Do meteorites contain stardust older than the Sun?
Yes. Primitive meteorites contain tiny presolar grains that formed around other stars before the Sun existed. Scientists recognise them by their unusual isotope ratios.
The cloud of gas and dust that collapsed to form the Sun was seeded with material from earlier generations of stars. Most of that dust was vaporised or thoroughly mixed, but a small fraction survived intact and became locked inside primitive meteorites. These survivors are called presolar grains, and they really are stardust.
They are minute, usually around a micrometre across or smaller, and they make up only a tiny fraction of the host rock, typically parts per million. Known types include silicon carbide, graphite, nanodiamonds, oxides such as corundum and spinel, and silicate minerals. They were first isolated in the late 1980s, when researchers dissolved samples of carbonaceous chondrites in powerful acids to remove the ordinary minerals and leave the tough survivors behind. Today, they are also found directly in polished samples using instruments that map isotopes grain by grain.
How do scientists know they are older than the Sun? The giveaway is their isotopes, versions of the same element with different masses. Material formed in the Solar System has fairly uniform isotope ratios, but presolar grains can be wildly different, matching the nuclear reactions in particular kinds of star. Many silicon carbide grains formed in the cool, swollen outer layers of ageing stars known as red giants; others carry signatures of supernova explosions.
Some grains even have measurable ages. In 2020, scientists studying silicon carbide from the Murchison meteorite, which fell in Australia in 1969, used isotopes produced by cosmic rays in interstellar space to estimate how long the grains had drifted between the stars. Many were older than the Sun by up to a few hundred million years, and a few appear to be roughly 7 billion years old, making them among the oldest solid material known.