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.

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