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The panchromatic JWST dayside spectrum of WASP-121 b reveals a refractory-rich formation

By combining new JWST observations with archival data to detect high abundances of refractory SiO in WASP-121 b's atmosphere, this study reveals that the ultra-hot Jupiter's composition resulted from accretion from multiple reservoirs and its current high-obliquity orbit can be explained by post-formation dynamical events.

Original authors: K. Angelique Kahle, Paul Mollière, Laura Kreidberg, Bertram Bitsch, Mara Attia, Silke S. Dainese, Nicholas Storm, Daniel Valentine, Thomas M. Evans-Soma, H. J. Hoeijmakers, Stefan Pelletier, Ludmila C

Published 2026-06-23✓ Author reviewed ⓘ

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CC BY 4.0

Original authors: K. Angelique Kahle, Paul Mollière, Laura Kreidberg, Bertram Bitsch, Mara Attia, Silke S. Dainese, Nicholas Storm, Daniel Valentine, Thomas M. Evans-Soma, H. J. Hoeijmakers, Stefan Pelletier, Ludmila Carone, Cyril Gapp, Yoav Rotman, Sophia R. Vaughan, D. A. Christie, Louis-Philippe Coulombe, Christiane Helling, Thomas Henning

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a giant, scorching-hot planet named WASP-121 b orbiting a star so close that its "day" side is hotter than the surface of most stars. It's a "Ultra-Hot Jupiter," a place where the weather is extreme, and the chemistry is wild.

A team of astronomers used the James Webb Space Telescope (JWST)—our most powerful space eye—to take a complete "panchromatic" snapshot of this planet. Think of this as taking a photo that captures everything from the deep reds of visible light all the way to the invisible heat signatures of the mid-infrared. By stitching together data from different instruments, they created a full spectrum, like a musical chord containing every note from low bass to high treble, to figure out what this planet is made of and where it came from.

Here is the story of what they found, told in simple terms:

1. The Cosmic Recipe: Rocks vs. Gases

To understand how a planet is born, scientists usually look at the ratio of Carbon to Oxygen (like checking the ratio of flour to sugar in a cake). But the authors say this method is tricky; different recipes can end up tasting the same.

Instead, they looked for "refractory" ingredients. In everyday language, refractory elements are the "rocks" of the universe—things like silicon, magnesium, and iron that turn into solid rocks or dust at high temperatures. Volatile elements are the "gases" or "ices," like water vapor or carbon dioxide.

The Discovery:
The team found that WASP-121 b is rich in both rocks and gases, but it has a lot more rocks than you'd expect for a gas giant.

The Analogy: Imagine baking a cake. Usually, a gas giant is like a fluffy sponge cake (mostly gas). But WASP-121 b is like a sponge cake that someone accidentally dropped a whole bag of gravel into. The "rocky" ingredients (specifically Silicon) are about 3.5 times more abundant relative to the "gassy" ingredients than they are in the star it orbits.

2. The Detective Work: How Did It Get There?

If a planet is full of rocky dust, it means it must have eaten a lot of solid material while it was growing. The authors used this "rocky recipe" to solve a mystery: Where was this planet born?

They proposed two main scenarios, like two different travel stories:

Story A (The Local Builder): The planet was born close to its star, inside the "water ice line" (the zone where it's too hot for ice to exist). It ate up the dry, rocky dust available there, and then later sucked up some gas to become huge.
Story B (The Migrant): The planet was born far away in the cold, icy outer regions, ate some icy rocks, and then migrated inward. As it moved closer to the hot star, it ate more dry, rocky dust on its journey.

The Verdict: The data suggests the planet likely formed closer to the star (inside the water ice line) and ate a mix of dry rocks and gas. It didn't just drift in from the cold outer edges; it was built right there, gathering heavy elements as it grew.

3. The Hot Spot and the Wind

The team also mapped the temperature of the planet's day side.

The Hot Spot: On Earth, the hottest part of the day is usually directly under the sun. On WASP-121 b, the hottest spot is shifted slightly to the East (about 5 degrees).
The Analogy: Imagine a spinning top. If you blow air on it, the heat doesn't stay exactly where the heat source is; it gets pushed slightly ahead. This suggests that even on this super-hot world, there are winds trying to move the heat around, though they are weaker than we might expect for such a hot planet.

4. The Mystery of the Missing Titanium

Titanium is a metal that usually glows brightly in the atmosphere of hot planets. But on WASP-121 b, it's missing from the day side.

