These strange worlds are too hot to be planets, and too cool to be stars

Ultra-hot Jupiters are a new class of exoplanets that astronomers are increasingly finding scattered throughout the cosmos. These incredibly hot gas giants sit much closer to their host stars than Mercury does to the Sun, which invariably leads to tidal locking — meaning the same side of the planet always faces the star. This causes dayside temperatures to exceed 3,500 degrees Fahrenheit (1,900 Celsius), while nightside temperatures hover around 1,800 F (1,000 C). Furthermore, ultra-hot Jupiters display unique atmospheric characteristics that are not seen in other types of planets, such as an apparent lack of most molecules.

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