Giant planets can form in two ways. In core accretion, a rocky core slowly accumulates from dust and pebbles in a protoplanetary disk until it becomes massive enough to gravitationally capture surrounding hydrogen and helium gas. In gravitational instability, a dense region of the disk collapses directly into a gas giant — the same mechanism that forms stars. Both pathways produce roughly similar end products: massive planets dominated by hydrogen and helium. Distinguishing which pathway formed a specific planet has been difficult because the bulk compositions overlap.
JWST detected sulfur in the atmospheres of the giant planets orbiting HR 8799 — a directly imaged multi-planet system about 130 light-years away. Sulfur is the discriminator. In the core accretion model, the growing rocky core accumulates sulfur-bearing minerals from the disk. When the core becomes massive enough to capture gas, the sulfur is already present in the core and outgasses into the atmosphere. In the gravitational instability model, the planet forms from gas that collapses all at once. The gas-phase sulfur abundance in a protoplanetary disk is much lower than the solid-phase sulfur abundance, because sulfur preferentially condenses into refractory minerals at the relevant temperatures. A planet formed by direct collapse should have less atmospheric sulfur than one formed by core accretion.
The HR 8799 planets have atmospheric sulfur. They formed by core accretion — slowly, like Jupiter, building solid cores before capturing their envelopes. This matters because the HR 8799 planets are massive — five to ten times Jupiter's mass — and orbit far from their star. Both characteristics were traditionally associated with gravitational instability, which works best for massive planets at large orbital distances. Core accretion was thought to be too slow at those distances, because the orbital timescales are longer and the disk material is sparser. The sulfur detection says core accretion worked anyway, even in conditions where theory suggested it should be difficult.
The structural insight is about what a single measurement can resolve. The question “how did this planet form?” seems like it should require reconstructing the entire formation history — the initial disk conditions, the accretion timeline, the migration pathway. Instead, a single element in the atmosphere acts as a fossil record of the formation process. Sulfur remembers whether it was incorporated as a solid or captured as a gas. The measurement is in the present; the information is from four billion years ago. The atmosphere is a geological record of planetary assembly.
This is the power of a diagnostic trace — a quantity that is easy to measure now and hard to erase, carrying information about a process that cannot be directly observed. Carbon isotope ratios in atmospheric CO2 distinguish fossil fuel burning from natural sources. Helium-3 in ocean sediments traces extraterrestrial dust influx. Sulfur in exoplanet atmospheres traces formation mechanism. In each case, the trace element is not the main constituent. It is a minor component whose presence or absence distinguishes between pathways that produce similar major outcomes. The answer is in the impurity, not the bulk.