The Explanation: The authors believe the titanium isn't gone; it's just frozen out on the cold night side of the planet. It's like water vapor turning into ice on a cold windowpane. The day side is too hot to hold the ice, so the titanium has "rained out" and settled on the dark, cold side, leaving the day side clear.

5. The Bounce: How the Planet Got Tilted

WASP-121 b has a very strange orbit. It doesn't orbit neatly around the star's equator; it's tilted almost 90 degrees, like a wheel rolling on its side. This is called a "high-obliquity" orbit.

The Cause: The authors suggest this tilt happened after the planet was formed. They propose a dramatic event, like a cosmic pool game.
- The Analogy: Imagine a billiard table. A third, invisible ball (a massive companion planet or star) might have bumped into WASP-121 b, knocking it off its straight path and sending it into a chaotic, tilted orbit. Over millions of years, the star's gravity smoothed out the orbit, but the tilt remained.
- They checked the data and found no evidence of this "bumping" companion right now, but it could have been there in the past or be too faint to see.

6. The Shiny Coat

Finally, the planet is surprisingly reflective. It bounces back about 22% of the starlight that hits it.

The Mystery: Since the day side is too hot for normal clouds to form, what is reflecting the light? The authors suggest it might be clouds drifting over from the cold night side (like fog rolling over a mountain) or perhaps a giant, glowing halo of gas escaping the planet that scatters the light.

Summary

In short, this paper tells us that WASP-121 b is a unique world. It's a gas giant that was built with a heavy dose of rocky dust, likely formed close to its star, and was later knocked into a sideways orbit by a cosmic collision. By looking at the "ingredients" in its atmosphere, the astronomers were able to reconstruct its entire life story, from its birth in a dusty disk to its current, scorching existence.

Technical Summary: The Panchromatic JWST Dayside Spectrum of WASP-121 b Reveals a Refractory-Rich Formation

Problem and Motivation
Understanding the formation history of exoplanets requires linking their present-day atmospheric compositions to formation models. Traditional diagnostics relying solely on the carbon-to-oxygen (C/O) ratio and overall metallicity are often degenerate, as specific ratios can arise from multiple evolutionary pathways. Furthermore, uncertainties in disk composition and the presence of unobserved clouds can render C/O ratios ambiguous in space-based data. To resolve these degeneracies, recent literature suggests utilizing refractory-to-volatile abundance ratios as additional formation tracers. However, measuring refractory inventories in gas giants is challenging because refractory species typically condense into deep atmospheric layers or clouds. Ultra-hot Jupiters (UHJs), with dayside temperatures exceeding 3000 K, offer a unique opportunity to observe refractory elements in the gas phase, as silicate clouds evaporate on the dayside. WASP-121 b, a UHJ with a dayside temperature inversion and previously detected silicon monoxide (SiO), serves as a prime target to test these formation tracers and resolve conflicting literature results regarding its refractory-to-volatile ratios.

Methodology
The authors present a comprehensive analysis of the panchromatic dayside emission spectrum of WASP-121 b, spanning 0.6 to 12 µm. The study combines:

New Observations: A secondary eclipse observation using the JWST Mid-Infrared Instrument (MIRI) in Low-Resolution Spectroscopy (LRS) mode (5–12 µm), obtained in April 2024.
Archival Data: Existing JWST eclipse spectra from the Near-Infrared Imager and Slitless Spectrograph (NIRISS) and the Near-Infrared Spectrograph (NIRSpec/G395H).
Stellar Characterization: Re-analyzed high-resolution ESPRESSO/VLT observations to derive precise stellar abundances (Fe, C, O, Mg, S, Si) using NLTE synthesis, providing a baseline for planetary enrichment calculations.

Data Reduction and Retrieval Analysis

MIRI/LRS Reduction: The data were reduced using the Eureka! pipeline. The authors modeled the light curve using a batman eclipse model combined with a sinusoidal phase variation and a systematics model including linear trends and exponential ramps.
Joint Retrieval: The authors performed atmospheric retrievals using petitRADTRANS coupled with easyCHEM for equilibrium chemistry. The model included 100 pressure layers and retrieved a flexible temperature-pressure (T-P) profile at 11 fixed pressures. Gaussian Processes (GPs) were employed during the joint retrieval to handle correlated noise, specifically identifying a local mismatch around 8–9 µm.
Chemical Parametrization: To avoid biases from dissociation at high temperatures, the authors retrieved elemental enrichments relative to solar values: volatile enrichment ([Vol/H]), refractory enrichment ([Ref/H]), and titanium enrichment ([Ti/H]). The C/O ratio was retrieved as a free parameter. Titanium was decoupled from other refractories due to known depletion on the dayside.
Model Variations: The study tested various model configurations, including different temperature parametrizations (Guillot vs. free T-P), inclusion of additional atomic opacities, and different treatments of reflected light and correlated noise.

