KELT-7b: Benchmark Transiting Hot Jupiter

• KELT-7b is a short-period transiting hot Jupiter known for its inflated radius, high irradiation, and orbit around a bright, rapidly rotating F-type star.
• Research on KELT-7b employs an array of methods including Doppler tomography, Rossiter–McLaughlin analysis, and multi-wavelength transmission spectroscopy to probe its atmospheric and orbital properties.
• Analyses reveal conflicting temperature profiles and chemical abundance interpretations, highlighting challenges such as stellar contamination and model dependency in hot Jupiter studies.

KELT-7b is a short-period transiting gas giant orbiting the bright F-type star KELT-7 (HD 33643). It was discovered by the Kilodegree Extremely Little Telescope survey as a hot Jupiter around a rapidly rotating, above-Kraft-break host star with $V = 8.54$, and it has subsequently become a benchmark system for Doppler tomography, Rossiter–McLaughlin analysis, high-resolution transmission spectroscopy, HST spectroscopy, JWST transmission retrievals, and optical eclipse studies. Across the literature, KELT-7b is consistently described as a highly irradiated, inflated giant planet with an orbital period near $2.73477$ d, a mass near $1.28\,M_{\rm Jup}$, a radius near $1.5\,R_{\rm Jup}$, and an equilibrium temperature near $2028$–$2048$ K, while the host star is characterized by rapid rotation and a near-aligned planetary orbit (Bieryla et al., 2015, Tabernero et al., 2022).

1. Discovery, survey context, and basic system properties

KELT-7b was reported as a transiting hot Jupiter by the KELT survey, which was designed to be optimally sensitive to planets transiting bright stars in the approximate range $V \sim 8$–$10$. Within that programmatic context, KELT-7b is close to the center of KELT’s core science goals: it transits a very bright, hot, and rapidly rotating F-type star, and the system exemplifies KELT’s emphasis on bright hosts that were historically difficult for classical precision-RV confirmation (Bieryla et al., 2015, Pepper et al., 2018).

In the discovery analysis, the planet was measured to have $M_p = 1.28 \pm 0.18\,M_{\rm J}$, $R_p = 1.533^{+0.046}_{-0.047}\,R_{\rm J}$, and $2.73477$0 d, with $2.73477$1 K under the standard assumption of zero Bond albedo and full heat redistribution. The host star was measured as an F star with $2.73477$2 K, $2.73477$3, $2.73477$4, $2.73477$5, and $2.73477$6 (Bieryla et al., 2015).

Later joint modeling of TESS, KELT photometry, and TRES radial velocities refined several parameters, giving $2.73477$7 d, $2.73477$8 BJD, $2.73477$9 au, $1.28\,M_{\rm Jup}$0, $1.28\,M_{\rm Jup}$1 deg, $1.28\,M_{\rm Jup}$2, $1.28\,M_{\rm Jup}$3, and $1.28\,M_{\rm Jup}$4 K, with no transit timing variations detected and an ephemeris precision improved by approximately $1.28\,M_{\rm Jup}$5 relative to the discovery paper (Tabernero et al., 2022).

The basic transit and irradiation relations used throughout the KELT-7b literature are the standard ones,

$1.28\,M_{\rm Jup}$6

$1.28\,M_{\rm Jup}$7

with $1.28\,M_{\rm Jup}$8 commonly assumed, and

$1.28\,M_{\rm Jup}$9

Using the discovery values, the planet’s surface gravity is reported as $1.5\,R_{\rm Jup}$0, corresponding to $1.5\,R_{\rm Jup}$1 (cgs) (Bieryla et al., 2015).

At discovery, KELT-7 was described as the fifth most massive, fifth hottest, and the ninth brightest star known to host a transiting planet, and KELT-7b was the brightest star around which KELT had discovered a transiting planet. That combination of brightness, transit depth, and low planetary density established the system early as a high-priority target for atmospheric follow-up (Bieryla et al., 2015).

2. Stellar rotation, orbital architecture, and spin–orbit geometry

A defining feature of KELT-7b is that it orbits a rapidly rotating F star. The discovery paper reported $1.5\,R_{\rm Jup}$2 from SPC and noted that the rapid rotation produces a Rossiter–McLaughlin signal with an unusually large amplitude of several hundred $1.5\,R_{\rm Jup}$3; the approximation

$1.5\,R_{\rm Jup}$4

gives $1.5\,R_{\rm Jup}$5, while the detailed global fit yields $1.5\,R_{\rm Jup}$6 (Bieryla et al., 2015).

