Hinna Shivkumar

Hot-Jupiters, gas giant planets with equilibrium temperatures above 1000 K, host large temperature gradients between their permanent day- and nightsides, resulting in circulation regimes that have no solar system analogs. Past Kepler and Spitzer measurements have shown that the phase curves of these objects typically peak westward of the substellar point at optical wavelengths and eastward in the infrared, indicative of a possible interaction between circulation and cloud coverage. However, few hot-Jupiters have joint reflected light and thermal emission measurements, preventing a definitive statement as to the link between these phenomena. Here, we present the first phase curve of a hot-Jupiter that separates the contributions of thermal emission and reflected light through JWST observations of NGTS-10 Ab with the NIRSpec PRISM instrument ( λ = 0.5–5.5 μ m). Using the spectrally resolved phase curve, we jointly retrieve the planet’s thermal and reflected light maps. We show that the temperature and reflectance distributions are anti-correlated, which is best explained by clouds evaporating from the eastern substellar region ( φ ≈ −17° to 48°) where temperatures are highest. This is further confirmed by comparisons of the phase-resolved spectra with three-dimensional circulation models, which show that NGTS-10 Ab’s atmosphere hosts inhomogeneous cloud coverage, likely made-up of μ m-sized silicate particles, and weak atmospheric drag ( τ _drag > 10 ⁶ s). Finally, by measuring the spectral variation of the thermal phase curve offsets, we infer a slope of 7.1 ± 1.9 degrees per pressure dex, indicative of heat transport that becomes more efficient at depth. Future optical and infrared hot-Jupiter phase curve measurements over a wide range of equilibrium temperatures will enable a complete mapping of the interplay between heat transport and cloud formation in highly irradiated exoplanet atmospheres.

While recent JWST observations of mature super-Earths and sub-Neptunes have frequently revealed featureless transmission spectra, their inflated progenitors offer a unique window into understanding their primordial compositions. As part of the Keys to Revealing the Origin and Nature of Sub-Neptune Systems (KRONOS) JWST program, we present the Near Infrared Imager and Slitless Spectrograph Single Object Slitless Spectroscopy transmission spectrum of V1298 Tau c, an ∼23 Myr super-Earth progenitor orbiting a young solar analog. We detect H ₂ O in V1298 Tau c’s atmosphere with a log 10 volume mixing ratio of − 1.8 3 − 0.77 + 0.68 , but no additional molecules from these data alone. We find consistent results for the planetary atmospheric properties in both retrievals with and without informed priors on stellar heterogeneities based on the observed stellar spectrum. We infer an atmospheric metallicity [O/H] of 14 . 8 − 12.28 + 56.0 × the solar value. This metallicity is similar to literature measurements for other young planets, including its massive outer companion V1298 Tau b. In contrast, this measured metallicity is systematically lower than the metallicities of mature planets of similar mass and temperature. Altogether, these results provide tentative but growing evidence that the exoplanet mass–metallicity relation evolves with planetary age.

The Galaxy’s most common planetary systems consist of several Earth- to Neptune-sized planets on compact orbits, yet the young (10–30 Myr) star V1298 Tau hosts an unusual compact system of four large planets (≈5–10 Earth radii) arranged in a chain of near-mean-motion resonances that generate transit-timing variations of several hours. This system provides a rare opportunity to jointly investigate early dynamical, interior, and atmospheric evolution. We present new CHEOPS observations of the three innermost planets, delivering high-precision radius measurements that significantly improve constraints on their bulk densities and internal structures. Combined with recent mass estimates from transit-timing variations, we test whether the system formed in a resonant chain and subsequently evolved through tidal migration. We find that the planets are currently too far from exact commensurabilities for tidal dissipation to have driven them out of resonance, disfavouring a primordial full resonant chain. Accounting for post-formation planetary contraction further modifies the rate and direction of tidal migration, reducing the likelihood of resonance capture and suggesting formation with period ratios already below resonance. We also present complementary Hubble and JWST observations that reveal an extended hydrogen–rich atmosphere, with unexpectedly low metallicity for V1298 Tau b and a lack of methane, pointing to strong atmospheric mixing. Evolution models suggest substantial atmospheric loss over the next gigayear, potentially transforming the planet into a Neptune-sized world. V1298 Tau thus offers a benchmark for linking dynamical history with atmospheric and interior evolution, a synergy that will be greatly expanded by the large sample of young planets expected from ESA’s forthcoming PLATO mission.

