Hot Jupiters

Lore & Background

Though there is diversity among hot Jupiters, they do share some common properties. Their defining characteristics are their large masses and short orbital periods, spanning 0.36–11.8 Jupiter masses and 1.3–111 Earth days. The mass cannot be greater than approximately 13.6 Jupiter masses because then the pressure and temperature inside the planet would be high enough to cause deuterium fusion, and the planet would be a brown dwarf. Most have nearly circular orbits (low eccentricities). It is thought that their orbits are circularized by perturbations from nearby stars or tidal forces. Many have unusually low densities. The lowest one measured thus far is that of TrES-4b at 0.222 g/cm³. The large radii of hot Jupiters are not yet fully understood but it is thought that the expanded envelopes can be attributed to high stellar irradiation, high atmospheric opacities, possible internal energy sources, and orbits close enough to their stars for the outer layers of the planets to exceed their Roche limit and be pulled further outward. Usually they are tidally locked, with one side always facing its host star. They are likely to have extreme and exotic atmospheres due to their short periods, relatively long days, and tidal locking. Atmospheric dynamics models predict strong vertical stratification with intense winds and super-rotating equatorial jets driven by radiative forcing and the transfer of heat and momentum. Recent models also predict a variety of storms (vortices) that can mix their atmospheres and transport hot and cold regions of gas. The day-night temperature difference at the photosphere is predicted to be substantial, approximately 500 K (500 °C; 900 °F) for a model based on HD 209458 b. They appear to be more common around F- and G-type stars and less so around K-type stars. Hot Jupiters around red dwarfs are very rare. There are three schools of thought regarding the possible origin of hot Jupiters. One possibility is that they were formed in situ at the distances at which they are currently observed. Another possibility is that they were formed at a distance but later migrated inward. Such a shift in position might occur due to interactions with gas and dust during the solar nebula phase. It might also occur as a result of a close encounter with another large object destabilizing a Jupiter's orbit. In the migration hypothesis, a hot Jupiter forms beyond the frost line, from rock, ice, and gases via the core accretion method of planetary formation. The planet then migrates inwards to the star where it eventually forms a stable orbit. The planet may have migrated inward smoothly via type II orbital migration. Or it may have migrated more suddenly due to gravitational scattering onto eccentric orbits during an encounter with another massive planet, followed by the circularization and shrinking of the orbits due to tidal interactions with the star. A hot Jupiter's orbit could also have been altered via the Kozai mechanism (also called Lidov-Kozai mechanism) causing an exchange of inclination for eccentricity resulting in a high eccentricity low perihelion orbit, in combination with tidal friction. This requires a massive body—another planet or a stellar companion—on a more distant and inclined orbit; approximately 50% of hot Jupiters have distant Jupiter-mass or larger companions, which can leave the hot Jupiter with an orbit inclined relative to the star's rotation. Instead of being gas giants that migrated inward, in an alternate hypothesis the cores of the hot Jupiters began as more common super-Earths which accreted their gas envelopes at their current locations, becoming gas giants in situ. If the atmosphere of a hot Jupiter is stripped away via hydrodynamic escape, its core may become a chthonian planet. No such objects have been found yet and they are still hypothetical. If you plot all Hot-Jupiters on a line representing orbital period there is a pile-up of Jupiter-size planets in orbits with a 3 day period. Several hot Jupiters, such as HD 80606 b, have orbits that are misaligned with their host stars, including several with retrograde orbits such as HAT-P-14b.

Reader's Guide

Hot Jupiters are significant because they were among the first exoplanets discovered and remain the easiest to detect via radial velocity, providing key insights into planetary system formation and evolution. Their existence challenged traditional models of planet formation, which predicted that gas giants could not form so close to their stars. The three proposed formation mechanisms—in situ formation, inward migration, and gravitational scattering—each have implications for the architecture of planetary systems and the prevalence of terrestrial planets. Simulations have shown that the migration of a Jupiter-sized planet through the inner protoplanetary disk (the region between 5 and 0.1 AU from the star) is not as destructive as expected. More than 60% of the solid disk materials in that region are scattered outward, including planetesimals and protoplanets, allowing the planet-forming disk to reform in the gas giant's wake. In the simulation, planets up to two Earth masses were able to form in the habitable zone after the hot Jupiter passed through and its orbit stabilized at 0.1 AU. Due to the mixing of inner-planetary-system material with outer-planetary-system material from beyond the frost line, simulations indicated that the terrestrial planets that formed after a hot Jupiter's passage would be particularly water-rich. According to a 2011 study, hot Jupiters may become disrupted planets while migrating inwards; this could explain an abundance of "hot" Earth-sized to Neptune-sized planets within 0.2 AU of their host star. One example of these sorts of systems is that of WASP-47. There are three inner planets and an outer gas giant in the habitable zone. The innermost planet, WASP-47e, is a large terrestrial planet of 6.83 Earth masses and 1.8 Earth radii; the hot Jupiter, b, is little heavier than Jupiter, but about 12.63 Earth radii; a final hot Neptune, c, is 15.2 Earth masses and 3.6 Earth radii. A similar orbital architecture is also exhibited by the Kepler-30 system. The pile-up of hot Jupiters at a 3-day orbital period remains a notable observational feature. Their misaligned orbits, including retrograde orbits, further complicate understanding of planetary dynamics. Hot Jupiters also serve as laboratories for studying extreme atmospheric physics, including tidal locking, strong winds, and large temperature contrasts. Their legacy includes informing the search for habitable planets and refining theories of planetary migration and atmospheric escape.

Did You Know?

• The mass of a hot Jupiter cannot exceed approximately 13.6 Jupiter masses, or it would become a brown dwarf due to deuterium fusion.
• The lowest density measured for a hot Jupiter is TrES-4b at 0.222 g/cm³.
• Approximately 50% of hot Jupiters have distant Jupiter-mass or larger companions.
• If you plot all hot Jupiters on a line representing orbital period, there is a pile-up of Jupiter-size planets in orbits with a 3-day period.
• Several hot Jupiters, such as HD 80606 b, have orbits misaligned with their host stars, including retrograde orbits like HAT-P-14b.