Hot Jupiter

General characteristics

Though there is diversity among hot Jupiters, they do share some common properties.

Formation and evolution

There are two general schools of thought regarding the origin of hot Jupiters: formation at a distance followed by inward migration and in-situ formation at the distances at which they're currently observed. The prevalent view is migration.

Migration

In the migration hypothesis, hot Jupiters are thought to form at a distance from the star beyond the frost line, where the planet can form from rock, ice and gases. The planets then migrate inwards to the star where they eventually form a stable orbit. The planets usually move by type II orbital migration, or possibly via interaction with other planets or a stellar companion. It has been shown that approximately 50% of hot Jupiters have distant Jupiter-mass or larger companions. The 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.

If their atmospheres are stripped away via hydrodynamic escape, their cores may become chthonian planets. The amount of gas removed from the outermost layers depends on the planet size, the envelope gases, the orbital distance from the star, and on the stellar luminosity. In a typical system a gas giant orbiting 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. It should be noted that none of such objects have been found yet and they are still hypothetical.

In situ

It is also theorized that a substantial fraction of hot Jupiters may have formed in-situ via the core accretion method of planetary formation. This theory is particularly attractive because it has measurable consequences including the expectation that hot Jupiters should frequently be accompanied by additional low-mass planets with periods shorter than ~100 days. Traditionally, this mode of conglomeration has been disfavored due to the fact that there may not be enough solid material orbiting close to the star to allow for the in situ assembly of massive cores, which are necessary for the formation of hot Jupiters. Recent surveys, however, have found that the inner regions of planetary systems are not empty, and are frequently occupied by super-Earth type planets. Yet, direct calculations indicate that in situ formation of super-Earths in the close proximity of a solar-mass star require surface densities of solids ≈ 10⁴ g/cm², or larger.

Terrestrial planets in systems with hot Jupiters

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 one might assume. 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.

In 2015, two planets were discovered around WASP-47. One was potentially a large terrestrial planet, with less than 22 Earth masses and 1.8 Earth radii, the other is of similar mass at 15.2 Earth masses but with 3.6 Earth radii it is almost certainly a gas giant. They orbit on either side of a previously discovered hot Jupiter, the smaller, terrestrial planet closer in. A similar orbital architecture is also exhibited by the Kepler-30 system.

Retrograde orbit

It has been found that several hot Jupiters have retrograde orbits and this calls into question the theories about the formation of planetary systems, although rather than a planet's orbit having been disturbed, it may be that the star itself flipped over early in their system's formation due to interactions between the star's magnetic field and the planet-forming disc. By combining new observations with the old data it was found that more than half of all the hot Jupiters studied have orbits that are misaligned with the rotation axis of their parent stars, and six exoplanets in this study have retrograde motion.

Recent research has found that several hot Jupiters are in misaligned systems. This misalignment may be related to the heat of the photosphere the hot Jupiter is orbiting. There are many proposed theories as to why this might occur. One such theory 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 (explaining why hot Jupiters orbiting cooler stars are well aligned) while hotter stars do not damp the obliquity (explaining the observed misalignment).

Ultra-short-period Jupiters

Ultra-short-period Jupiters are a class of hot Jupiters with orbital periods below 1 day and occur only around stars of less than about 1.25 solar masses.

Five ultra-short-period planet candidates have been identified in the region of the Milky Way known as the galactic bulge. They were observed by the Hubble Space Telescope and first described by researchers from the Space Telescope Science Institute, the Universidad Catolica de Chile, Uppsala University, the High Altitude Observatory, the INAF–Osservatorio Astronomico di Padova, and the University of California, Los Angeles.

Confirmed transiting hot Jupiters that have orbital periods of less than one day include WASP-18b, WASP-19b, WASP-43b and WASP-103b.

Puffy planets

Gas giants with a large radius and very low density are sometimes called "puffy planets" or "hot Saturns", due to their density being similar to Saturn's. Puffy planets orbit close to their stars so that the intense heat from the star combined with internal heating within the planet will help inflate the atmosphere. Six large-radius low-density planets have been detected by the transit method. In order of discovery they are: HAT-P-1b, COROT-1b, TrES-4, WASP-12b, WASP-17b, and Kepler-7b. Some hot Jupiters detected by the radial-velocity method may be puffy planets. Most of these planets are below two Jupiter masses as more massive planets have stronger gravity keeping them at roughly Jupiter's size.

Even when taking surface heating from the star into account, many transiting hot Jupiters have a larger radius than expected. This could be caused by the interaction between atmospheric winds and the planet's magnetosphere creating an electric current through the planet that heats it up, causing it to expand. The hotter the planet, the greater the atmospheric ionization, and thus the greater the magnitude of the interaction and the larger the electric current, leading to more heating and expansion of the planet. This theory matches the observation that planetary temperature is correlated with inflated planetary radii.

Moons

Theoretical research suggests that hot Jupiters are unlikely to have moons due to both a small Hill sphere and the tidal forces of the stars they orbit, which would destabilize the satellites' orbits, the latter process being stronger for larger moons. This means that for most hot Jupiters stable satellites would be small, asteroid-sized bodies. In spite of this, observations of WASP-12b suggest that it is orbited by at least 1 large exomoon.

Hot Jupiters around red giants

It has been proposed that, even though no planet of this type has been found until now, gas giants orbiting red giants at distances similar to that of Jupiter could be hot Jupiters due to the intense irradiation they would receive from their stars. It is very likely that in the Solar System Jupiter will become a hot Jupiter after the transformation of the Sun into a red giant.

Hot Jupiters orbiting red giants would differ from those orbiting main-sequence stars in a number of ways, most notably the possibility of accreting material from the stellar winds of their stars and, assuming a fast rotation (not tidally locked to their stars), a much more evenly distributed heat with many narrow-banded jets. Their detection using the transit method would be much more difficult due to their tiny size compared to the stars they orbit, as well as the long time needed (months or even years) for one to transit their star as well as to be occulted by it.