Hot Jupiters (also called roaster planets,[1] epistellar jovians,[2][3] pegasids[4][5] or pegasean planets) are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter (1.9×1027 kg). While Jupiter orbits its parent star (the Sun) at 5.2 astronomical units (780×106 km), the planets referred to as hot Jupiters orbit between approximately 0.015 and 0.5 astronomical unit (2.2×106 and 75×106 km) of their parent stars.[6]
One of the most well-known hot Jupiters is 51 Pegasi b, nicknamed Bellerophon. Discovered in 1995, it was the first extrasolar planet found orbiting a Sun-like star.
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Hot Jupiters have some common characteristics:
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 other known types of planets.
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 sun where they eventually form a stable orbit.[7] The planets usually move by type 2 migrations, or possibly via interaction with other planets. The migration happens during the solar nebula phase, and will typically stop when the sun enters its T-Tauri phase. The strong stellar winds at this time remove most of the remaining nebula.
After hot Jupiters get their atmospheres and outer layers stripped away (hydrodynamic escape), their cores may become chthonian planets. Losing of the outermost layers depends on the size and the material of the planet and the distance from the star. 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 the whole planet except for its core.[8]
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.[9] 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 solar system material with outer solar 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.[9]
It has been found that several hot Jupiters have retrograde orbits and this calls into question the theories about the formation of planetary systems,[10] 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.[11] 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.
Ultra-short-period planets are a class of hot Jupiters with orbital periods below 1 day and occur only around stars of less than about 1.25 solar mass.[12] They orbit closer to stars than any other described planetary object.
Five ultra-short-period planets 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 at Los Angeles.[12]
More transiting hot Jupiters have been discovered, such as Wasp-18b, that have orbital periods of less than one day that do not support the hypothesis of the research above.
Gas giant planets with a large radius and very low density are sometimes called "puffy planets"[14] or "hot Saturns", due to their similar density to Saturn. Puffy planets may orbit close to their stars since the intense heat from the star and internal heating within the planet will help inflate the planet's atmosphere. Six large-radius and low-density planets have been detected by the transit method. In order of discovery they are: HAT-P-1b,[15][16] 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.
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