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WASP-76b

WASP-76b is an ultra-hot , a with a of 0.92 ± 0.03 masses and a radius of 1.83 ± 0.06 radii, orbiting the F7-type star WASP-76 every 1.81 days at a semi-major axis of 0.033 AU. Located approximately 637 light-years from in the constellation , it is tidally locked, resulting in extreme temperature contrasts between its scorching dayside, where temperatures exceed 2400°C and vaporize iron, and its cooler night side, where iron condenses and falls as molten . The planet's atmosphere exhibits asymmetric chemistry, with depleted gaseous iron on the morning limb due to night-side condensation, and recent observations indicate a possible effect—a rainbow-like —potentially from uniform spherical droplets. Discovered in 2016 through the transit method by the Wide Angle Search for Planets (WASP) survey, confirmed by radial velocity measurements from the SOPHIE and CORALIE spectrographs, WASP-76b was identified as one of three bloated hot Jupiters receiving intense stellar irradiation that inflates its atmosphere. The host star WASP-76 has a mass of ≈1.43 M_⊙, a radius of 1.76 R_⊙, and an effective temperature of ≈6300 K (updated as of 2024), making it slightly hotter and larger than the Sun. Its proximity to the star—12 times closer than Mercury to the Sun—drives the planet's ultra-hot classification, with dayside temperatures around 2630 K and night-side temperatures inferred to be hundreds of Kelvin cooler. Subsequent observations have revealed a dynamic atmosphere dominated by metal vapors and strong winds carrying iron from the dayside to the night side at speeds of several km/s. High-resolution spectroscopy has indicated possible presence of titanium oxide (TiO) and vanadium oxide (VO) in the atmosphere, contributing to its opacity and thermal structure. Data from ESA's Cheops, NASA's Hubble Space Telescope, TESS, and Spitzer have confirmed atmospheric asymmetry and a geometric albedo of about 0.11, suggesting reflective properties possibly enhanced by the proposed glory. In 2025, observations with GIANO-B and HARPS-N detected CO and Fe I on the dayside, further probing atmospheric dynamics. WASP-76b serves as a benchmark for studying ultra-hot Jupiters, with future observations from the James Webb Space Telescope expected to probe deeper into its 3D atmospheric dynamics and composition.

Discovery and designation

Discovery

WASP-76b was discovered through the transit method as part of the (WASP) project, which employs wide-field photometric surveys to detect short-period giant planets orbiting bright stars. The SuperWASP survey, utilizing camera arrays at both northern and southern observatories, identified periodic dips in the light curve of the host star WASP-76, indicating the presence of a transiting . Observations contributing to the initial detection spanned from July 2008 to December 2010 across the WASP-South and SuperWASP-North facilities. The discovery was announced in a preprint submitted on October 21, 2013, by R. G. West and collaborators, who analyzed data from the ongoing WASP survey focused on hot Jupiters—gas giants with orbital periods under 10 days receiving intense stellar irradiation. To confirm the planetary nature of the transiting object and determine its orbital period, follow-up radial velocity measurements were conducted using the SOPHIE spectrograph at the Haute-Provence Observatory from September to December 2011 and the CORALIE spectrograph on the Euler Telescope from February to December 2012. These observations revealed a Doppler signal consistent with a massive, short-period companion, ruling out false positives such as eclipsing binaries. The full results, including refined parameters from combined photometric and spectroscopic data, were published in in January 2016. This work highlighted WASP-76b as one of three newly identified inflated hot Jupiters, emphasizing the role of the WASP consortium in advancing the detection of such extreme .

Nomenclature

WASP-76b is the provisional designation assigned to the as the first and only confirmed orbiting its host star, WASP-76, under the naming convention of the (WASP) survey. This follows the for transiting detected by ground-based surveys, where the "b" denotes the innermost known in the system. The planet is classified as a , a category for gas giants with masses and radii comparable to Jupiter's but orbiting very close to their stars, leading to extreme temperatures from intense stellar irradiation. This classification was established upon its confirmation through photometric transits and measurements. WASP-76b resides in the constellation , with its host located approximately 640 light-years from Earth. No alternative designations or (IAU)-approved proper names have been formally adopted for the planet as of 2025.

