Astronomers have detected chemical precursors of building blocks of life in the large disk of dust and gas whirling around a young nearby star. These complex organic molecules, two forms of cyanide and one chemically related compound, likely formed after the protoplanetary disk collapsed, the researchers say. The same chemicals are found in roughly similar proportions in comets circling our sun, which may have brought them to Earth billions of years ago.
“We know that the solar system isn’t unique in its number of planets or abundance of water,” says Karin Öberg, an astrochemist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. “Now we know that we’re not unique in organic chemistry. From a life in the universe point of view, this is great news.”
The star the researchers studied, dubbed MWC 480, lies about 450 light-years from Earth in the constellation Taurus. About 1 million years old, the star—which is about 1.8 times the mass of our sun—is surrounded by a disk of dust and gas that altogether contains approximately two-tenths a solar mass, Öberg says. The team’s observations, as well as previous studies, haven’t spotted any nascent planets inside the protoplanetary disk, she notes: Either those measurements haven’t had high enough resolution to discern the objects, or it’s too early in the star’s evolution for such bodies to have formed.
What the team has seen, however, are the chemical signs of three complex organic molecules in the cyanide family—an astronomical first, Öberg says. Although astronomers have spotted hydrogen cyanide in other star systems, the two more complicated chemical relatives detected in this study have never been seen in a protoplanetary disk. Those substances—hydrogen cyanide (HCN), methyl cyanide (CH3CN), and cyanoacetylene (HC3N)—were discerned through the microwave radiation they emitted at several millimeter-scale wavelengths. The researchers found them in a zone of the disk lying between 4.5 billion and 15 billion kilometers from the parent star in roughly the same proportions seen in comets circling our sun, the researchers report online today in Nature.
The same sort of reactions took place around our sun when it was young, Öberg says. “We now have evidence that this same chemistry exists elsewhere in the universe, in regions that could form solar systems not unlike our own.”
The portion of the disk where the chemicals were observed is roughly analogous to our solar system’s Kuiper belt, the realm of comets and icy bodies that lies beyond Neptune. That’s also where the substances would be present in the gas form, says Geoffrey Blake, a planetary scientist at the California Institute of Technology in Pasadena, who was not part of this study. There, the chemicals are being liberated from the surfaces of ice grains where they were forged by light-driven reactions, he notes. Closer to the star, the chemicals would typically be destroyed by intense radiation, and farther away they’d remain frozen onto ice grains and therefore undetectable.
Ironically, cyanides are incredibly toxic to life once it’s established, but they could be key ingredients in the chemical precursors for life. Last year, researchers found that in conditions mimicking those sparked by a comet or meteorite impact, intense heat and pressure converted formamide (which forms when hydrogen cyanide reacts with water) and other simple substances into the four information-bearing nucleobases in RNA, a likely genetic precursor to DNA.
The material in the protoplanetary disk around MWC 480 will suffer several fates, Öberg suggests. Some will fall into the star, adding to its heft, and some will be blown out of the system into interstellar space. The rest will coalesce into planets, asteroids, and comets—some of which will, as happened in our solar system billions of years ago, carry water and chemicals to rocky bodies lying within the star’s habitable zone.
“From studies such as these,” Blake says, “we are learning that the pivotal chemical compounds needed to seed early worlds with the volatiles required for life are universal.”