TOI-5205 b orbits a small star roughly four times the size of Jupiter, with a mass about 40 percent that of our Sun. Despite its relatively small host star, TOI-5205 b blocks an extraordinary six percent of the star’s light during transit events, which makes it a highly favorable candidate for detailed spectroscopic study. Using transit spectroscopy, researchers analyzed starlight filtered through the planet’s atmosphere to decode the chemical makeup and gain insights into its formation history and atmospheric processes. These spectroscopic observations provide a direct window into exoplanetary atmospheres—a frontier that has expanded dramatically with JWST’s advanced instruments.

Planets form within the protoplanetary disks of gas and dust that encircle newborn stars. It is well accepted in astrophysics that giant planets emerge from these disks in the early stages of star formation, accreting large amounts of gas and heavy elements from their surroundings. However, the discovery of gas giants like TOI-5205 b orbiting close to cool, low-mass M dwarf stars raises compelling questions about the efficiency and mechanisms of giant planet formation, migration, and chemical differentiation around such stars. This particular system poses a unique puzzle due to the planet’s considerable mass relative to its host star.

The study, published in The Astronomical Journal, represents a collaboration led by NASA’s Goddard Space Flight Center’s Caleb Cañas alongside Carnegie Science’s Shubham Kanodia, and forms part of the ambitious “Red Dwarfs and the Seven Giants” JWST Cycle 2 program. This initiative is specifically designed to investigate the so-called “GEMS” planets—giant exoplanets orbiting M dwarf stars—whose formation and evolutionary histories remain largely enigmatic. The TOI-5205 b observations mark a pioneering step in characterizing these extreme worlds.

Initial identification of TOI-5205 b came from NASA’s Transiting Exoplanet Survey Satellite (TESS), with confirmation and subsequent JWST observations spearheaded by Kanodia and colleagues. Their concerted effort involved capturing the planet’s transit events across different wavelengths, allowing them to apply transmission spectroscopy techniques that dissect the atmospheric signatures imprinted on starlight. This approach revealed a surprisingly metal-poor atmosphere—a finding that defied expectations based on comparisons to giant planets within our own Solar System such as Jupiter.

Detailed analysis uncovered that the planet’s atmosphere contains significantly lower abundances of heavier elements relative to hydrogen than its host star’s composition. This low metallicity atmosphere is unprecedented among gas giants studied thus far and contradicts standard planet formation models predicting enrichment of heavy elements in giant planet atmospheres due to core accretion and gas accumulation processes. Moreover, JWST spectra showed notable presence of methane (CH₄) and hydrogen sulfide (H₂S), offering additional clues to the chemical pathways shaping this exotic atmosphere.

To better understand these surprising results, researchers from the University of Zurich, including Simon Muller and Ravit Helled, employed sophisticated planetary interior models. Their computations suggest that although the planetary atmosphere is metal-poor, the overall composition of TOI-5205 b—including its interior—is approximately 100 times richer in heavy elements. This dichotomy implies that during its formation, heavy elements migrated toward the planet’s core or inner layers, decoupling from the atmosphere. Such heavy-element sequestration may indicate inhibited mixing between the planet’s interior and atmospheric layers, shedding light on complex evolutionary processes previously unexplored in such systems.

Kanodia emphasized the implication of these findings: the evident disparity between interior metallicity and atmospheric metal content signals that planets like TOI-5205 b possess atmospheres dominated by carbon-rich, oxygen-poor chemistry. This altered elemental balance contrasts with giant planets like Jupiter, which exhibit enhanced atmospheric metallicities consistent with their host stars and solar system formation models. Such atmospheric compositions challenge existing theoretical frameworks and invite new explorations into chemical dynamics and planetary differentiation influenced by host star properties and protoplanetary disk environments.

Additionally, the study painstakingly addressed the confounding influence of starspots on the host star, which can distort transit spectroscopy data. TOI-5205’s star exhibits significant spot activity, causing brightening effects at certain wavelengths that could mask or mimic atmospheric features. Carnegie astronomers Nicole Wallack and Shubham Kanodia developed corrective methodologies to eliminate the influence of stellar magnetic activity on the data. This refinement enhances the reliability of the atmospheric characterizations and sets a new standard for future JWST observations of planets orbiting magnetically active stars.

The GEMS survey team comprises a broad international coalition of astronomers, including specialists from institutions like Johns Hopkins University, University of Maryland, Caltech, University of St. Andrews, University of Amsterdam, and Penn State University. This transdisciplinary collaboration integrates observational astronomy, theoretical modeling, and data analysis techniques to decode the multiplicity of physical processes shaping giant exoplanets around red dwarfs—a stellar population comprising a significant fraction of the Milky Way’s stars but largely unexplored in giant planet context.