Projected obliquity estimates have been refined several times. The discovery RM analysis found $1.5\,R_{\rm Jup}$7 deg, consistent with alignment (Bieryla et al., 2015). A later Doppler-tomographic reanalysis of archival TRES spectra reported $1.5\,R_{\rm Jup}$8, with $1.5\,R_{\rm Jup}$9, $2028$0, $2028$1, and $2028$2, again indicating a prograde, well-aligned configuration (Zhou et al., 2016). A subsequent HORuS Doppler-shadow analysis found $2028$3 deg and $2028$4, with the authors emphasizing Doppler tomography rather than classical RM as the more reliable route for such a rapid rotator (Tabernero et al., 2022).

The three-dimensional obliquity was estimated in the HORuS reanalysis by combining the projected angle with a TESS-based stellar rotation period. The relations used were

$2028$5

and

$2028$6

With $2028$7 d, the result was $2028$8 deg, or $2028$9 deg because of the inclination degeneracy, reinforcing the conclusion that KELT-7 is a well-aligned planetary system (Tabernero et al., 2022).

The stellar characterization has also evolved. Spectral synthesis of the co-added HORuS spectrum gave $2048$0 K, $2048$1, $2048$2, $2048$3, $2048$4, and $2048$5. Although KELT-7 is cataloged as F2 V, the large radius and surface gravity were interpreted as indicating that the star is slightly evolved and entering the subgiant phase (Tabernero et al., 2022).

The rapid stellar rotation is also central to dynamical interpretation. The tomographic study reported an upper-limit rotation period of $2048$6 d for $2048$7 and argued that KELT-7 is in a super-synchronous, spin-orbit aligned state. Using equilibrium-tide arguments, it estimated a spin-evolution timescale of $2048$8 yr, concluding that there is no evidence the stellar spin has been modified by star–planet tidal interactions and that the observed alignment may represent a primordial configuration (Zhou et al., 2016).

3. Observational coverage and analysis frameworks

KELT-7b has been studied with a notably broad instrumental set. In the optical at high resolution, HARPS-N observations covered two transits on 2017-12-06 and 2017-12-17, with $2048$9 s and $V \sim 8$0 s exposures, respectively; HARPS-N provided $V \sim 8$1 over approximately $V \sim 8$2–$V \sim 8$3 nm, and the mean per-exposure S/N around $V \sim 8$4 nm was $V \sim 8$5 on the first night and $V \sim 8$6 on the second (Stangret et al., 2021). A separate high-resolution optical campaign with HORuS on the 10.4-m GTC observed two transits on 2019 November 17 and 2020 January 30, obtaining $V \sim 8$7 and $V \sim 8$8 consecutive spectra with individual exposure times of $V \sim 8$9 s, at $10$0 over $10$1–$10$2 Å and average S/N per pixel at $10$3 Å of approximately $10$4 (Tabernero et al., 2022).

In space, HST/WFC3 G141 observed two visits in 2017, each with five HST orbits, spanning $10$5–$10$6m in spatial-scan mode (Pluriel et al., 2020). HST/WFC3/UVIS G280 later provided one transit on 2022-12-29 over $10$7–$10$8m, with 127 exposures of 16 s over five HST orbits and two independent reductions, lluvia and Hazelnut (Gascón et al., 23 Jun 2025). JWST/NIRSpec G395H then observed one transit on 2024-02-03 in BOTS mode over $10$9–$M_p = 1.28 \pm 0.18\,M_{\rm J}$0–$M_p = 1.28 \pm 0.18\,M_{\rm J}$1m, with 4976 integrations during a 7.51 h sequence (Ahrer et al., 15 Sep 2025). On the dayside, CHEOPS and TESS photometry, together with HST/WFC3 and Spitzer/IRAC occultations, were used to constrain emission, albedo, and atmospheric structure (Garai et al., 25 Jun 2025).

The high-resolution optical studies used standard transmission and cross-correlation formalisms. The transmission spectrum was defined as

$M_p = 1.28 \pm 0.18\,M_{\rm J}$2

with $M_p = 1.28 \pm 0.18\,M_{\rm J}$3 constructed as a master out-of-transit spectrum (Stangret et al., 2021). In the HARPS-N cross-correlation analysis, the planet radial velocity during transit was written

$M_p = 1.28 \pm 0.18\,M_{\rm J}$4

and $M_p = 1.28 \pm 0.18\,M_{\rm J}$5 was scanned from $M_p = 1.28 \pm 0.18\,M_{\rm J}$6 to $M_p = 1.28 \pm 0.18\,M_{\rm J}$7; for KELT-7b, the predicted value was $M_p = 1.28 \pm 0.18\,M_{\rm J}$8 (Stangret et al., 2021).

