Hot Jupiters

In 1995, when Michel Mayor and Didier Queloz announced the discovery of 51 Pegasi b, the astronomical community was shocked. A gas giant — a world the size of Jupiter — orbiting its star in just 4.2 days, at a distance of 0.05 AU. This violated everything theorists believed about planetary formation.

In our solar system, gas giants orbit far from the Sun: Jupiter at 5.2 AU, Saturn at 9.5 AU. Theory predicted that gas giants must form in the cool, outer regions of protoplanetary disks, where ice accumulates and cores can rapidly accrete hydrogen and helium. They were not supposed to exist at 0.05 AU, where temperatures exceed 1000 K.

Yet 51 Pegasi b was unmistakable. Since that discovery, thousands of hot Jupiters have been found. They comprise a significant fraction of detected exoplanet populations, particularly around older, metal-rich stars. This apparent paradox led to a revolution in planetary migration theory and our understanding of planetary system formation.

The resolution to the hot Jupiter puzzle is planetary migration. Gas giants do indeed form in the outer disk, where ice is abundant and cores can rapidly accrete hydrogen. But after formation, planets interact gravitationally with the protoplanetary disk and with one another. These interactions transfer angular momentum, causing orbital decay.

Type II migration — the movement of massive planets through the disk — can drag gas giants inward rapidly. A Jupiter-mass planet, formed at 5 AU, can spiral inward to 0.1 AU in just a few million years, all while the protoplanetary disk remains. Once the disk dissipates, migration halts. If a planet migrated inward and the disk subsequently vanished, the hot Jupiter remains orbiting close to its star.

This theory predicts that hot Jupiters should be more common around metal-rich stars — metallicity correlates with planet formation efficiency and more rapid core growth. Observations confirm this: hot Jupiters preferentially orbit metal-rich stars. This is powerful evidence that migration, not in situ formation, explains their existence.

WASP-12 b orbits its star every 1.09 days at a distance of 0.023 AU. Its equilibrium temperature exceeds 2500 K — hotter than the photosphere of some stars. Tidal heating from its proximity to the star inflates its radius to 1.9 times Jupiter's radius, despite its higher mass. It is being tidally disrupted and will eventually be consumed by its host star within a billion years.

HD 189733 b, orbiting at 0.03 AU with a period of 2.2 days, has an equilibrium temperature of 1449 K. Its atmosphere shows water vapor, methane, and other molecules. Wind speeds in its atmosphere exceed 8,700 kilometers per hour — faster than Earth's escape velocity. It may rain molten glass in one direction due to the extreme heat differential between dayside and nightside.

KELT-9 b, discovered in 2016, has an equilibrium temperature of 4600 K — hotter than many main-sequence stars. It orbits an F-type star (more luminous than the Sun) with a period of just 1.5 days. Its atmosphere is being stripped at an estimated rate of 10,000 tons per second.