These observations and subsequent analyses herald a new era in exoplanetary science, unveiling how planetary formation physics can diverge remarkably depending on the host star’s characteristics and environment. As JWST continues to provide unprecedented spectroscopic sensitivity, astronomers anticipate uncovering further examples of metal-poor giant atmospheres and anomalous chemical signatures, offering deep insight into the diversity of planetary systems and their evolutionary trajectories. The implications extend to refining core accretion theories, migration models, and the chemical evolution of planets orbiting different types of stars, reshaping our understanding of planet formation in the cosmos.

This remarkable study underscores the transformational impact of JWST’s capabilities in transcending previous observational limitations and delivering the detailed characterization required to probe planetary atmospheres directly. By revealing the intriguing chemical fingerprint of TOI-5205 b, it opens new frontiers in exoplanetary research and ignites fresh theoretical challenges. Continued observations of similar systems promise to expand our knowledge of the profound mechanisms governing planetary emergence, chemical evolution, and atmospheric dynamics on worlds beyond our Solar System.

Subject of Research: Not applicable

Article Title: GEMS JWST: Transmission Spectroscopy of TOI-5205b Reveals Significant Stellar Contamination and a Metal-poor Atmosphere

Web References:
https://iopscience.iop.org/article/10.3847/1538-3881/ae4976

References:
Cañas, C., Kanodia, S., et al. (2024). GEMS JWST: Transmission Spectroscopy of TOI-5205b Reveals Significant Stellar Contamination and a Metal-poor Atmosphere. The Astronomical Journal. DOI: 10.3847/1538-3881/ae4976

Image Credits: Katherine Cain, Carnegie Science

Keywords

TOI-5205 b, exoplanet atmosphere, JWST, transmission spectroscopy, giant exoplanets, M dwarf stars, metal-poor atmosphere, planet formation, stellar contamination, methane, hydrogen sulfide, planetary interiors, protoplanetary disks

Thermal phase curves represent a powerful observational tool, measuring the planet’s emitted infrared radiation as it orbits its star, thus mapping temperature variations across its surface. For TRAPPIST-1 b, the data paints a vivid picture of an intensely hot dayside with a brightness temperature measured at approximately 490 Kelvin, contrasted by near-zero emission from its nightside. Such an extreme temperature contrast is a hallmark of a body lacking a substantial atmosphere, which would otherwise redistribute heat to the nightside. A lack of phase offset further corroborates the notion of a dark, desolate surface, absorbing stellar radiation without the moderating influence of atmospheric circulation.

In stark contrast, TRAPPIST-1 c displays a somewhat cooler dayside, with temperatures measured around 369 Kelvin alongside an equally frigid nightside. This thermal profile suggests that the planet either possesses a tenuous atmosphere, potentially oxygen-rich but very thin, or alternatively has an airless surface with higher reflectivity. The possibility of a sparse oxygen atmosphere, perhaps a remnant of past atmospheric loss or photodissociation processes, hints at evolutionary divergence from TRAPPIST-1 b, despite their shared stellar environment and similar bulk compositions.

Intriguingly, the data decisively disfavors atmospheric models with surface pressures exceeding approximately one bar for both planets. This threshold, close to Earth’s atmospheric pressure at sea level, is significant, as gases under such conditions would redistribute heat efficiently, smoothing out temperature contrasts and creating measurable phase offsets, which the JWST observations do not detect. This observation forces scientists to reconsider mechanisms by which planets around red dwarfs can lose or fail to accumulate thick atmospheres over geological timescales.

The harsh radiation environment generated by low-mass red dwarfs is well-known to impact planetary atmospheres profoundly. High-energy stellar winds and frequent flares can strip away atmospheric gases, an effect exacerbated by close orbital proximities needed to maintain temperate conditions. TRAPPIST-1 b and c, residing very close to their host star, presumably endure intense stellar activity, which might explain the absence or extreme depletion of robust atmospheres. However, the presence of any oxygen or reflective surfaces on TRAPPIST-1 c opens questions about atmospheric chemistry and planetary surface interactions that merit further exploration.

The implications of these findings ripple across multiple domains of astrophysics and astrobiology. Firstly, the stark contrast with Earth-like thick atmospheres challenges assumptions about habitability around M-dwarf stars. Such stars constitute the majority of stars in the galaxy and are prime targets for searching potentially life-supporting planets. The absence of thick atmospheres on planets like TRAPPIST-1 b and c suggests that habitability criteria must take into account the difficulty of atmospheric retention under intense stellar radiation fluxes, altering priorities for biosignature searches.

Moreover, the data showcases the exceptional capability of JWST’s mid-infrared instruments to probe planetary climates and surface conditions remotely. This method provides not only temperature measurements but also constraints on atmospheric composition and pressure, offering a new lens through which to study exoplanetary atmospheres in exquisite detail. Future observational campaigns will likely exploit these methods for a broader sample of rocky exoplanets, potentially categorizing them by atmospheric presence and evolutionary history.

