A hot Jupiter is a giant gas planet, similar in mass to Jupiter, that orbits extremely close to its host star. While Jupiter sits about five times farther from the Sun than Earth does and takes 12 years to complete one orbit, a hot Jupiter can circle its star in as few as four days. That proximity superheats these worlds to thousands of degrees, creating some of the most extreme environments ever observed in planetary science.
How Hot Jupiters Were First Found
The first hot Jupiter, 51 Pegasi b (nicknamed “Dimidium”), was detected in October 1995 by Swiss astronomers Michel Mayor and Didier Queloz. It was also the first exoplanet ever confirmed around a Sun-like star, a discovery that earned both scientists a share of the 2019 Nobel Prize in Physics. 51 Pegasi b has roughly the mass of Jupiter but orbits twenty times closer to its star than Earth is to the Sun, completing a full orbit every four Earth days.
The discovery was shocking. At the time, no one expected a Jupiter-sized planet could exist so close to a star. It challenged everything astronomers thought they knew about how planetary systems form and immediately opened a new field of research.
Hot Jupiters were the first type of exoplanet found not because they’re common, but because they’re the easiest to spot. Their large mass and tight orbits create a strong gravitational tug on their host star, producing a large signal in the radial velocity method (which measures a star’s wobble). They also block more light during a transit, making them easier to pick up with space telescopes like Kepler and TESS. Smaller, more distant planets require far greater precision to detect.
How Rare They Actually Are
Despite being easy to find, hot Jupiters are genuinely rare. A survey using TESS data found that only about 0.33 percent of Sun-like and hotter stars host one. The rate varies slightly by star type: roughly 0.55 percent of G-type stars (similar to our Sun), 0.36 percent of F-type stars, and 0.29 percent of A-type stars. So for every 300 Sun-like stars you look at, you’d expect to find about one or two hosting a hot Jupiter.
Why They Orbit So Close
Giant planets need large amounts of gas and ice to form, materials that only exist in abundance far from a star where temperatures are low enough for them to condense. That means hot Jupiters almost certainly didn’t form where we find them today. Instead, they formed several astronomical units out from their star (similar to where Jupiter sits in our solar system) and then migrated inward.
Several mechanisms can drive this migration. The leading theory involves the young planet interacting with the disk of gas and dust it was born in. As the planet plows through this disk, gravitational torques slowly drain its orbital energy, causing it to spiral closer to the star. This is called disk migration. Another possibility involves gravitational interactions with other planets in the system: one planet can scatter another onto a highly elongated orbit that gradually shrinks and circularizes through tidal interactions with the star. A distant companion star or planet can also tilt and shrink an orbit through a process called the Kozai mechanism.
What stops the planet from falling all the way into its star is less clear. Proposed explanations include tidal forces from the star, magnetic interactions between the star and the inner edge of the disk, and regions in the disk where migration naturally stalls.
Extreme Temperatures and Tidal Locking
Sitting so close to their stars, hot Jupiters absorb enormous amounts of radiation. Most are tidally locked, meaning one hemisphere permanently faces the star while the other faces deep space, the same way our Moon always shows the same face to Earth. This creates a dramatic temperature split between the permanent dayside and nightside.
The most extreme example discovered so far is KELT-9b, whose dayside reaches about 7,800°F (4,600 Kelvin). That makes it hotter than the surfaces of most stars in the galaxy. As NASA’s Jet Propulsion Laboratory put it, KELT-9b is “a planet that is hotter than most stars,” with an atmosphere unlike anything else ever observed.
Violent Atmospheres and Supersonic Winds
The massive temperature difference between day and night drives ferocious atmospheric circulation. Heat from the dayside rushes toward the cooler nightside, generating powerful winds that whip around the planet. Observations of WASP-76 b, an ultra-hot Jupiter, measured a day-to-nightside wind of about 5.5 kilometers per second (roughly 12,300 miles per hour) in its lower atmosphere. Higher up, vertical winds were clocked at around 23 kilometers per second, or more than 50,000 miles per hour. Earlier observations of another hot Jupiter, HD 209458 b, detected a 2 kilometer per second wind blowing from day to night.
These winds create a phenomenon called super-rotation, where the hottest point on the planet gets pushed downwind from the spot directly facing the star. So instead of the peak temperature sitting at “high noon,” it shifts eastward, something space telescopes have confirmed by mapping the infrared glow of several hot Jupiters.
What Their Atmospheres Contain
Because hot Jupiters are large and puffed up by heat, their atmospheres are relatively easy to study when the planet passes in front of or behind its star. Starlight filtering through the atmosphere leaves chemical fingerprints that telescopes can read.
Water vapor and carbon monoxide are among the most commonly detected molecules. Observations of the ultra-hot Jupiter WASP-121b, for instance, measured the absorption signals of both water and carbon monoxide and found they behave differently at different points in the planet’s orbit, revealing how chemistry shifts between the dayside and nightside. Iron vapor has also been detected in the atmosphere of WASP-121b using ground-based telescopes. On cooler hot Jupiters, sodium and potassium have been identified in the upper atmospheres.
On the most extreme worlds like KELT-9b, temperatures are high enough to rip apart water molecules entirely and even vaporize metals that would be solid rock on Earth.
Atmospheric Escape and Long-Term Fate
Intense radiation from the nearby star doesn’t just heat a hot Jupiter’s atmosphere. It can strip it away entirely over time. High-energy X-ray and ultraviolet light ionizes hydrogen in the upper atmosphere, giving gas particles enough energy to escape the planet’s gravity. This process, called photoevaporation, creates a comet-like tail of material streaming away from the planet.
How fast a hot Jupiter loses atmosphere depends on several factors, including the strength of its own magnetic field. Modeling shows that the total mass-loss rate can vary by more than a factor of ten depending on the planet’s magnetic field strength. At weak field strengths (below about 2.5 Gauss), the magnetic field actually accelerates escape by pushing ionized material outward. At stronger field strengths, the field can trap and confine some of the escaping gas, partially shielding the atmosphere.
For most known hot Jupiters, this mass loss is significant in scientific terms but not enough to destroy the planet over the lifetime of its star. These are massive worlds, and even losing material steadily for billions of years typically strips only a small fraction of their total mass. Smaller, less massive planets in similar orbits may not be so lucky, which could explain why there’s a noticeable gap in the population of medium-sized planets found very close to their stars.