The origin of life on Earth remains a mystery. The most popular theory is that it arose through chemical processes on the early Earth. But another speculative possibility is that simple forms of life, like bacteria, can spread through space from one planet to another. This theory is called panspermia.
Panspermia is only a possibility if at least some bacteria that can survive prolonged exposure to the harsh conditions of outer space, including airlessness, extreme temperatures, and radiation. In twenty-twenty, a team of Japanese scientists published results showing that a species of bacteria survived up to three years of exposure to space outside the International Space Station.
The scientists chose a species of bacteria called Deinococcus radiodurans. These bacteria are classified as extremophiles because they can withstand extreme conditions. They have been found floating on dust particles more than seven miles up in the stratosphere, and can survive a dose of radiation three thousand times larger than that lethal to a human. Spacewalking astronauts placed pallets covered with dried aggregates of these bacteria, brought from Earth, on the outside of the space station.
When the pallets were returned to Earth, researchers cultured the bacteria on them in the lab. They found that if an aggregate of bacteria was large enough, its dead outer layers protected bacteria on the inside from space conditions. Bacteria from aggregates at least half a millimeter in diameter survived three years of exposure to space, and were capable of repairing the genetic damage they suffered. The results show that bacteria could survive space long enough to make the trip between Mars and Earth. If life arose on either planet, it might have spread to the other.