2/ As the planet crosses in front of its host star, atoms in the planet's atmosphere absorb specific colours of the star's light. These missing colours tell us how fast the atoms are moving and how deep in the atmosphere they are.
The weather in WASP-121b is fascinating:
- There is a deep layer of iron wind blowing away from the point where the star is directly overhead.
- Above it there's a fast jet stream of sodium that gets faster and hotter as it crosses the planet's day side.
- Finally, there's an upper layer of hydrogen wind blowing outwards.
WASP-121b is tidally locked –– one side always faces its host star –– and these results tell us how energy is transferred from the day to the night side.
🎨 ESO/M. Kornmesser