Key Results

Refractory Detection: The analysis robustly detects SiO gas at high confidence ( $\Delta \ln Z > 23$ ), confirming the presence of refractory species in the observable atmosphere.
Elemental Enrichments: The atmosphere is enriched in both volatile and refractory species relative to the host star.
- Volatile enrichment: $[Vol/H] \approx 0.80 \times$ stellar.
- Refractory enrichment: $[Ref/H] \approx 1.22 \times$ stellar.
- Derived ratios: $Si/O = 3.54^{+0.86}_{-0.69} \times$ stellar and $Si/C = 3.05^{+1.12}_{-0.80} \times$ stellar.
C/O Ratio: Unlike previous studies that suggested a highly super-stellar C/O ratio, this joint retrieval finds a planetary C/O of $0.57^{+0.07}_{-0.08}$ , which is only slightly enhanced compared to the stellar value ( $0.48 \pm 0.05$ ). The authors note that the C/O ratio is highly model-dependent, varying between 0.36 and 0.69 depending on the temperature parametrization and uncertainty treatment.
Temperature Structure: The retrieved T-P profile reveals a strong temperature inversion with a complex structure, including distinct gradients between $10^{-2}$ – $10^{-3}$ bar and $10^{-3}$ – $10^{-4}$ bar. This complexity cannot be captured by standard parametrized profiles (e.g., Guillot 2010).
Reflected Light: The model requires an additional source of reflected light opacity to explain the flux at short wavelengths, with a retrieved geometric albedo of $0.22 \pm 0.03$ .
Eclipse Mapping: Analysis of the MIRI/LRS light curve reveals a small eastward hotspot offset of $4.8^{+2.7}_{-2.8}$ degrees, consistent with previous NIRSpec findings. The dayside brightness temperature is $\sim 2889$ K, with a hotspot temperature of $\sim 3444$ K.
Titanium Depletion: The study confirms the depletion of TiO on the dayside, consistent with cold-trapping on the nightside.

Formation and Dynamical Implications

Formation Scenario: The measured enrichment pattern (high refractory-to-volatile ratios) suggests the planet accreted material from multiple reservoirs. The authors propose two likely scenarios:
- Scenario A: Formation interior to the water ice line, accreting volatile-poor planetesimals and volatile-rich gas.
- Scenario B/C: Migration from beyond the water ice line, accreting volatile-rich solids and subsequently migrating inward to accrete silicon-rich material.
  The authors favor a scenario involving accretion of solids both inside and outside the water ice line, or accretion of both solids and gas between the water ice line and the soot line.
Dynamical History: To explain the planet's current high-obliquity (near-polar) orbit, the authors model high-eccentricity migration (HEM). They find that a post-formation dynamical event, such as Von Zeipel–Lidov–Kozai (ZLK) cycles driven by a hidden sub-stellar companion (e.g., a $5 M_J$ object at 22 AU), is a viable mechanism. Gaia DR3 astrometry does not rule out such a companion. The modeling indicates that the planet's atmosphere would survive this migration with negligible mass loss ( $<0.1\%$ ), preserving the elemental inventory imprinted during accretion.

Significance and Claims
The paper claims to provide the first panchromatic (0.6–10.5 µm) estimate of the refractory-to-volatile abundance ratio for WASP-121 b, resolving discrepancies in previous single-instrument studies. By combining JWST datasets, the authors demonstrate that refractory species (specifically SiO) are significantly enhanced relative to volatiles, providing a robust tracer for the planet's formation location. The study highlights that while the C/O ratio remains model-dependent, the refractory-to-volatile ratio is a stable and powerful constraint on formation history. Furthermore, the work validates the use of ultra-hot Jupiters for directly measuring refractory inventories and confirms that dynamical migration mechanisms like ZLK cycles can transport planets to their current orbits without stripping their atmospheres, thereby preserving the chemical signatures of their formation. The authors modestly conclude that while their results strongly support specific formation pathways, the exact C/O ratio and the detailed nature of the scattering opacity (clouds vs. outflow) require further investigation and more sophisticated 3D modeling.