Half a century ago, the notion of shaking hands with aliens was treated as a joke - what if they were made of antimatter? A touch with any of them could lead to mutual annihilation. Fortunately, advances in gamma-ray and cosmic-ray astronomy have shown that antimatter is an insignificant component of the cosmic mass budget, making such a scenario highly improbable.

However, says a scientist whose life goal has been finding alien civilizations, the risks posed by extraterrestrial life may be far more insidious and existential than mere physical annihilation. The cause of the new concern is extraterrestrial mirror life, a phenomenon explained in a new study published recently by an international team of 38 researchers in the journal Science.

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Professor Avi Loeb, head of the Galileo Project and founding director of Harvard University’s Black Hole Initiative, says in a blog post that the dangers of mirror life stem from its potential incompatibility with terrestrial biology. Immune systems, which rely on interactions between chiral molecules, may be unable to recognize or combat mirror organisms.

What is mirror life?

Mirror life refers to organisms that are biochemically similar to Earth life but with reversed molecular chirality. Chirality is a property of an object or molecule that makes it asymmetrical in a way that it cannot be superimposed on its mirror image. It’s similar to the difference between your left and right hands: they are mirror images of each other, but you cannot place one over the other so that all parts align perfectly. Objects or molecules with this property are called chiral.

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Chirality, or "handedness," describes how certain molecules exist in mirror-image forms, akin to left and right hands.

On Earth, life exhibits homochirality: DNA, RNA, and their building blocks are right-handed, while amino acids and proteins are left-handed. This biochemical uniformity enables life's intricate processes and immune defenses.

What if extraterrestrial life evolved with the opposite handedness? Loeb asks in his analysis.

Mirror bacteria

Mirror bacteria, for instance, could evade detection in such case, spreading unchecked through Earth's ecosystems. Such pathogens could wreak havoc on plants, animals, and humans alike, undermining the foundations of terrestrial life, the scientist explains with reference to a 299-page technical report titled “Mirror Bacteria: Feasibility and Risks,” which highlights the existential threat posed by these organisms, warning that their introduction into Earth’s biosphere could lead to catastrophic ecological and health consequences.

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While the report primarily addresses the risks of human-engineered mirror bacteria through synthetic biology, the possibility of encountering naturally occurring mirror life from extraterrestrial sources cannot be dismissed, according to Professor Loeb.

A lifeform evolving in a space lab - an episode from the 2017 SF movie Life. 

If a Mars sample return mission or a meteorite containing traces of mirror life were to introduce these organisms to Earth, the consequences could be disastrous. Unlike engineered mirror bacteria, natural mirror life would be beyond human control, and Earth's biology would have no inherent defenses against it.

The interplanetary delivery of such life forms would be impossible to handle safely, with the plot in Daniel Espinosa’s 2017 science fiction movie Life rendering a fortunate scenario for humankind.

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The absence of widespread mirror life on Earth suggests one of two possibilities: either a universal mechanism favors the same handedness for life everywhere, or Earth has been extraordinarily lucky in avoiding contamination over the past 4.2 billion years, Loeb notes, adding that we cannot afford to rely on luck alone.

The risks posed by extraterrestrial mirror life demand rigorous precautions. NASA’s and ESA’s Mars sample return campaigns, as well as Elon Musk’s vision of human colonization of Mars, must consider the possibility that Martian soil contains relics of mirror life.

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Until we understand whether homochirality is universal or whether mirror life could evolve independently, caution must guide humans’ approach to extraterrestrial exploration, the scientist said – yet without clarifying whether there’ll be any changes in his quest for alien life after this revelation.

If aliens exist and they consist of mirror life, they have probably realized the danger of physical contact with Earth’s biology. Perhaps that’s the reason we haven’t seen them yet - they simply avoid us for the sake of self-preservation.

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