The planet WASP-121b is extreme. It’s a gas giant almost twice as big as Jupiter orbiting extremely close to its star–50 times closer than the Earth does around the Sun. WASP-121b is so close to its star that tidal forces have locked its rotation in a “resonance”: the planet always shows the same face to its star, like the Moon to the Earth. Therefore, one side of WASP-121b constantly bakes in light whereas the other is in perpetual night. This difference causes huge variations in temperature across the planet. It can be more than 3,000°C on one side and drop 1,500°C on the other.
This huge temperature contrast is the source of violent winds, blowing several kilometres per second, which try to redistribute the energy from day to night. Until now, we had to guess the strength and direction of the winds with indirect measurements, such as measurements of the planet’s temperature. In recent years, with the arrival of new instruments on giant telescopes, we’ve been able to directly measure the wind speed of certain exoplanets, including WASP-121b.
