WASP-121b is a hot Jupiter — a gas giant orbiting so close to its star that its dayside temperature exceeds 3,000 kelvin, hot enough to vaporize iron and corundum. JWST observed the planet continuously over a full orbit and found two enormous streams of helium stretching across more than half the orbital path. The planet is losing its atmosphere in real time, and for the first time, the loss has been tracked continuously rather than sampled at isolated transit events.
Previous observations of atmospheric escape relied on single transits — the moment the planet passes in front of its star, allowing starlight to filter through the escaping gas. A single transit shows that gas is escaping. It does not show how the escape evolves over the orbit, whether the streams are steady or pulsating, or how far they extend. JWST's continuous observation reveals the full geometry: two helium tails, not one, trailing the planet like a comet's twin ion tails, shaped by the interplay of the stellar wind and the planet's orbital motion.
The structural insight is about the difference between snapshot and trajectory. A single transit observation is a snapshot — it captures the state of the atmosphere at one orbital phase. The continuous observation is a trajectory — it captures the evolution of the atmosphere through the full range of stellar radiation, magnetic field orientation, and orbital dynamics. The snapshot said “this planet is losing atmosphere.” The trajectory says “this is how the atmosphere leaves.” The distinction matters because the physics of escape is phase-dependent: the dayside radiation drives the escape, the nightside cooling partially recondenses it, and the orbital motion sweeps the escaped gas into structured streams. None of this is visible in a snapshot.
The two-tail geometry is itself informative. One tail leads the planet; the other trails behind it. The leading tail is swept forward by the stellar wind. The trailing tail follows the planet's orbital motion. The relative extent and density of the two tails encode the ratio of stellar wind pressure to orbital momentum — a measurement of the stellar environment that is otherwise difficult to obtain. The escaping atmosphere is a probe of the conditions that are destroying it.
WASP-121b will eventually lose its atmosphere entirely — the escape is faster than any plausible replenishment mechanism. What remains will be a stripped core: a dense, rocky or metallic remnant where a gas giant once existed. The planet is in the process of becoming something else. JWST is watching the transformation in progress.