Hot Jupiters

Giant planets orbiting scorchingly close to their stars — the surprise that greeted the very first exoplanet discoveries and rewrote how we think planets form.

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The first exoplanets found around Sun-like stars were nothing like our own giants. Instead of a Jupiter parked far out in the cold, they were Jupiters roasting a few stellar radii from their stars, whipping around in days.

Nobody expected them. Gas giants were supposed to form far out, beyond the 'snow line' where ice can condense. A giant planet skimming its star violated the textbook — and forced a new idea: planets can migrate inward after they form.

Hot Jupiters are rare in absolute terms but easy to find — huge, close, and deeply transiting — so they dominated the early years and remain the best-studied exoplanets of all.

Hot Jupiters are gas giants on orbits of only a few days, with dayside temperatures often exceeding 1,000 degrees. Their existence implies large-scale orbital migration: they likely formed beyond the snow line and spiralled inward through interactions with the protoplanetary disk or with other bodies, ending up where no giant could have assembled in place.

Their size and proximity make them the workhorses of exoplanet atmosphere science. Inflated by their stars' heat, they produce deep transits and strong signals, and they were the first worlds to have their atmospheres, winds, and even escaping tails detected.

51 Pegasi b, the first planet found around a Sun-like star, is the archetype — and the reason planet-formation theory had to grow a chapter on migration.

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  • /explore 51 Pegasi b — the original hot Jupiter

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