Alien organisms or aliens could hitch a ride on an astronaut's spacecraft and pollute Earth, scientists warn.
They said, the rise of space exploration also increases the possibility of alien organisms attacking Earth, and Earth-based organisms attacking other planets.
The researchers referred to the human record of the displacement of species to new environments on Earth, where the organisms can become invasive and harm native species.
In the scientific journal BioScience they say, such behavior suggests the same thing could happen with alien life from other planets polluting Earth and vice versa.
"The search for life outside our world is an exciting endeavor that could lead to major discoveries in the not too distant future," said study lead author Anthony Ricciardi, professor of invasion biology at McGill University in Montreal, Canada.
"However, in the face of increasing space missions, including those intended to return samples to Earth, it is critical to reduce the risk of biological contamination in both directions."
Ricciardi and his colleagues used the paper to call for more collaborative studies between astrobiologists looking for extraterrestrial life and invasion biologists studying invasive species on Earth.
"We can only speculate about what types of organisms might be encountered if astrobiologists discovered life. The most plausible forms of life are microbes and may resemble bacteria," Ricciardi said.
Risk of interplanetary contamination
Scientists consider the risk of interplanetary contamination to be extremely low, in part because the harsh conditions of outer space make it difficult for hitchhiking organisms to survive outside human spacecraft.
However, according to Ricciardi, we still have to be careful of interplanetary contamination based on the negative impact that invasive species have on Earth.
Humans have been destroying ecosystems around the world by allowing organisms to invade new environments they would never have reached naturally.
For example, a fungus from South America called Austrouccinia psidii was introduced to Australia under unknown circumstances and took over the country's native eucalyptus trees, inhibiting their growth, and sometimes killing them.
The researchers note that island ecosystems that thrive in geographic isolation, such as on islands and in countries such as Australia, are particularly vulnerable to invasive species, because native wildlife in those places has not yet developed adaptations to deal with these "colonizers".
"Biological invasions often destroy plants and animals in these systems. We argue that planets and moons that could potentially contain life should be treated as if they were island systems," said Ricciardi.
For evidence of interplanetary contamination, the researchers cited Israel's Beresheet spacecraft crashing into the Moon in 2019 carrying thousands of tardigrades, microscopic animals that can survive extreme conditions, including a vacuum.
A 2021 study published in the journal Astrobiology concluded that the creatures might not have survived the impact of a lunar collision, but the incident did point to the potential for a biological spill.
Space agencies like NASA have long been aware of the potential risks of biological contamination, and planetary protection policies have been in place since the 1960s.
"However, an unprecedented risk is posed by a new era of space exploration, aimed at targeting areas most likely to contain life," Ricciardi said.
This includes increasing private space exploration such as SpaceX which makes space more accessible. SpaceX, for example, aims to travel to Mars and beyond with its SpaceX Starship program.
The researchers suggest improving biosecurity protocols related to space travel, focusing on early detection of potential biological contaminants and developing plans for rapid response to such detections.
"The planet and the Moon are always exchanging matter via meteorites, but human space exploration can accelerate contamination," said Jennifer Wadsworth, an astrobiologist at Lucerne University of Applied Sciences and Arts in Switzerland who was not involved in the paper.
"The line between exploration and conservation is thin. One should not be abandoned at the expense of the other, but both require careful consideration and, most importantly, compliance," he stressed.
