Line-of-sight velocity in the vicinity of the planet in the orbital midplane for simulation model S2*. The observer is positioned along the negative x-axis, with the velocity component in the x direction representing the observer’s perspective during mid-transit. The black and red circles indicate the planetary radius and the Hill radius, respectively. This analysis focuses solely on the main component of the metastable helium line. Black, gray, and white contours mark the cumulative optical depth of τ = 10, 1, and 0.1, respectively, along the radial direction toward the observer. The dots indicate high optical depth values (Δτ) in the simulation grid. This figure highlights the primary region of He I 1083 nm line formation.