WASP-76b: Ultra-Hot Jupiter

• WASP-76b is an inflated ultra-hot Jupiter with a 1.81-day orbit, low density, and high equilibrium temperature, making it an ideal target for atmospheric studies.
• High-resolution spectroscopy has revealed diverse species such as Fe I, Na I, and Ca II, uncovering pronounced terminator asymmetry, strong winds, and evidence of potential 'iron rain'.
• Multi-wavelength analyses from transmission and emission spectra have provided insights into thermal inversions, vertical circulation, and the mechanisms behind its radius inflation.

WASP-76b is an inflated ultra-hot Jupiter orbiting a bright F7 star on a $1.809886 \pm 0.000001$ d orbit at $0.0330 \pm 0.0005$ AU. In the discovery solution it had $M_p = 0.92 \pm 0.03\,M_{\mathrm{Jup}}$, $R_p = 1.83^{+0.06}_{-0.04}\,R_{\mathrm{Jup}}$, $\rho_p = 0.151 \pm 0.010\,\rho_J$, and $T_{P,A=0} = 2160 \pm 40$ K, placing it among the most irradiated and most inflated hot Jupiters then known. It subsequently became a benchmark ultra-hot Jupiter for studies of transmission spectroscopy, phase curves, thermal inversions, atmospheric dissociation chemistry, and 3D circulation because the system combines a bright host star, a large atmospheric scale height, and large orbital Doppler shifts (West et al., 2013).

1. Discovery and fundamental properties

WASP-76b was discovered by the WASP transit surveys and confirmed with OHP/SOPHIE and Euler/CORALIE radial velocities together with EulerCAM and TRAPPIST follow-up photometry. The discovery analysis used a simultaneous MCMC fit with a circular orbit and found $P = 1.809886 \pm 0.000001$ d, $T_{14} = 0.1539 \pm 0.0008$ d, $\Delta F = 0.01189 \pm 0.00016$, $b = 0.14^{+0.11}_{-0.09}$, $0.0330 \pm 0.0005$0 deg, and $0.0330 \pm 0.0005$1 km s$0.0330 \pm 0.0005$2. The host star was characterized as an F7 star with $0.0330 \pm 0.0005$3 K, $0.0330 \pm 0.0005$4, $0.0330 \pm 0.0005$5, $0.0330 \pm 0.0005$6, and $0.0330 \pm 0.0005$7 (West et al., 2013).

Quantity	Value	Source context
Orbital period	$0.0330 \pm 0.0005$8 d	Discovery fit
Semi-major axis	$0.0330 \pm 0.0005$9 AU	Discovery fit
Planet mass	$M_p = 0.92 \pm 0.03\,M_{\mathrm{Jup}}$0	Discovery fit
Planet radius	$M_p = 0.92 \pm 0.03\,M_{\mathrm{Jup}}$1	Discovery fit
Mean density	$M_p = 0.92 \pm 0.03\,M_{\mathrm{Jup}}$2	Discovery fit
Equilibrium temperature	$M_p = 0.92 \pm 0.03\,M_{\mathrm{Jup}}$3 K	Discovery fit
Host spectral type	F7	Discovery spectroscopy

Later high-resolution studies commonly adopted closely related literature values such as $M_p = 0.92 \pm 0.03\,M_{\mathrm{Jup}}$4, $M_p = 0.92 \pm 0.03\,M_{\mathrm{Jup}}$5, and a planetary radial-velocity semi-amplitude $M_p = 0.92 \pm 0.03\,M_{\mathrm{Jup}}$6 km s$M_p = 0.92 \pm 0.03\,M_{\mathrm{Jup}}$7. The orbital geometry was also described as misaligned, with $M_p = 0.92 \pm 0.03\,M_{\mathrm{Jup}}$8 $M_p = 0.92 \pm 0.03\,M_{\mathrm{Jup}}$9, which is relevant for interpreting resolved in-transit velocity fields (Deibert et al., 2021).

