Hot Jupiter


Hot Jupiters are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods. The close proximity to their stars and high surface-atmosphere temperatures resulted in their informal name "hot Jupiters".
Hot Jupiters are the easiest extrasolar planets to detect via the radial-velocity method, because the oscillations they induce in their parent stars' motion are relatively large and rapid compared to those of other known types of planets. One of the best-known hot Jupiters is. Discovered in 1995, it was the first extrasolar planet found orbiting a Sun-like star. has an orbital period of about four days.

General characteristics

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. It is thought that their orbits are circularized by perturbations from nearby stars or tidal forces. Whether they remain in these circular orbits for long periods of time or collide with their host stars depends on the coupling of their orbital and physical evolution, which are related through the dissipation of energy and tidal deformation.
  • 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 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 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. Generalizations about the distribution of these planets must take into account the various observational biases, but in general their prevalence decreases exponentially as a function of the absolute stellar magnitude.

    Formation and evolution

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.

Migration

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 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.
The type II migration happens during the solar nebula phase, i.e. when gas is still present. Energetic stellar photons and strong stellar winds at this time remove most of the remaining nebula. Migration via the other mechanism can happen after the loss of the gas disk.

In situ

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. The super-Earths providing the cores in this hypothesis could have formed either in situ or at greater distances and have undergone migration before acquiring their gas envelopes. Since super-Earths are often found with companions, the hot Jupiters formed in situ could also be expected to have companions. The increase of the mass of the locally growing hot Jupiter has a number of possible effects on neighboring planets. If the hot Jupiter maintains an eccentricity greater than 0.01, sweeping secular resonances can increase the eccentricity of a companion planet, causing it to collide with the hot Jupiter. The core of the hot Jupiter in this case would be unusually large. If the hot Jupiter's eccentricity remains small the sweeping secular resonances could also tilt the orbit of the companion. Traditionally, the in situ mode of conglomeration has been disfavored because the assembly of massive cores, which is necessary for the formation of hot Jupiters, requires surface densities of solids ≈ 104 g/cm2, or larger. Recent surveys, however, have found that the inner regions of planetary systems are frequently occupied by super-Earth type planets. If these super-Earths formed at greater distances and migrated closer, the formation of in situ hot Jupiters is not entirely in situ.

Atmospheric loss

If the atmosphere of a hot Jupiter is stripped away via hydrodynamic escape, its core may become a chthonian planet. The amount of gas removed from the outermost layers depends on the planet's size, the gases forming the envelope, the orbital distance from the star, and the star's luminosity. In a typical system, a gas giant orbiting at 0.02 AU around its parent star loses 5–7% of its mass during its lifetime, but orbiting closer than 0.015 AU can mean evaporation of a substantially larger fraction of the planet's mass. No such objects have been found yet and they are still hypothetical.
File:Clear to cloudy hot Jupiters.jpg|thumb|center|400px| From top left to lower right: WASP-12b, Boinayel, WASP-31b, Bocaprins, HD 189733b, Puli, Ditsö̀, Banksia, HAT-P-1b and HD 209458b

Terrestrial planets in systems with hot Jupiters

Simulations have shown that the migration of a Jupiter-sized planet through the inner protoplanetary disk 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.

Misaligned orbits

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. This misalignment may be related to the heat of the photosphere the hot Jupiter is orbiting. There are several proposed hypotheses as to why this might occur. One such hypothesis involves tidal dissipation and suggests there is a single mechanism for producing hot Jupiters and this mechanism yields a range of obliquities. Cooler stars with higher tidal dissipation damps the obliquity while hotter stars do not damp the obliquity. Another hypothesis is that the host star sometimes changes rotation early in its evolution, rather than the orbit changing. Yet another hypothesis is that hot Jupiters tend to form in dense clusters, where perturbations are more common and gravitational capture of planets by neighboring stars is possible.

Ultra-hot Jupiters

Ultra-hot Jupiters are hot Jupiters with a dayside temperature greater than. In such dayside atmospheres, most molecules dissociate into their constituent atoms and circulate to the nightside where they recombine into molecules again.
One example is TOI-1431b, announced by the University of Southern Queensland in April 2021, which has an orbital period of just two and a half days. Its dayside temperature is, making it hotter than 40% of stars in our galaxy. The nightside temperature is.