The Milky Way and Galactic Center rise over Lake Waiau on the summit of Mauna Kea. Astrochemist P. Brandon Carroll stands in the foreground. Image credit: Brett A. McGuire
You may think of life as we know it as being dependent on water or air. But life really depends on two things: chiral molecules—which are distinct, non-superimposable mirror images of each another, much like your right and left hands—and nature’s absolute use of “one handedness,” which makes it possible to create key biological structures. For example, there are only “left-handed” amino acids in left-handed DNA. The origins of this handedness, or homochirality, is one of biology’s biggest unsolved mysteries.
Now, astrochemists have made a discovery in space that could offer a clue about how life on Earth came to favor one “hand”: a chiral molecule found in a cloud of dust and gas near the center of the Milky Way.
Though chiral molecules have been discovered on meteorites before, this work is the first instance of chirality in interstellar space. The authors present their findings today at the American Astronomical Society meeting in San Diego and will publish their work in the June 17 issue of Science.
Scientists have proposed many possible pathways for homochirality to arise, from hydrothermal vents to interstellar clouds, and now they may be able to put some of these hypotheses to the test.
The research team detected the molecule, a small triangle-shaped compound with a tail called propylene oxide, by pointing powerful radiotelescopes towards the star-forming cloud Sagittarius B2, known as a hot spot for detecting new molecules because of its brightness. Of the approximately 180 compounds that have been discovered in space, about a third have been found in Sagittarius B2.
“The telescope we used, deep down the way it works is very much like an FM radio,” Brandon Carroll, co-first author on the paper and a graduate student at Caltech, told mental_floss. “We’re literally tuning the telescope to a specific frequency and listening.”
What they were listening for was a set of three very specific spectral signals that make up a signature unique to propylene oxide. These signals correspond to the molecule’s rotational transitions, or the way the molecule rotates, which is dictated by quantum mechanics. The researchers cleanly observed two of the three telltale signals using the Green Bank Telescope at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. Because the third signal was obstructed by satellite interference, they traveled to the Parkes Radio Telescope in New South Wales, Australia, where they confirmed the detection of the last signal.
One possible interpretation of the finds, says Seth Shostak, senior astronomer at SETI, is that homochiral molecules may have been present in the dust cloud that formed our solar system. (Shostak was not involved in the current study.) That could complicate the search for signs of life on other planets and moons.
“I was just speaking to a professor from Univ. of Arizona who was talking about looking for life on Mars or under the icy carapace of Europa, and I said, ‘So how will you know it’s life—especially if it’s not life as we know it?'” Shostak recounted in an email to mental_floss. “His answer was to appeal to homochirality—which is to say, look to see if the molecules are all either left- or right-handed.”
However, Shostak said, if such handed molecules were part of the ingredients present in the solar system from the beginning, “then there might be lots of such molecules around that might not indicate life on worlds such as Europa, but rather a common legacy from the dust cloud from which the planets and moons were born.”
Carroll noted that while homochirality is “indeed probably a fantastic indicator of life … the trick here is that the cloud really only needs, and likely only can, produce a small, say a few percent, difference in the amount of each handedness to tip things in one direction.”
The next step in the research is to attempt to identify the specific “hand” of the propylene oxide. Brett McGuire, co-first author and Jansky post-doctorate fellow at the NRAO, told mental_floss that the technique they used in this research doesn’t reveal whether you’re seeing the right or left forms. McGuire compared their spectral data of the molecule to the shadows that your hands might cast if you spread them out in front of you with both palms facing down and then flipped one hand over. “If you put a light source behind your hands, you can’t tell if the shadow is coming from your right or left hand,” McGuire said.
But there is a way to find out which form you’re looking at—and importantly, if one form of the molecule exists more abundantly than the other in the star-forming cloud.
It’s an experiment that relies on circularly polarized light, which can also be thought of as left- and right-handed. Compounds whose handedness matches the light will absorb more strongly.
Determining the handedness won’t be an easy task, said Alexander Tielens, an astronomer at Leiden University, who wasn’t part of the study. “Detection will thus require the presence of a (background) circularly polarized source at submillimeter wavelengths; a magnetic white dwarf, for example. It would be a chance occurrence, and we have to be lucky to find this situation,” he told mental_floss in an email. “The detection of a chiral molecule in space is a very interesting result that opens new avenues of research. But it is really only a first step on a long road.”
The researchers say determining the "handedness" of the molecule will be a challenging and time-consuming task. For now, the team is excited to have found the chiral molecule and about the opportunities it presents for studying the origins of an essential aspect of biology. Carroll said, “We can actually think about understanding how a really fundamental mystery in biology might be answered way out in space.”