The combination of low gravity, a highly inflated radius, and intense irradiation made WASP-76b a natural target for atmospheric work from the outset. A plausible implication is that the planet sits at a regime boundary where bulk-structure inflation, metal-rich optical opacity, thermal dissociation, and strong day-to-night transport all become simultaneously observable.

2. Space-based spectra and the low-resolution atmospheric picture

Space-based spectroscopy established the broad thermochemical framework against which later high-resolution work was interpreted. The PanCET program combined seven transits and five eclipses from HST and Spitzer over $R_p = 1.83^{+0.06}_{-0.04}\,R_{\mathrm{Jup}}$0–$R_p = 1.83^{+0.06}_{-0.04}\,R_{\mathrm{Jup}}$1m and reported TiO and H$R_p = 1.83^{+0.06}_{-0.04}\,R_{\mathrm{Jup}}$2O absorption in transmission, additional short-wavelength absorption by heavy metals such as Fe, Ni, Ti, and SiO, and a secondary-eclipse spectrum with muted water but strong CO emission in the $R_p = 1.83^{+0.06}_{-0.04}\,R_{\mathrm{Jup}}$3m band, indicating an inverted temperature-pressure profile (Fu et al., 2020).

Interpretation of the HST/WFC3 transmission spectrum, however, depends sensitively on treatment of the unresolved companion. A reanalysis that explicitly corrected for the fainter stellar companion found that removing the background-star contribution changes the slope of the transmission spectrum, eliminating the need for TiO, VO, or a non-grey cloud model in transmission while preserving a strong water feature. The same study still found the emission spectrum consistent with TiO and a thermal inversion, with retrieved dayside quantities including $R_p = 1.83^{+0.06}_{-0.04}\,R_{\mathrm{Jup}}$4, $R_p = 1.83^{+0.06}_{-0.04}\,R_{\mathrm{Jup}}$5, and an inverted profile reaching $R_p = 1.83^{+0.06}_{-0.04}\,R_{\mathrm{Jup}}$6 K (Edwards et al., 2020).

Full-orbit Spitzer phase curves supplied the first global circulation constraints. At $R_p = 1.83^{+0.06}_{-0.04}\,R_{\mathrm{Jup}}$7m, the dayside and nightside brightness temperatures were $R_p = 1.83^{+0.06}_{-0.04}\,R_{\mathrm{Jup}}$8 K and $R_p = 1.83^{+0.06}_{-0.04}\,R_{\mathrm{Jup}}$9 K; at $\rho_p = 0.151 \pm 0.010\,\rho_J$0m they were $\rho_p = 0.151 \pm 0.010\,\rho_J$1 K and $\rho_p = 0.151 \pm 0.010\,\rho_J$2 K. Both channels showed very small eastward phase offsets, $\rho_p = 0.151 \pm 0.010\,\rho_J$3 and $\rho_p = 0.151 \pm 0.010\,\rho_J$4, respectively. Comparison with cloud-free GCMs implied strong frictional drag, $\rho_p = 0.151 \pm 0.010\,\rho_J$5 s, and favored a cold interior adiabat for reproducing the large $\rho_p = 0.151 \pm 0.010\,\rho_J$6m amplitude and cold nightside (May et al., 2021).

Taken together, the low-resolution space-based results established three recurrent themes. First, WASP-76b has a strongly irradiated, spectroscopically active atmosphere with prominent optical/near-infrared features. Second, the dayside is inverted and hotter than the global equilibrium temperature. Third, apparently modest phase offsets coexist with later high-resolution evidence for strong winds, suggesting a vertically structured circulation rather than a single drag regime.

3. High-resolution transmission spectroscopy and the species inventory

Optical and near-infrared high-resolution transmission spectroscopy transformed WASP-76b from a chemically interesting target into a kinematically resolved atmosphere. ESPRESSO observations over $\rho_p = 0.151 \pm 0.010\,\rho_J$7–$\rho_p = 0.151 \pm 0.010\,\rho_J$8 Å detected Li I, Na I, Mg I, Ca II, Mn I, K I, and Fe I. In that analysis Na I reached $\rho_p = 0.151 \pm 0.010\,\rho_J$9, Fe I cross-correlation detections reached $T_{P,A=0} = 2160 \pm 40$0 and $T_{P,A=0} = 2160 \pm 40$1 in two transits, and the procedure was reported to detect individual transmission features down to $T_{P,A=0} = 2160 \pm 40$2 and cross-correlation signals down to $T_{P,A=0} = 2160 \pm 40$3 ppm (Tabernero et al., 2020).

