A basic property of Earth's organic molecules could be caused by supernova explosions. That means massive stellar explosions indirectly governed the building blocks of life on Earth...and could be doing it elsewhere in the cosmos.
In chemistry, molecules are said to be chiral if they lack symmetry - in other words, if a molecule and its mirror image can't be superimposed one on top of the other, then that molecule is chiral. Because our hands are the most obvious example of chirality in the human experience, chemists informally call chiral molecules "left-handed" and "right-handed", depending on their orientation.
Many chiral molecules are the result of an asymmetric carbon atom, which forces the rest of the molecule's atoms into two mirrored configurations. Since carbon is also the most crucial element to life as we know it, a lot of vital organic molecules happen to be chiral. None of that is particularly strange. What is mysterious is the fact that virtually all of Earth's amino acid molecules are left-handed when one chiral configuration should be just as common as the other. In fact, amino acids found on the Murchison meteorite show precisely the same overabundance of left-handed molecules, suggesting this bias is found throughout the universe.
Now astrophysicists think they might have the answer, and it's all to do with supernovas. When these stellar explosions occur, a lot of different particles are released in powerful bursts. One such particle is the antineutrino, the antimatter counterpart of the neutrino. Antineutrinos are themselves fairly mysterious particles, but we do know one thing for a fact - every antineutrino we have ever observed had a rightward spin, and that means it wants to interact with other particles that have a similarly rightward spin.
That's where right-handed amino acids reenter the picture. All amino acids feature not only a carbon atom but also a nitrogen atom, and the spin of that atom is determined by the chiral configuration of the amino acid as a whole. In left-handed amino acids, the nitrogen has a leftward spin, while it has a rightward spin in right-handed amino acids. Those rightward-spinning nitrogen atoms are the perfect candidates to align with antineutrinos as they shoot away from the supernova.
This alignment between antineutrinos and nitrogen atoms causes the nitrogen to be converted into carbon, which in the process destroys the original amino acid. This antineutrino wave would provide a natural sorting mechanism that breaks apart right-handed amino acids while leave the left-handed amino acids intact, leading to the eventual dominance of left-handed amino acids - precisely what's happened here on Earth.
You might wonder if there's a similar process that can destroy left-handed amino acids in much the same way. There is, and it's caused by antineutrino's opposite number, the neutrino. Supernovas can also emit bursts of neutrinos, and these left-spinning particles would convert nitrogen into oxygen in left-handed amino acids. However, this process requires way more energy, and so it would naturally happen far, far less often than the right-handed process.
Of course, the range of these antineutrino waves wouldn't be infinite. Most likely, the antineutrinos would only destroy the right-handed amino acids in a few molecular clouds that happened to be near the supernova. But then those molecular clouds would mix throughout the galaxy, diffusing a small but still noticeable bias toward left-handed amino acids. Even if the imbalance started as just 1 more left-handed molecule in every million, that would be enough to lead to their eventual dominance throughout space, including on any planet where life developed.
If this theory is true - and it happens to be the best and only explanation for why we see a left-handed imbalance on Earth - then that means the original building blocks of life on this planet must have originated elsewhere. It's possible that these supernova-altered molecules got thrown into the initial mixture of materials that formed our solar system, or perhaps meteorites brought these amino acids from the molecular clouds to Earth.
Either way, the left-handed nature of our amino acids puts some fairly fundamental constraints on what life on this planet can look like, and it's possible that any alien life might operate under similar guidelines. As nuclear astrophysicist Richard Boyd puts it:
"I find it really mind-boggling that the same constraints that exist on our chemicals of life might also exist for every other entity in the universe. If other entities are out there, the constraints on their chemistry appear to be sufficiently similar to ours that we may have lots of things in common with them."