We really are made of stardust. New research suggests the majority of Earth's carbon came from the interstellar medium, the diffuse supply of gas and dust found between a galaxy's stars.
According to a new study, published Friday in the journal Science Advances, carbon from the interstellar medium became incorporated into the solar system's protoplanetary disk just a million years after the sun was born.
Previously, scientists hypothesized most of Earth's organic molecules were sourced from nebular gas. As gas from the stellar nebula cooled, researchers surmised, carbon and other molecules precipitated out of the cloud and became incorporated into rocky planets.
The problem with this theory is that once carbon vaporizes, it's unable to condense back into a solid.
"The condensation model has been widely used for decades," lead study author Jie Li, planetary scientist at the University of Michigan, said in a news release.
"It assumes that during the formation of the sun, all of the planet's elements got vaporized, and as the disk cooled, some of these gases condensed and supplied chemical ingredients to solid bodies. But that doesn't work for carbon," said Li.
Most of the carbon pulled from the interstellar medium came in the form of organic molecules. However, vaporized carbon yields more volatile varieties, which can only condense back into solids at very low temperatures.
To avoid this pitfall, scientists considered the possibility that Earth's carbon was never vaporized at all -- that it was acquired directly from the interstellar medium.
Researchers began by using seismic waves to study the contents of Earth's core and gather data they helped them estimate the maximum amount of carbon the planet might contain.
"We asked a different question: We asked how much carbon could you stuff in the Earth's core and still be consistent with all the constraints," said co-author Edwin Bergin, professor of astronomy at Michigan.
"There's uncertainty here. Let's embrace the uncertainty to ask what are the true upper bounds for how much carbon is very deep in the Earth, and that will tell us the true landscape we're within," Bergin said.
To support life, a planet must have just the right amount of carbon. If a newborn planet acquires too much carbon, it's likely to overheat, yielding a hothouse planet like Venus. If an infant planet doesn't get enough carbon, it's likely to host more Mars-like conditions, frigid and dry.
In a related study, published earlier this year in the journal PNAS, scientists analyzed iron meteorites to better understand how much carbon might have survived the planet formation process.
Their findings showed early planetesimals likely lost most of their carbon as a planet's building blocks melted, formed cores and expelled gas.
"Most models have the carbon and other life-essential materials such as water and nitrogen going from the nebula into primitive rocky bodies, and these are then delivered to growing planets such as Earth or Mars," said co-author Marc Hirschmann.
"But this skips a key step, in which the planetesimals lose much of their carbon before they accrete to the planets," said Hirschmann, a professor of earth and environmental sciences at the University of Minnesota.
Together, the Science Advances and PNAS papers suggest both carbon acquisition and carbon loss play important roles in setting the stage for life on rocky planets.
"Answering whether or not Earth-like planets exist elsewhere can only be achieved by working at the intersection of disciplines like astronomy and geochemistry," said co-author Fred Ciesla, professor of geophysical sciences at the University of Chicago.
"While approaches and the specific questions that researchers work to answer differ across the fields, building a coherent story requires identifying topics of mutual interest and finding ways to bridge the intellectual gaps between them. Doing so is challenging, but the effort is both stimulating and rewarding," Ciesla said.