Ionized calcium became a particularly important tracer of the extended upper atmosphere. CARMENES observations of two transits yielded a $T_{P,A=0} = 2160 \pm 40$4 detection of the Ca II infrared triplet and only an upper limit on He I $T_{P,A=0} = 2160 \pm 40$5 Å because of strong telluric contamination. In the same work, the strongest IRT component at $T_{P,A=0} = 2160 \pm 40$6 Å had a contrast of $T_{P,A=0} = 2160 \pm 40$7, a FWHM of $T_{P,A=0} = 2160 \pm 40$8 Å, and $T_{P,A=0} = 2160 \pm 40$9 km s$P = 1.809886 \pm 0.000001$0 (Casasayas-Barris et al., 2021). GRACES later resolved the triplet with line depths $P = 1.809886 \pm 0.000001$1, $P = 1.809886 \pm 0.000001$2, and $P = 1.809886 \pm 0.000001$3 at $P = 1.809886 \pm 0.000001$4, $P = 1.809886 \pm 0.000001$5, and $P = 1.809886 \pm 0.000001$6 nm, corresponding to effective radii $P = 1.809886 \pm 0.000001$7, $P = 1.809886 \pm 0.000001$8, and $P = 1.809886 \pm 0.000001$9, close to but within a transit-equivalent Roche radius $T_{14} = 0.1539 \pm 0.0008$0 (Deibert et al., 2021).

Near-infrared molecular work showed that refractory species do not exhaust the observable chemistry of the planet. A CARMENES NIR transmission analysis detected OH with peak S/N $T_{14} = 0.1539 \pm 0.0008$1, retrieving $T_{14} = 0.1539 \pm 0.0008$2 km s$T_{14} = 0.1539 \pm 0.0008$3, $T_{14} = 0.1539 \pm 0.0008$4 km s$T_{14} = 0.1539 \pm 0.0008$5, $T_{14} = 0.1539 \pm 0.0008$6 km s$T_{14} = 0.1539 \pm 0.0008$7, and a weak temperature constraint of $T_{14} = 0.1539 \pm 0.0008$8–$T_{14} = 0.1539 \pm 0.0008$9 K at the pressures probed by the OH signal (Landman et al., 2021).

The species inventory continued to expand as masks, wavelength coverage, and detrending strategies improved. A reanalysis of ESPRESSO data reported a new detection of Ba$\Delta F = 0.01189 \pm 0.00016$0, described as the heaviest species detected to date in any exoplanet atmosphere, with amplitudes $\Delta F = 0.01189 \pm 0.00016$1 ppm and $\Delta F = 0.01189 \pm 0.00016$2 ppm on two nights, and confirmed Ca$\Delta F = 0.01189 \pm 0.00016$3, Cr, Fe, H, Li, Mg, Mn, Na, and V in WASP-76b (Silva et al., 2022). GRACES/ExoGemS later recovered Fe I, Na I, and Ca II at $\Delta F = 0.01189 \pm 0.00016$4, with tentative signals from Li I, K I, Cr I, and V I, while injection/recovery tests showed that several non-detections were expected from the instrument’s wavelength-dependent sensitivity (Deibert et al., 2023).

A recurring consequence of this work is that WASP-76b is not merely “metal-rich” in a generic sense. It is an atmosphere in which neutral atoms, ions, and dissociation products are all spectroscopically accessible, often with distinct velocity centroids and line widths.

