Asteroids are old. Really, really old. They formed at the same time as the planets, so when their pieces fall to Earth, they deliver 4.56 billion-year-old mementos that allow us to touch the birth of the Solar System. But some small bits in those meteorites are even older, if you’re limber enough for a little more mind-bendingly deep time.

Some mineral grains that ended up in the Solar System formed in other stars. Most that have been found so far formed in certain types of red giant stars. As these stars age and run out of fuel, they swell in size. In the diffuse envelope around the star, elements can interact and cool enough to form very tiny particles—cosmic dust. Some of those particles were present in the nebula that gave birth to our Solar System, and they have survived to the present day.

Other grains appear to have a different origin—the collapse of a high mass star in a supernova. The grains we’ve found have varied chemically (some made of carbon, some of silicate or oxide minerals) but recently discovered grains from two different meteorites add a new composition to the list—SiO 2 . This isn’t just stamp-collecting for the fun of it—silica had been predicted to form in supernovae, but none had ever been found.

The silica grains plucked from a pair of rocky meteorites were analyzed using an instrument that fires heavy cesium ions at a material and identifies the atoms that are blasted out. In this case, they were mainly interested in the isotopes of oxygen that were present. The relative proportions of 16O, 17O, and 18O can fingerprint the source of the grains.

In terms of their oxygen composition, both of the two silica grains were distinct from the signature of the materials that formed during the birth of our Solar System. But whereas previously identified silica grains fit the red giant profile, these contained less 17O, putting them in the range of a supernova source.

So a massive star went supernova, casting its guts into our nebular neighborhood, and now we’re examining little pieces of those guts and extracting that history. But these little time capsules present a puzzle.

Massive stars (pre-explosive supernova) are composed of concentric layers of gas, each with different elements. According to our understanding of these stars, silica should only be able to solidify in oxygen-rich layers near the core. But if that’s the case, our models would predict a much higher proportion of 16O than was found in the silica grains. In fact, the same is true of other mineral grains we’ve identified as coming from supernovae.

In order for the oxygen isotope numbers to come out, material from those oxygen-rich inner layers would have to mix with a much larger share of the outermost layers before solidifying. But because of the abundance of other elements in those outer layers, plain vanilla silica shouldn’t form.

Similar problems arise when trying to account for the oxygen isotope signatures of other types of grains thought to come from such stars. But astronomers think they’ve detected all these substances, including silica, in the dusty remnants of supernovae, so it’s unlikely that we’re simply wrong about where these grains formed. There’s probably something about the physics and chemistry of massive stars that we’re missing.

It’s handy, then, that we can find pieces of those stars in our Solar System to help us put together the puzzle.

Astrophysical Journal Letters, 2013. DOI: 10.1088/2041-8205/768/1/L17 (About DOIs).