Left-handed Molecules from Outer Space

A new study, conducted by University of California, Los Angeles and the European Southern Observatory, found the building blocks of life orbiting a white dwarf star about 200 light-years away from Earth. The scientists discovered carbon, nitrogen, hydrogen and oxygen in a zone similar to the Kuiper-Belt in our solar system, a vast region of icy objects beyond Neptune.

This finding followed the discovery, in June last year, of a complex organic molecule in deep space that is key to life on Earth. This molecule, propylene oxide, was detected in a giant cloud of gas and dust called Sagittarius B2, which is located near the center of the Milky Way galaxy about 25,000 light years away. It is a chiral molecule, which means that it exists in two forms that mirror each other.

Just like your right-hand mirrors your left but will never fit comfortably into a left-handed glove, life’s molecular building blocks come in both right and left versions. While still can't be determined whether there are more of one hand than the other, the discovery of the molecule could illuminate more about the mystery of how life began on Earth or maybe elsewhere in the Universe.

Although most amino acids exist in both left and right-handed forms, life on Earth is almost exclusively made of "left-handed" amino acids. Meanwhile, all sugars characteristic of life on Earth are "right-handed." No one knows why this is the case.

This phenomenon is called “chirality”—from the Greek word for handedness. The big questions still remain: How and why did life choose only one of two mirror reflections to construct every single creature in her menagerie? Did the seeds of homochirality originate in the depths of interstellar space, or did they evolve here on Earth?

To look for clues, researchers focus on meteorites—time capsules from 4.5 billion years ago that find their way down to solid ground. Many meteorites contain carbon-based molecules including amino acids and sugars, which are just the right ingredients for life. Scientists analyzed the composition of these 'organic' compounds in dozens of meteorites, and came to a surprising conclusion.

Oftentimes both the left- and right-handed versions of, for example, an amino acid, were found in equal amounts—exactly what might be expected. But in many cases, one or more organic molecule was found with an excess of one hand, sometimes a very large excess. In each of those cases, and in every meteorite studied so far by other researchers in the field, the molecule in excess was the left-handed amino acid that is found exclusively in life on Earth.

Some research suggests that the meteorites with the highest left-handed enrichment had the longest exposure to liquid water, over time periods ranging from 1,000 to 10,000 years. This was being estimated based on the presence of clays and minerals. Previous lab experiments have shown that liquid water can amplify any inequality in amino acids, whether a small bias exists toward left-handed or right-handed types.

A supernova, of the sort that could have triggered the formation of our solar system, may also have caused a left-handed molecular preference. Radiation from the newly formed neutron star at the core of the supernova may have caused the scattering of ultraviolet radiation in dust particles, causing the selection of one hand over another.

Some astrobiologists have used chirality as a way to judge if life is likely to be ubiquitous in the galaxy. If life somewhere else would be based on the same molecules as us with the same sense of symmetry, it should be chiral too. And although all the current evidence suggests that both Earth and the solar system lean left, that still doesn't mean right-handed life never existed on primitive Earth, or doesn't exist elsewhere in the Universe.

A conclusive answer is going to be hard to come by. In a few years, however, we might know more: In September 2016, NASA launched the OSIRIS-REx mission to study near-Earth asteroid Bennu. After completing its measurements it will collect the ultimate prize: a sample from the surface of the 500-meter asteroid, which it will bring back to the Earth in the year 2023 so that scientists can study its chemical composition.

Unlike meteorites, which are separate from their parent body and might get contaminated when they strike Earth, with Bennu, the scientists will know exactly where the sample came from. Hopefully, the sample will learn us more about the origins of homochirality or even life itself.
(February 2017)