4. Terminator asymmetry, winds, and the “iron rain” debate

WASP-76b is best known for a resolved asymmetry in Fe I absorption across transit. HARPS observations of four transits confirmed that the iron signal is near the planetary rest velocity at ingress and strongly blueshifted toward egress. In the HARPS combination, the beginning-of-transit Gaussian centroid was $\Delta F = 0.01189 \pm 0.00016$5 km s$\Delta F = 0.01189 \pm 0.00016$6, while the end-of-transit centroid was $\Delta F = 0.01189 \pm 0.00016$7 km s$\Delta F = 0.01189 \pm 0.00016$8; the evening-side absorption was also stronger by about $\Delta F = 0.01189 \pm 0.00016$9 ppm, and the integrated Fe detection reached S/N $b = 0.14^{+0.11}_{-0.09}$0 (Kesseli et al., 2021).

The initial physical picture linked this asymmetry to condensation of iron on the nightside, so that the cooler morning limb becomes depleted in gaseous Fe whereas the hotter evening limb retains it. The OH detection fits naturally into that broader dynamical framework: its retrieved blueshift of $b = 0.14^{+0.11}_{-0.09}$1 km s$b = 0.14^{+0.11}_{-0.09}$2 was interpreted as predominantly originating from the evening terminator, where tidal rotation and day-to-night winds contribute constructively, and where thermal dissociation of water enhances OH abundance (Landman et al., 2021).

Subsequent 3D modeling showed that the observations do not isolate a single mechanism. Monte Carlo radiative-transfer calculations coupled to SPARC/MITgcm demonstrated that the Fe I cross-correlation trail is primarily driven by temperature structure, rotation, and dynamics, and is less sensitive to the precise iron distribution. In that framework, the previously observed signal can be reproduced either by iron condensation on the leading limb or by a substantial temperature asymmetry between the trailing and leading limbs, so that iron condensation is not strictly required to match the data (Wardenier et al., 2021). A different post-processed GCM study went further, arguing that gas-phase iron condensation alone cannot reproduce either the magnitude or the time dependence of the phase-resolved Doppler signature; it identified low atmospheric drag, a deep radiative-convective boundary, and high-altitude optically thick clouds as more successful ingredients, with the fit further improved by allowing a small eccentricity $b = 0.14^{+0.11}_{-0.09}$3 (Savel et al., 2021).

Multi-transit limb-resolved ESPRESSO retrievals added an important constraint: Fe and Mg were found to be longitudinally uniform across the limbs in all three analyzed transits, whereas V and VO were unconstrained on the leading limb in two of the transits, suggesting possible depletion there due to recombination and condensation. The same retrievals found consistent net blueshifts, $b = 0.14^{+0.11}_{-0.09}$4 km s$b = 0.14^{+0.11}_{-0.09}$5, across the transits (Maguire et al., 2024).

The common shorthand “iron rain” therefore captures only part of the current picture. The observations require a strongly asymmetric terminator, but current forward models and retrievals attribute that asymmetry to a combination of thermal structure, condensation chemistry, clouds, and dynamics rather than to a uniquely diagnosed rainout process.

5. Dayside emission, thermal inversion, and circulation structure

Dayside emission studies established that the atmosphere above the substellar hemisphere is inverted and dynamically active. CRIRES+ observations in the $b = 0.14^{+0.11}_{-0.09}$6–$b = 0.14^{+0.11}_{-0.09}$7 nm range detected CO emission at S/N $b = 0.14^{+0.11}_{-0.09}$8, with a $b = 0.14^{+0.11}_{-0.09}$9 peak at $0.0330 \pm 0.0005$00 km s$0.0330 \pm 0.0005$01 and a dayside temperature-pressure retrieval showing $0.0330 \pm 0.0005$02 K at $0.0330 \pm 0.0005$03 and $0.0330 \pm 0.0005$04 K at $0.0330 \pm 0.0005$05. The retrieved $0.0330 \pm 0.0005$06 was broadly consistent with solar-abundance expectations, and the equatorial rotation speed $0.0330 \pm 0.0005$07 km s$0.0330 \pm 0.0005$08 matched the tidally locked value (Yan et al., 2023).