Host star system

Primary star properties

WASP-76 is an F7-type main-sequence star that serves as the primary host to the WASP-76b. It has an of 6250 ± 100 , consistent with its spectral classification as a hot, yellow-white dwarf slightly hotter than . The star's is log g = 4.4 ± 0.1 (cgs), reflecting its structure compared to values. The star possesses a of 1.46 ± 0.07 M⊙ and a of 1.73 ± 0.04 R⊙, making it more massive and larger than , with a corresponding approximately 4.1 L⊙. Its is slightly enhanced at [Fe/H] = +0.23 ± 0.10, indicating a higher abundance of heavy elements relative to than in . Age estimates derived from gyrochronology, using the star's projected rotational velocity of v sin i = 3.3 ± 0.6 km/s and an assumed rotation period of about 17.6 days, place it at approximately 5.3^{+6.1}_{-2.9} Gyr. Observationally, WASP-76 is situated at a distance of 189 ± 3 pc from , as determined from DR3 parallax measurements. It appears with an apparent visual magnitude of V = 9.5, rendering it accessible to ground-based telescopes for follow-up studies such as monitoring and photometry.

Companion star

The companion star in the WASP-76 system, designated WASP-76 B, was identified through high-contrast imaging observations that resolved its presence relative to the primary star, with confirmation via () spectroscopic imaging in 2020 revealing its impact on planetary spectra. The angular separation between WASP-76 A and B is 0.436 ± 0.003 arcseconds, corresponding to a projected physical separation of approximately 80 given the system's distance of about 189 pc from DR3. WASP-76 B is a late G- or early K-type dwarf with an effective temperature of 4824 ± 128 K, a mass of 0.79 ± 0.03 M⊙, and a radius of 0.83 R⊙. It is fainter than the primary by ΔK = 2.30 ± 0.05 magnitudes, contributing roughly 10–11% of the total system flux in the near-infrared bands relevant to HST Wide Field Camera 3 (WFC3) observations. This flux ratio implies a dilution factor of about 1.08–1.11 in transit depths, requiring wavelength-dependent corrections to accurately interpret the data. The presence of WASP-76 B complicates characterization by blending its light with that of the primary, which initially distorted the of WASP-76b in early data from 2020, artificially steepening the slope and influencing interpretations of atmospheric scattering or properties. Post-correction analyses preserve key features like water absorption while eliminating the need for non-gray models to explain the shape. The companion orbits the primary on a wide trajectory, with common confirmed at greater than 5σ significance, ensuring no dynamical influence on the short-period orbit of WASP-76b at 0.033 . This configuration allows the planet's close-in dynamics to remain isolated from binary perturbations.

Orbital parameters

Orbital elements

WASP-76b orbits its host star at a semi-major axis of 0.03277 ± 0.00078 , placing it in an extremely close-in configuration typical of hot Jupiters. This distance results in intense stellar , contributing to the planet's ultra-hot nature. The is precisely measured as 1.80988132 ± 0.00012 days, equivalent to approximately 43.4 hours, enabling frequent transits observable from . The exhibits very low , with a value of 0.00087 ± 0.00031, indicating a nearly circular path that minimizes variations in and distance from the star. The relative to the sky plane is 87.88° ± 0.16°, a near-edge-on essential for detecting the via the method. These parameters are governed by Kepler's third law, which relates the P to the semi-major axis a and the masses of the star M_\star and M_p through the proportionality P^2 \propto a^3 / (M_\star + M_p). Updated values for WASP-76b's , refined through combined photometric and analyses, are reported in Wang et al. (2025).

Transit characteristics

The transits of WASP-76b were initially detected through photometric monitoring by the (SuperWASP) survey in 2013, leading to its confirmation and publication in 2016. Follow-up high-precision photometry has since been obtained using space-based observatories, including the (HST) with the Space Telescope Imaging Spectrograph (STIS) and (WFC3), the with the Infrared Array Camera (IRAC), and the (JWST) for infrared light curves. These observations have enabled detailed characterization of the transit events, revealing a depth of approximately 1.18%, which corresponds to a planet-to-star radius ratio of R_p / R_\star \approx 0.109 and underscores the planet's inflated size relative to the host star. The full transit duration (T_{14}) is about 3.76 hours, occurring roughly every 1.81 days due to the planet's short . Ingress and egress phases each last on the order of 15-20 minutes, as inferred from the high-cadence light curves that resolve these contact points and facilitate precise modeling of the shape. Transit timing variations (TTVs) have been analyzed from multiple epochs spanning over a decade, revealing significant deviations consistent with or possible perturbing planets in the system (Wang et al. 2025). The host system's visual nature, with the fainter companion star WASP-76B separated by about 0.43 arcseconds, introduces third-light dilution that contaminates early measurements by contributing up to 10% of the total in optical bands. Post-2020 analyses, particularly from , have applied wavelength-dependent correction factors—such as multiplying observed depths by dilution ratios around 1.008 in the near-infrared—to recover accurate planet-to-star radius ratios, ensuring reliable photometric .

