Simulated metastable helium number density (top) and the line-oſ-sight velocity (bottom) in the orbital mid-plane of the WASP-121 system. The host star, located at the coordinate origin, is orbited by the planet, located on the negative x-axis, in counterclockwise direction. The observer is also located on the negative x-axis. The bottom panel shows the velocity component in the x-direction, representing the observer’s view during mid-transit. Purple and white contours indicate the cumulative optical depth at the center of the He I λ10833 line, with τ = 10 and τ = 1, respectively; solid and dotted contours represent the density I and II setup. The planetary outflow forms a red-shifted, leading tail, which originates from the more pronounced day side outflow.