The reduction and modeling frameworks span several methodological traditions. HARPS-N spectra were reduced with HARPS-N DRS v3.7; tellurics were removed with Molecfit; RM and CLV models were generated with SME, ATLAS9 atmospheres, and VALD3; and cross-correlation templates were generated with petitRADTRANS using an isothermal $M_p = 1.28 \pm 0.18\,M_{\rm J}$9 K profile and solar abundances (Stangret et al., 2021). The HORuS work used BT-Settl atmospheres, Turbospectrum, VALD3, ARoME, Doppler-shadow fitting, and disk-integrated Turbospectrum/PHOENIX RM+CLV models, while also tracking stellar activity indices in Ca II H&K, H$R_p = 1.533^{+0.046}_{-0.047}\,R_{\rm J}$0, Na I D, and Mg I b (Tabernero et al., 2022). HST/WFC3 G141 spectra were reduced with Iraclis and retrieved with TauREx3 (Pluriel et al., 2020). HST UVIS and combined UV–IR transmission retrievals used NEMESISPY and POSEIDON with stellar-contamination terms (Gascón et al., 23 Jun 2025). JWST/NIRSpec reductions were independently performed with Eureka!, ExoTiC-JEDI, and Tiberius, while atmospheric retrievals used petitRADTRANS, POSEIDON, and NEMESISPY (Ahrer et al., 15 Sep 2025). Emission analyses of the dayside employed Pyrat Bay, platon, gray-sky eclipse modeling, and a 3D expeRT/MITgcm simulation with magnetic drag (Garai et al., 25 Jun 2025).

This unusually heterogeneous observational and methodological base is central to the KELT-7b literature. It permits direct comparison between Doppler-tomographic, RM, high-resolution transmission, low-resolution space-based transmission, emission retrieval, and 3D circulation inferences within a single system.

4. Transmission spectroscopy: from optical non-detections to H$R_p = 1.533^{+0.046}_{-0.047}\,R_{\rm J}$1-dominated continua

The optical high-resolution record is dominated by non-detections and stellar contamination. In the HARPS-N study of six ultra-hot Jupiters, KELT-7b showed no significant absorption at H$R_p = 1.533^{+0.046}_{-0.047}\,R_{\rm J}$2, and no significant planetary features were found in H$R_p = 1.533^{+0.046}_{-0.047}\,R_{\rm J}$3, Na I D, or Ca II H&K. Cross-correlation searches also failed to detect any tested species, including Fe I, Fe II, Ti I, Ca I, Cr I, Mg I, Mg II, Si II, Na I, K I, Li I, H$R_p = 1.533^{+0.046}_{-0.047}\,R_{\rm J}$4O, TiO, and VO. After RM+CLV correction, no peak appeared at $R_p = 1.533^{+0.046}_{-0.047}\,R_{\rm J}$5 and $R_p = 1.533^{+0.046}_{-0.047}\,R_{\rm J}$6 at $R_p = 1.533^{+0.046}_{-0.047}\,R_{\rm J}$7 (Stangret et al., 2021).

The HORuS optical analysis reached the same qualitative conclusion, but with explicit upper limits after accounting for RM+CLV and stellar activity. For the co-added transmission spectrum from the two transits, the reported upper limits were $R_p = 1.533^{+0.046}_{-0.047}\,R_{\rm J}$8 for Na I D, $R_p = 1.533^{+0.046}_{-0.047}\,R_{\rm J}$9 for H$2.73477$00, $2.73477$01 for Mg I b, $2.73477$02 for Ca II H&K, and $2.73477$03 for Li I at 6103.65 Å. The authors also showed that modest, percent-level changes in stellar activity during transit sequences can produce spurious features at the planetary rest frame and reproduced apparent in-transit Na absorption/emission patterns using simulated variable stellar Na line profiles with no planetary signal included (Tabernero et al., 2022).

By contrast, the HST/WFC3 G141 transmission spectrum was reported as atmospheric rather than flat. In that work, the Atmospheric Detectability Index was $2.73477$04, corresponding to $2.73477$05 evidence for atmospheric features beyond a flat spectrum. The retrieval favored a cloud-free limb with a deep cloud parameter, an isothermal terminator temperature of $2.73477$06 K, mean molecular weight $2.73477$07, $2.73477$08, and $2.73477$09, with H$2.73477$10 continuum identified as an important contributor to the short-wavelength rise inside the WFC3 bandpass (Pluriel et al., 2020).

