on the planets and the thrive.
It is not what we typically think of when we consider "Life Elsewhere in the Universe". Many people do not classify human contamination extra-terrestrial life. The conditions observed in space are far more extreme than those in a cleanroom, and there is a good chance these are sufficient to kill hitchhiking organisms
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