We present the JWST/NIRSpec G395H transmission spectrum of the young (10–30 Myr) transiting planet V1298 Tau b (9.85 ± 0.35 R _⊕, T _eq = 670 K). Combined Hubble Space Telescope and JWST observations reveal a haze-free, H/He-dominated atmosphere with a large scale height (∼1500 km), allowing detection of CO₂ (35σ), H₂O (30σ), CO (10σ), CH₄ (6σ), SO₂ (4σ), and OCS (3.5σ). Our observations probe several scale heights (∼4.4 in the CO₂ 4.3 μm band and ∼3 in the 2.7 μm water band). The planet’s mass, inferred from atmospheric scale height using free retrieval and grid modeling, is 12 ± 1 M _⊕ and 15 ± 1.7 M _⊕, respectively, which is significantly lower than previous radial velocity estimates and confirms it as “gas-dwarf” sub-Neptune progenitor. We find an atmospheric metallicity (log Z = 0.6 − 0.6 + 0.4 × solar) and subsolar C/O ratio (0.22 − 0.05 + 0.06 ). The atmospheric metallicity is low compared to mature sub-Neptunes by an order of magnitude. The CH₄ abundance ([CH₄] = −6.2 − 0.5 + 0.3 ]) is ∼7σ lower than the equilibrium chemistry prediction. To adjust for the low methane abundance, the self-consistent grids favor a high internal temperature (∼500 K) and vertical mixing (K _zz ∼ 10⁷−10⁸ cm² s^–1). These internal temperatures are inconsistent with predictions from evolutionary models, which expect ∼100–200 K at the current system age. We estimate a gas-to-core mass fraction between 0.1% and 8%, with a core mass of 11–12 M _⊕, consistent with in-situ gas-dwarf formation. A deep atmospheric metallicity gradient may explain both the high internal temperature and low observable metallicity. Over time, mass loss from such an atmosphere could enhance its metallicity, potentially reconciling V1298 Tau b with mature sub-Neptunes.

High-eccentricity gas giant planets serve as unique laboratories for studying the thermal and chemical properties of H/He-dominated atmospheres. One of the most extreme cases is HD 80606 b—a hot Jupiter orbiting a Sun-like star with an eccentricity of 0.93—which experiences an increase in incident flux of nearly 3 orders of magnitude as the star–planet separation decreases from 0.88 au at apoastron to 0.03 au at periastron. We observed the planet’s periastron passage using JWST’s NIRSpec/G395H instrument (2.8–5.2 μm) during a 21 hr window centered on the eclipse. We find that, as the planet passes through periastron, its emission spectrum transitions from a featureless blackbody to one in which CO, CH₄, and H₂O absorption features are visible. We detect CH₄ during postperiapse phases at 4.1–10.7σ depending on the phase and on whether a flux offset is included to account for NRS1 detector systematics. Following periapse, H₂O and CO are also detected at 4.2–5.5σ and 3.7–4.4σ, respectively. Furthermore, we rule out the presence of a strong temperature inversion near the IR photosphere based on the lack of obvious emission features throughout the observing window. General circulation models had predicted an inversion during periapse passage. Our study demonstrates the feasibility of studying hot Jupiter atmospheres using partial phase curves obtained with NIRSpec/G395H.

Transit surveys have revealed young transiting exoplanets (

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The V1298 Tau system is a multi-planet system that provides the opportunity to perform comparative exoplanetology between planets orbiting the same star. Because of its young age (20–30 Myr), this system also provides the opportunity to compare the planet’s early evolutionary properties, right after their formation. We present the first atmospheric comparison between two transiting exoplanets within the same multiple planet system: V1298 Tau b and V1298 Tau c. We observed one primary transit for each planet with the Hubble Space Telescope (HST), using Grism 141 (G141) of Wide Field Camera 3 (WFC3). We fit the spectroscopic light curves using state-of-the-art techniques to derive the transmission spectrum for planet c and adopted the transmission spectrum of planet b obtained with the same observing configuration and data analysis methods from previous studies. We measured the mass of planet b and c (8₋₂⁺⁴, 17₋₆⁺¹³ M_⊕ ; respectively) from the transmission spectrum and found the two planets to have masses in the Neptune or sub-Neptune regime. Using atmospheric retrievals, we measured and compared the atmospheric metallicities of planet b and c (logZ/Z_⊙=−2.04_−0.59^0.69, logZ/Z_⊙ = −0.16_−0.94^1.15, respectively), and found them to be consistent with the solar or sub-solar, which is low (at least one order of magnitude) compared to known mature Neptune and sub-Neptune planets. This discrepancy could be explained by ongoing early evolutionary mechanisms, which are expected to enrich the atmospheres of such young planets as they mature. Alternatively, the observed spectrum of planet c can be explained by atmospheric hazes, which is in contrast to planet b, where efficient haze formation can be ruled out. Higher haze formation efficiency in planet c could be due to differences in atmospheric composition, temperature and/or higher UV flux compared to planet b. In addition, planet c is likely to experience a higher fraction of mass loss compared to planet b, given its proximity to the host star.