مشتری داغ (به انگلیسی: Hot Jupiter) یا سیاره تفتیده نوعی سیاره فراخورشیدی است. مشتریهای داغ سیاراتی گازی در ابعاد سیاره هرمز مشتری یا بزرگتر از آن هستند که فاصلهٔ بسیار کمی با ستاره خود دارند و به همین سبب دمای زیادی دارند و «مشتری داغ» نامیده میشوند. نخستین سیارات فراخورشیدی کشف شده جملگی از این نوع بودند.
درباره اینکه مشتریهای داغ چگونه در مدارهایی این چنین نزدیک به ستارهشان قرار گرفتهاند بیشتر اخترشناسان بر این باورند که چگالی سحابیهای پیشسیارهای -ابر گاز و غباری که فشرده میشود و سیارات را شکل میدهد- معمولاً در این نواحی درونی بسیار کمتر از آن است که این مشتریهای داغ در آنجا شکل بگیرند. آنها بایست خارج از این ناحیه شکل گرفته و سپس به این نواحی درونی مهاجرت کرده باشند.
در سحابی پیشسیارهای سامانه خورشیدی هرچه از مرکز فاصله میگرفتید دما کمتر میشد. در ناحیه فراتر از کمربند امروزی سیارکها دما چنان پایین بود که ذرات غبار و دانههای برف مانند هر دو منقبض میشدند. سپس این ذرات به هم پیوستند تا هستههای سیاراتی مانند مشتری را بسازند و وقتی این هستهها به اندازه مشخصی رسیدند شروع به جذب گازهای هیدروژن و هلیوم کردند. محصول نهایی سیارات هرمز، کیوان (زحل)، اورانوس و نپتون بودند.
در این مرحله، خورشید مرکزی وارد مرحلهای از تحول خود به نام مرحله تی-ثوری شد. ستارههای تی–ثوری ستارههای بسیار جوان و بسیار فعالی هستند که مقادیر عظیمی از مواد سطح شان را به فضا پرتاب میکنند. این تندباد ستارهای هر آنچه از سحابی سیاره ساز باقیمانده جاروب میکند و میبرد و به این ترتیب سیارات به ثبات میرسند. هرچند، اگر این مرحله تی-ثوری به هر علت به تأخیر بیفتد، سیارات تازه شکل گرفته در فضایی نسبتاً خالی در مدار نمیگردند بلکه در میان ابر گازی چگالی حرکت میکنند. اصطکاک بین این سیارات تازه شکل گرفته و ابر گازی، انرژی سیارهای را میگیرد و موجب میشود که مدار سیارات جدید دایرهایتر و به آهستگی کوچکتر شود. نتیجه این است که سیارههای جدید به سوی ستاره مرکزی کوچ میکنند و این روند فقط زمانی متوقف میشود که باد تی-ثوری وزیدن بگیرد و ابرگازی از میان برود. به سبب رخ دادن همین فرایند، طی بیش از چند میلیون سال، است که سیارهای همجرم مشتری از نواحی بیرونی سامانه سیارهای به مداری بسیار نزدیک به ستارهاش کوچ میکند. اگر مرحله تی-ثوری زود آغاز شود، غول گازی در نواحی بیرونی منظومه باقی میماند. مرحله تی-ثوری اگر زیاد به تأخیر بیفتد سیاره به درون ستاره سقوط میکند.
Hot Jupiters are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital period (P<10 days). The close proximity to their stars and high surface-atmosphere temperatures resulted in the moniker "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 51 Pegasi b. Discovered in 1995, it was the first extrasolar planet found orbiting a Sun-like star. 51 Pegasi b has an orbital period of about 4 days.
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.
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.
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.
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.
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. 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.
In 2015, two planets were discovered around WASP-47. One was potentially a large terrestrial planet, of 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, with the smaller, terrestrial planet closer in. A similar orbital architecture is also exhibited by the Kepler-30 system.
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 planets
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.
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.
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 any satellite's orbit, 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 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. The recent discovery of particularly low density gas giants orbiting red giant stars supports this theory.
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.