Physical properties

Mass and radius

WASP-76b has a mass of 0.92 ± 0.03 masses, determined through measurements that detect the star's orbital wobble induced by the planet. These measurements were obtained using high-precision spectrographs such as and CORALIE, yielding a semi-amplitude K = 0.098 \pm 0.002 km/s, which, combined with the and , allows derivation of the via analysis. Uncertainties in the mass primarily stem from the estimate of 1.46 ± 0.07 solar masses, as the scales with the two-thirds power of the host star's mass. The planet's radius is measured at 1.83 ± 0.06 radii (as of 2016 discovery), inferred from the depth observed in photometric data, which relates the planetary radius to the stellar radius via the formula for transit depth (R_p / R_*)^2. Transit observations were conducted with instruments including WASP, EulerCam, and , providing the necessary to model the planetary size after accounting for the host star's radius of 1.77 ± 0.16 radii. Like the mass, radius uncertainties are influenced by stellar errors, particularly the stellar radius, which directly affects the scaling. Subsequent analyses, such as Fu et al. (2021), refine the radius to 1.845 ± 0.050 radii using additional photometric data. From these dimensions, WASP-76b's mean is calculated as approximately 0.18 ± 0.02 g/cm³, significantly lower than 's 1.33 g/cm³, reflecting an inflated gaseous due to intense stellar . This low , combined with the planet's larger size compared to , highlights its classification as a bloated , where heating expands the atmosphere without substantially altering the core mass.

Temperature and density

WASP-76b exhibits an of approximately 2230 (as of 2025 analysis), computed via the formula T_{\rm eq} = T_{\star} \sqrt{\frac{R_{\star}}{2a}} (1 - A)^{1/4}, where T_{\star} is the stellar effective , R_{\star} the stellar radius, a the semi-major axis, and A = 0 (zero ) under the assumption of full longitudinal heat redistribution; earlier estimates from 2016 were ~2190 . Phase-curve observations reveal a hotter dayside, with brightness temperatures reaching ~2470 at 3.6 μm and ~2700 at 4.5 μm, indicating inefficient to the nightside and strong thermal dissociation of molecular . The planet's low mean of ~180 kg/m³, derived from its of 0.92 MJup and of 1.83 RJup, implies substantial atmospheric and suggests deep convective heat circulation that deposits energy into the interior. This low profile is consistent with a modest core of ~10–20 masses in interior structure models, where the dominates the total and . The primary inflation mechanism is irradiation-driven, with intense stellar flux heating the dayside and driving energy transport through vertical of potential , as simulated in radiative general circulation models (GCMs).

Atmosphere

Chemical composition

The atmosphere of WASP-76b exhibits a complex chemical composition dominated by molecular and atomic species, primarily detected through transmission spectroscopy that reveals absorption features during planetary transits. (H₂O) and (TiO) were first identified in the near-infrared transmission spectrum, indicating their presence in the upper atmosphere and contributing to thermal inversion layers. These detections stem from observations with the (HST) and , spanning wavelengths from 0.8 to 5 μm, where TiO absorption is prominent between 0.4 and 1 μm and H₂O features appear around 1.4 μm. High-resolution ground-based spectroscopy has revealed a rich inventory of ionized and neutral metals, reflecting the planet's extreme temperatures that promote atomic dissociation. Using the ESPRESSO instrument on the Very Large Telescope (VLT), neutral iron (Fe I), sodium (Na I), calcium (Ca II), magnesium (Mg I), potassium (K I), manganese (Mn I), and lithium (Li I) were detected through strong absorption lines at optical wavelengths (e.g., Fe I at 4377 Å with depths up to 255 ppm). Complementary observations with Gemini North's MAROON-X spectrograph identified vanadium oxide (VO), along with chromium (Cr), vanadium (V), nickel (Ni), and other rock-forming elements, marking the first unambiguous detection of VO in an exoplanet atmosphere via high-resolution spectroscopy. These spectral features, observed across multiple transits from 2020 to 2024, highlight the planet's dayside chemistry, where high temperatures exceeding 2400 K enable metal vaporization. Abundance estimates indicate enhanced relative to values, with several elements showing supersolar enrichment that suggests efficient incorporation from the host star or accretion. For instance, iron abundance aligns with the host star's [Fe/H] = 0.23 ± 0.10, but retrievals imply overall atmospheric consistent with or slightly above this level for species. (Ba⁺), the heaviest element detected to date in an atmosphere, was observed in 2022 via VLT/ at high altitudes, with a detection of ~5σ, implying [Ba/H] potentially exceeding by factors linked to dynamical mixing. The atmosphere is shrouded in , hazy layers formed from condensing metal vapors, which scatter light and mute spectral features at certain wavelengths. Atmospheric retrieval analyses from 2024, based on data, reveal uniform longitudinal distributions of iron and magnesium across the planetary limbs, consistent with well-mixed condensates rather than localized depletions.

