Chondrules look like frozen raindrops made of stone. They're about the size of a grain of sand, but they record the history of the first few million years of the Solar System. Once, these droplets floated in the scorching gas around the young Sun, then clumped into asteroids and fell to Earth as meteorites.
Scientists built a computer model of the disk where planets were born. They traced how oxygen traveled through the disk along with material falling in from the cloud. Oxygen is like a dye that marks each droplet: its shade tells you exactly where the droplet froze. The model showed that chondrules from carbonaceous meteorites could have formed if material fell onto the disk not too far out — roughly out to ten astronomical units from the Sun (that's about the distance from the Sun to Saturn). Or the disk could have been wider, but the parent cloud had little ice and less shielding from ultraviolet radiation.
Carbonaceous chondrules, on the other hand, got luckier: they could form in colder regions of the disk if some water vapor escaped during heating. This scenario explains why meteorites contain two distinct types of oxygen signatures, as if some droplets froze in an oven and others in a steam room.
The model isn't just about the past. It helps us understand how planets like Earth assembled from the chaos of gas and dust. After all, chondrules are the first building blocks from which entire worlds grew.
Chondrules are among the oldest solid objects in the Solar System: their age exceeds 4.5 billion years, making them older than Earth itself.
In Liu Cixin's novel "The Three-Body Problem," aliens study meteorites to understand the history of the Solar System, just as scientists today read the oxygen record of chondrules.