The HUSTLE program extended the transmission spectrum into the UV and optical and revised the physical interpretation. Combining HST/WFC3/UVIS G280, archival WFC3/IR G141, and Spitzer photometric points, it found a generally featureless transmission spectrum from $2.73477$11 to $2.73477$12m, with a slight downward slope toward shorter wavelengths and an asymmetric water feature in the $2.73477$13–$2.73477$14m region. Two independent retrieval frameworks converged on robust H$2.73477$15 detection, with $2.73477$16 in NEMESISPY and $2.73477$17 in POSEIDON, corresponding to Bayes factors of approximately $2.73477$18 ($2.73477$19) and $2.73477$20 ($2.73477$21), respectively. The same study also retrieved bright stellar inhomogeneities with covering fraction $2.73477$22 and temperature contrast $2.73477$23 K in NEMESISPY, arguing that the near-UV slope is best explained by stellar contamination rather than planetary haze (Gascón et al., 23 Jun 2025).

JWST/NIRSpec/G395H then extended the transmission spectrum to $2.73477$24m and reported a markedly muted near-infrared spectrum. The observed modulation amplitudes were $2.73477$25 ppm, with median spectroscopic transit-depth uncertainties of $2.73477$26–$2.73477$27 ppm and an NRS1/NRS2 offset of approximately $2.73477$28–$2.73477$29 ppm. Free-chemistry retrievals found only tentative evidence for H$2.73477$30O and CO$2.73477$31, with $2.73477$32–$2.73477$33 for H$2.73477$34O and $2.73477$35–$2.73477$36 for CO$2.73477$37, while CO showed no compelling evidence. Equilibrium-chemistry models were modestly favored over free chemistry, with $2.73477$38–$2.73477$39 (Ahrer et al., 15 Sep 2025).

Taken together, the transmission literature does not support a single simple description. At optical high spectral resolution, KELT-7b remains non-detected; in the HST near-IR and UV–optical low-resolution data, H$2.73477$40 is strongly implicated; and in JWST/NIRSpec/G395H the spectrum is sufficiently weak that both a high-altitude gray cloud deck and very low molecular abundances remain viable explanations.

5. Dayside emission, albedo, and the thermal-inversion problem

The first dayside spectroscopy with HST/WFC3 G141 showed that KELT-7b’s emission spectrum is not featureless in a trivial sense, but also is not well described by a simple blackbody. The spectroscopic eclipse depths span approximately $2.73477$41–$2.73477$42 across WFC3, and the Atmospheric Detectability Index against a blackbody was reported as $2.73477$43. A single-temperature blackbody fit yielded $2.73477$44 K but provided a poor match to the measured spectrum (Pluriel et al., 2020).

That same analysis retrieved a three-point dayside temperature–pressure structure interpreted as an inversion:

• $2.73477$45 K,
• $2.73477$46 K,
• $2.73477$47 K.

The emission spectrum was explained by a non-isothermal T–P profile together with collision-induced absorption and H$2.73477$48 continuum, while upper limits rather than detections were found for H$2.73477$49O, TiO, VO, FeH, CO, and $2.73477$50 in emission (Pluriel et al., 2020).

A later reanalysis substantially complicated that picture. Using CHEOPS, TESS, HST/WFC3, Spitzer/IRAC, and a coherent stellar-variability correction, the study detected optical secondary eclipses of $2.73477$51 ppm in CHEOPS and $2.73477$52 ppm in TESS. A gray-sky fit combining reflected light and thermal emission yielded a dayside temperature of $2.73477$53 K and a very low geometric albedo of $2.73477$54. Under the assumption of zero reflection, the band-specific thermal brightness temperatures were $2.73477$55 K and $2.73477$56 K (Garai et al., 25 Jun 2025).

The same work framed KELT-7b explicitly as a thermal-inversion conundrum. Thermochemical-equilibrium emission retrievals with Pyrat Bay and platon yielded a non-inverted T–P profile across approximately $2.73477$57 to $2.73477$58 bar, together with super-solar metallicity and C/O $2.73477$59; for example, Pyrat Bay gave $2.73477$60 dex and $2.73477$61, while platon gave $2.73477$62 dex and $2.73477$63. Free-chemistry retrievals, by contrast, yielded an inverted profile between approximately $2.73477$64 and $2.73477$65 bar, but only by invoking likely unphysical TiO and VO abundances, such as $2.73477$66 and $2.73477$67 in platon (Garai et al., 25 Jun 2025).

