Jan. 30 (UPI) -- Scientists have traced the grains that coalesced to form the planets in our solar system. They were formed by Asymptotic Giant Branch stars, or AGB stars.
AGB stars are six times larger than our sun and thousands of times brighter. These aging red dwarfs shed their outer layers as their inner cores expand. The expelled outer layers become interstellar clouds of gas and dust -- material which forms new stars as well as protoplanetary disks.
Though many of these ancient grains were destroyed in the planet-formation process, a small percentage survived and can be recovered from meteorites that hit Earth's surface.
Scientists have long theorized our solar system's building blocks were forged by AGB stars. But chemical analysis of ancient interstellar grains didn't match what scientists expected AGB grains to look like.
The latest research -- detailed in the journal Nature Astronomy -- suggests fusion inside AGB stars isn't quite as astronomers had thought. Fusion reactions between protons and an extremely heavy oxygen isotope, oxygen-17, are twice as abundant as previously estimated.
The frequency of the reaction during AGB fusion explains the composition of grains found among the remnants of ancient meteorites.
Scientists observed the fusion reaction during experiments at the Laboratory for Underground Nuclear Astrophysics, a lab buried a few thousand feet in the ground in Italy.
"It is a great satisfaction to know that we have helped to solve a long-standing puzzle on the origin of these key stardust grains," Marialuisa Aliotta, an astronomer at the University of Edinburgh, said in a news release. "Our study proves once again the importance of precise and accurate measurements of the nuclear reactions that take place inside stars."