The star AB Aur is young, about 4 million years old. Around it swirls a dense disk of gas and dust. Recently, a long gas stream, a flow of material from the surrounding cloud, crashed into it. Astronomers pointed the NOEMA radio telescope at this disk and saw that its chemistry is not uniform: the northern part smells of sulfur, while the southern part smells of hydrocarbons.

In the north, where the infalling stream struck, there are many SO (sulfur monoxide) molecules. There, the gas is hotter and denser. It seems the impact heated the dust, and icy grains released sulfur and oxygen, like defrosting a freezer, making smells stronger. In the south, by contrast, there is a lot of C2H (a molecule of two carbon atoms and one hydrogen atom). There, the chemistry is richer in carbon. The reason is not just temperature — possibly, ultraviolet radiation from the star or heat from a forming planet breaks down more complex molecules, leaving light hydrocarbons.

The ring of CS (carbon sulfide) molecules remains smooth and cold throughout: it is like an upper floor of the disk, where splashes from the impact do not reach. Meanwhile, HCO+ (formyl ion) shines brightly where dust has gathered into a dense ring. The carbon isotope ratio of 12C to 13C in the disk's cavity is higher than usual — a clear sign that the star's harsh ultraviolet light selectively destroys heavier molecules.

Comparison with chemical models showed that in the southern, hydrocarbon-rich sector, there is more carbon than oxygen in the gas. This is a rare state for protoplanetary disks. In the northern sector, by contrast, there is more oxygen — ice evaporated and mixed into the gas. Two mechanisms, the infall of external gas and the influence of a growing planet, together paint a chemical portrait of the disk, where north and south differ like day and night. Future planets, born under such different conditions, may inherit different chemical compositions.