Hot Jupiter

Hot Jupiters are a specific class of gas giant exoplanets. These planets are physically similar to our own Jupiter, which is why they are called Jupiter analogues. However, they orbit extremely close to their parent stars. This close proximity leads to very high surface-atmosphere temperatures. Because they are so large and close to their stars, they are easy to detect. Scientists often use the radial-velocity method to find them. This method measures the oscillations or wobbles that a planet induces in a star's motion. Because Hot Jupiters are massive and move quickly, these wobbles are relatively large and rapid.

Most Hot Jupiters share several defining physical characteristics. Their masses typically span from 0.36 to 11.8 Jupiter masses. There is a strict upper limit to this mass. If a planet reaches approximately 13.6 Jupiter masses, the internal pressure and temperature become too high. This triggers deuterium fusion, which would turn the object into a brown dwarf rather than a planet. Their orbital periods, or the time it takes to complete one orbit, range from 1.3 to 111 Earth days. Most of these planets maintain nearly circular orbits, known as low eccentricities. Scientists believe tidal forces or perturbations from nearby stars help circularize these orbits.

Some of these worlds are known as "puffy planets" or "hot Saturns." These planets have unusually low densities, similar to the planet Saturn. For example, the planet TrES-4b has a density of only 0.222 g/cm3. This low density happens because the planets have very large radii. High stellar irradiation, which is the intense light from the star, helps expand their atmospheres. Internal energy sources and high atmospheric opacities may also contribute to this inflation. In some cases, the outer layers of the planet exceed their Roche limit. This means the star's gravity pulls the outer layers further outward, expanding the planet even more.

There are three main scientific theories regarding how Hot Jupiters form. The first is the migration hypothesis. In this model, a planet forms far away from the star, beyond the frost line, using the core accretion method. It then moves inward toward the star. This might happen smoothly through type II orbital migration during the solar nebula phase. Alternatively, it might move suddenly due to gravitational scattering after encountering another massive planet. A third theory is in situ formation. This suggests that the cores of these planets began as super-Earths. These cores then gathered gas envelopes in their current locations.

Extreme weather and atmospheric dynamics define these planets. Most Hot Jupiters are tidally locked. This means one side always faces the host star while the other side faces away. This creates a massive temperature difference between the day and night sides. For the planet HD 209458 b, models predict a substantial temperature gap at the photosphere. Scientists expect intense winds and super-rotating equatorial jets. These jets are driven by radiative forcing and the transfer of heat and momentum. Models also predict various storms, or vortices, that mix the atmosphere and transport gas between hot and cold regions.

Some planets are even more extreme, known as ultra-hot Jupiters. These planets have dayside temperatures exceeding 2,500 Kelvin. In these incredibly hot atmospheres, most molecules dissociate into individual atoms. These atoms circulate to the cooler nightside, where they recombine into molecules again. One example is TOI-1431b, discovered in April 2021. Its dayside temperature is so high that it is hotter than 40% of the stars in our galaxy. There is also a class called ultra-short period planets. These have orbital periods of less than one day. There are 12 known Jovian ultra-short period planets currently identified.

Finally, the evolution of these planets can lead to total change. If a planet's atmosphere is stripped away through hydrodynamic escape, its core may remain. This leftover core is called a hypothetical chthonian planet. The amount of mass lost depends on the planet's size, the gases in its envelope, and the star's luminosity. A gas giant orbiting at 0.02 AU might lose 5% to 7% of its mass over its lifetime. However, if it orbits closer than 0.015 AU, it may evaporate much more significantly. This process shows how the intense environment of a star can completely reshape a planetary system.