Optical dayside spectroscopy then extended the inversion signature to metal lines. ESPRESSO observations before and after secondary eclipse detected Fe I emission at $0.0330 \pm 0.0005$09, with a co-added blueshift $0.0330 \pm 0.0005$10 km s$0.0330 \pm 0.0005$11 and FWHM $0.0330 \pm 0.0005$12 km s$0.0330 \pm 0.0005$13. Pre-eclipse and post-eclipse combinations were both blueshifted, at $0.0330 \pm 0.0005$14 and $0.0330 \pm 0.0005$15 km s$0.0330 \pm 0.0005$16, respectively. In that interpretation, the Fe I emission confirms a thermal inversion, while the persistent blueshift suggests bulk motion of the emitting gas toward the observer, plausibly linked to material rising from the hot spot into the inversion layer (Silva et al., 2024).

These dayside results connect naturally to the phase-curve constraints. The Spitzer curves require strong drag and produce near-zero hotspot offsets, yet optical and near-infrared high-resolution data reveal line-of-sight wind signatures of several km s$0.0330 \pm 0.0005$17. One explanation proposed in the phase-curve study is that ion drag and electrodynamics act differently at the high altitudes sampled by optical high-resolution spectroscopy than at the deeper, largely neutral levels probed by Spitzer, so a simple Rayleigh-drag prescription may over-damp upper-atmosphere dynamics (May et al., 2021).

A plausible synthesis is that WASP-76b hosts a vertically differentiated circulation. Deeper photospheric layers can exhibit small phase offsets and strong drag, while the upper atmosphere simultaneously maintains large resolved Doppler shifts, asymmetric terminator chemistry, and blueshifted dayside emission.

6. Radius inflation, elemental ratios, and benchmark status

WASP-76b is also a test case for the long-standing radius-inflation problem. A non-grey 3D GCM reanalysis focused on vertical enthalpy transport concluded that, after long integrations to near steady state, all the WASP-76b models considered exhibit significant downward enthalpy transport into the deep atmosphere, warming the internal adiabat relative to 1D radiative-convective models. Matching the deep atmosphere to an interior structure model yielded $0.0330 \pm 0.0005$18, compared with the observational benchmark $0.0330 \pm 0.0005$19, leading the authors to conclude that vertical advection of potential temperature alone plausibly explains the inflated radius (Sainsbury-Martinez et al., 2023).

High-resolution abundance work has begun to extend the planet’s importance from atmospheric dynamics into formation diagnostics. Gemini-S/IGRINS observations between $0.0330 \pm 0.0005$20 and $0.0330 \pm 0.0005$21m detected H$0.0330 \pm 0.0005$22O, CO, and OH at signal-to-noise ratios of $0.0330 \pm 0.0005$23, $0.0330 \pm 0.0005$24, and $0.0330 \pm 0.0005$25, respectively. A free retrieval returned a volatile metallicity $0.0330 \pm 0.0005$26 and $0.0330 \pm 0.0005$27, while a chemically self-consistent retrieval gave $0.0330 \pm 0.0005$28 and $0.0330 \pm 0.0005$29. The two retrievals agreed within $0.0330 \pm 0.0005$30, and the study noted that the inferred systemic and Keplerian velocity offsets were broadly consistent with expectations from 3D GCMs, apart from a redshifted $0.0330 \pm 0.0005$31 for H$0.0330 \pm 0.0005$32O that remains to be explained (Mansfield et al., 2024).

WASP-76b’s scientific importance therefore spans three linked domains. In atmospheric spectroscopy, it provides one of the clearest cases of species-resolved limb asymmetry, metal ionization, and molecular dissociation products such as OH. In circulation studies, it ties together phase curves, transmission asymmetries, and dayside emission in a single UHJ. In bulk-structure theory, it is one of the few planets for which long-timescale 3D enthalpy advection has been quantitatively connected to the observed radius.

The cumulative result is a planet that remains central to current work on ultra-hot Jupiters precisely because it resists reduction to a single simplified narrative. Its atmosphere is simultaneously inflated, inverted, metal-rich in the observable upper layers, dynamically asymmetric across the terminator, and chemically stratified with altitude and longitude.