A 3D expeRT/MITgcm simulation added yet another layer. With solar metallicity, equilibrium chemistry, TiO/VO opacity tables, and a strong magnetic-drag term of $2.73477$68 s, the GCM produced a TiO-driven inversion, suppressed equatorial superrotation, predicted minimal hotspot offset, and matched the CHEOPS and TESS eclipse depths and Spitzer $2.73477$69m within $2.73477$70, while underpredicting HST/WFC3 and lying approximately $2.73477$71 low at Spitzer $2.73477$72m (Garai et al., 25 Jun 2025).

The present state of the dayside problem is therefore internally tensioned. One branch of the literature supports an inversion through emission retrievals and 3D circulation modeling; another supports a non-inverted profile once thermochemical equilibrium is enforced. The studies agree more securely on lower-order properties: KELT-7b has a hot dayside, low geometric albedo, and inefficient day-to-night heat redistribution.

6. Classification, interpretation, and outstanding issues

KELT-7b occupies a boundary regime in hot-Jupiter atmospheric classification. It has been described both as a hot Jupiter and as part of ultra-hot Jupiter samples. In the HARPS-based census, the empirical frontier between hot and ultra-hot planets, based on the detection of atomic and ionized species in their atmospheres, was placed at $2.73477$73 K, and with $2.73477$74 K KELT-7b was said to lie below that frontier, in the “colder UHJ” regime where detections of ionized metals are rarer (Stangret et al., 2021). This suggests that KELT-7b is especially valuable as a transition object rather than as an extreme member of the hotter ultra-hot Jupiter population.

The dominant observational complication is the host star. In the HARPS-N data, a strong Rossiter–McLaughlin signature and stellar pulsations produced residual structures that could block or mimic planetary signals; the cross-correlation maps showed a strong, nearly vertical feature on both nights, consistent with pulsations, while the RM trail partially overlapped the expected planetary trail (Stangret et al., 2021). In the HORuS analysis, even modest, percent-level changes in stellar activity during transit sequences were sufficient to generate spurious transmission features at the planetary rest frame (Tabernero et al., 2022). In the HUSTLE UV–optical retrievals, bright stellar inhomogeneities were favored at approximately $2.73477$75 and were necessary to explain the blueward negative slope (Gascón et al., 23 Jun 2025). The recurring methodological lesson is that stellar contamination in KELT-7 is not a secondary nuisance but a first-order component of the inference problem.

The atmospheric picture is correspondingly composite. Optical high-resolution spectroscopy has not confirmed Na, Mg, Ca, H$2.73477$76, Fe, Ti, or molecular absorption in transmission (Stangret et al., 2021, Tabernero et al., 2022). Low-resolution space-based transmission studies indicate H$2.73477$77 continuum and at least some evidence for H$2.73477$78O in the WFC3 era (Pluriel et al., 2020), but the UV–optical plus IR synthesis argues that H$2.73477$79 dominates the continuum and that H$2.73477$80O is not required by the data once stellar contamination is included (Gascón et al., 23 Jun 2025). JWST/NIRSpec/G395H weakens the case for strong near-infrared molecular features still further, finding only tentative H$2.73477$81O and CO$2.73477$82 and no robust CO, while allowing either a high-altitude gray cloud deck with $2.73477$83 to $2.73477$84 or a clear atmosphere with very low molecular abundances (Ahrer et al., 15 Sep 2025).

The formation and composition inferences therefore remain broad. The JWST/NIRSpec study reported C/O values in the range $2.73477$85–$2.73477$86 and metallicity spanning $2.73477$87–$2.73477$88; it noted that a solar-to-super-solar metallicity would imply the accretion of solid material during formation, but also emphasized that the present uncertainties preclude detailed conclusions about formation history (Ahrer et al., 15 Sep 2025). The emission-side reanalysis found equilibrium-chemistry solutions with super-solar metallicity and C/O $2.73477$89, whereas the free-chemistry inverted solutions required likely unphysical abundances of optical absorbers (Garai et al., 25 Jun 2025). These are not trivially reconcilable, and they underscore that the dominant uncertainty is not merely photon noise but model dependence.

The forward path proposed across the literature is technically consistent. Recommended directions include higher S/N multi-epoch high-resolution spectroscopy, improved joint modeling of RM, CLV, and stellar pulsations, contemporaneous monitoring of activity indices, broader overlapping wavelength coverage with JWST, and extension of coherent stellar-variability corrections across instruments and epochs (Stangret et al., 2021, Tabernero et al., 2022, Garai et al., 25 Jun 2025, Ahrer et al., 15 Sep 2025). A plausible implication is that KELT-7b will remain important less because it yields a single stable atmospheric narrative than because it exposes the coupling between stellar variability, chemical assumptions, cloud degeneracies, and spin–orbit geometry in the analysis of hot-Jupiter atmospheres.