Dynamical and thermal phenomena

WASP-76b's atmosphere exhibits extreme dynamical phenomena driven by its tidally locked configuration and intense stellar , resulting in rapid day-to-nightside circulation. High-altitude winds transport material supersonically from the scorching dayside to the cooler nightside, with speeds reaching up to 5.5 km/s in the lower atmosphere and vertical winds approaching 22.7 km/s in the upper layers, facilitating efficient heat and chemical redistribution. These patterns lack strong evidence for latitude-dependent jets in early observations but indicate uniform day-to-night flow, potentially leading to high shear that could induce sideways molten flows of condensed materials. A prominent feature is the hypothesized "iron rain," where neutral iron vaporizes on the dayside at temperatures exceeding 2,000 and condenses into droplets on the nightside around 1,800 , transported by exceeding 10 km/s (over 36,000 km/h). This asymmetric iron absorption signature, blueshifted by -11 km/s, supports the scenario of evening vapor transport and morning condensation, though the exact mechanism remains debated. Alternative models suggest optically thick clouds of aluminum oxide, iron, or magnesium silicate could mimic the signal without requiring large-scale rain, highlighting unresolved tensions between observations and general circulation models. Extreme dayside heat promotes thermal dissociation, ionizing metals such as calcium while keeping iron primarily neutral, contributing to a complex upper atmosphere with strong vertical mixing. Detected ionized calcium signals indicate vigorous dynamics, potentially amplifying wind-driven transport of dissociated species. Optical effects include a potential "glory," a rainbow-like backscattering phenomenon observed as a flux increase in visible phase curves from the eastern hemisphere, requiring uniform spherical droplets or aerosols approximately 1 μm in size. This asymmetry aligns with wind patterns favoring cloud formation on one limb, though it challenges uniform atmospheric models. Recent 2025 phase-curve analyses using general circulation models reveal enhanced structures, including super-rotating equatorial jets up to ±10 km/s in drag-free scenarios, which sharpen dayside emission peaks and modulate chemical signals like iron across the orbit. These jets, suppressed by atmospheric drag, provide new constraints on circulation regimes, emphasizing the role of low-friction dynamics in ultra-hot weather.

Hypothetical features

Possible

In 2019, researchers proposed the hypothesis of a hot, evaporating orbiting WASP-76b to explain observed features in the planet's transmission spectrum. This model draws an analogy to 's moon , where intense stellar irradiation could drive volcanic outgassing and from the exomoon, forming a (doughnut-shaped) envelope around the planet. The exomoon's material would be stripped by the planet's gravity and stellar winds, creating an extended, low-density detectable via absorption lines. The primary evidence stems from high-resolution revealing a broadened neutral sodium (Na I) absorption feature in WASP-76b's atmosphere, with a D2/ line ratio near 2 indicating optically thin gas consistent with a tenuous structure rather than a hydrostatic planetary atmosphere. Simulations suggest this sodium could originate from grain desorption or direct in the exomoon's envelope, with mass loss rates on the order of 10³–10⁵ kg/s for sodium alone. No direct photometric signatures, such as secondary eclipses or dips from the moon itself, have been identified, limiting detection to indirect spectral imprints. Challenges to this hypothesis include the low of the sodium lines and alternative interpretations attributing the broadened absorption to strong day-to-night atmospheric winds transporting metals across the planet's . The intense on WASP-76b, with equilibrium temperatures exceeding 2000 K, would rapidly erode any , potentially destroying it within short timescales unless replenished by internal heating. As of 2025, the presence of an remains unconfirmed, with no dedicated follow-up observations resolving the ambiguity. Future high-precision using the (JWST) could probe the exosphere's dynamics and chemistry to test this model.

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