hot Jupiter

When astronomers found the first planet around a Sun-like star in 1995, it broke every expectation: a gas giant the heft of Jupiter, but whipping around its star in just four days, orbiting closer than Mercury orbits the Sun, with a surface roasted to thousands of degrees. Nothing in our tidy Solar System looked like that. A hot Jupiter is a giant, gaseous planet on a scorchingly close orbit around its star — the surprising, attention-grabbing class of world that opened the whole field of exoplanets.

A typical hot Jupiter has roughly the mass of Jupiter but circles its star at a small fraction of the Earth-Sun distance, completing an orbit in just days. So close to the furnace of its star, its outer layers can be heated to over 1,000 degrees Celsius, sometimes far hotter, and it is usually tidally locked, keeping one blazing face permanently toward the star. Many are puffed up larger than they should be, bloated by the relentless heat. Because they are big and close, they tug their stars hard and block a lot of starlight, which is exactly why the radial-velocity and transit methods spotted them first, even though they turn out to be fairly rare.

Hot Jupiters posed a deep puzzle: a giant planet cannot form so close to its star, where it is too hot and there is too little material. Their existence is the clearest evidence for planet migration — they must have formed farther out, beyond the snow line, and then spiraled inward. They remain prized targets because their large size and high temperature make their atmospheres the easiest of any exoplanet to study, even though they are utterly unlike Earth and host no prospect of life.