These discoveries also raise profound questions about the formation and evolution of planets around faint red dwarfs. That two neighboring planets within the same system — possibly having formed from similar primordial material — exhibit such divergent atmospheric states suggests complex evolutionary pathways. Differences could stem from stochastic events such as giant impacts, variable stellar activity phases, or even divergent internal geophysical processes affecting outgassing and atmospheric escape rates.

To understand the absence of thick atmospheres further, scientists rely on atmospheric escape models incorporating hydrodynamic blow-off driven by extreme ultraviolet and X-ray irradiation, combined with photochemical modeling of atmospheric constituents like oxygen, carbon dioxide, and water vapor. The observed constraints on surface pressure imply that either these models need refinement or that other unaccounted-for processes, such as magnetic field shielding, replenishment cycles, or planetary magnetospheres, play significant roles in shaping atmospheric fate.

The lack of observable phase offset in TRAPPIST-1 b’s thermal emission is particularly telling. In planets with substantial atmospheres, especially those with zonal winds and heat transport, the hottest region tends to be displaced eastward of the substellar point, generating a measurable phase shift in the infrared light curve. The zero phase offset measured here reiterates the conclusion of an airless or nearly airless world, casting light on the thermal dynamics of these distant terrains.

For TRAPPIST-1 c, the possibility of a reflective surface or a tenuous atmosphere adds an exciting dimension. Surface reflectivity can be influenced by high-albedo materials such as silicates or ice, which might exist given the planet’s cooler dayside temperature relative to TRAPPIST-1 b. Alternatively, oxygen-rich but rarefied atmospheres may represent a late evolutionary stage, where volatile loss has stripped away lighter species, leaving behind residual oxygen produced by photodissociation, a phenomenon that complicates the interpretation of oxygen as a bioindicator.

The comparison between planets within the same system underlines the dynamic complexity that awaits characterization in exoplanetary science. TRAPPIST-1 remains a cornerstone system for these inquiries, as its proximity, favorable inclination, and the number of Earth-sized planets provide unmatched opportunities for comparative exoplanetology. Upcoming JWST observations and next-generation telescopes will likely deepen this understanding and explore potential atmospheres on its more distant, potentially habitable worlds.

Ultimately, these findings redefine current paradigms about atmospheric survival in extreme environments, pushing the envelope of our understanding of exoplanet habitability. They highlight the necessity to integrate observational data with advanced modeling of stellar-planet interactions, atmospheric chemistry, and planetary interiors. As we accumulate more data from TRAPPIST-1 and similar systems, we continue to refine the conditions under which rocky planets can sustain atmospheres and, by extension, life.

Intriguingly, the study also emphasizes the importance of context when interpreting atmospheric signatures. The mere detection or absence of an atmosphere does not unambiguously indicate habitability or biological potential but must be situated within a framework that considers stellar characteristics, orbital dynamics, and planetary properties. TRAPPIST-1 b and c exemplify worlds where similar starting conditions can yield dramatically different present states, challenging the notion of “Earth-like” planets based solely on size and location.

This pioneering research utilizing JWST thermal phase curves stands as a testament to the transformative power of cutting-edge space telescopes in exoplanetary science. It sets the stage not only for more refined studies of TRAPPIST-1 but also for extensive surveys of rocky planets around nearby red dwarfs, ultimately enhancing our grasp on planet formation, atmospheric evolution, and the potential for life beyond our solar system.

As humanity’s search for understanding moves forward, the barren, dark surface of TRAPPIST-1 b and the enigmatic, cool TRAPPIST-1 c remind us that each exoplanet possesses a unique narrative shaped by complex cosmic forces. Only by unraveling these stories through meticulous observation and innovative analysis can we piece together the intricate puzzle of planetary atmospheres and the prospects for life in the cosmos.

Subject of Research: Atmospheric characterization of rocky exoplanets TRAPPIST-1 b and TRAPPIST-1 c using JWST thermal phase curve observations.

Article Title: No thick atmosphere around TRAPPIST-1 b and c from JWST thermal phase curves.

Article References:
Gillon, M., Ducrot, E., Bell, T.J. et al. No thick atmosphere around TRAPPIST-1 b and c from JWST thermal phase curves. Nat Astron (2026). https://doi.org/10.1038/s41550-026-02806-9

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41550-026-02806-9

The exoplanet in question, designated L 98-59 d, lies approximately 35 light-years from Earth, orbiting a modest red dwarf star. Its physical dimensions, about 1.6 times larger than Earth, alongside its notably low density, initially baffled astronomers. Recent measurements captured by the James Webb Space Telescope (JWST) and corroborated by terrestrial observatories indicated the unexpected presence of abundant hydrogen sulfide—a sulphur compound—in its atmosphere, a finding that defies conventional planetary models.

Traditionally, astrophysicists categorized planets like L 98-59 d as either rocky “gas dwarfs” enveloped by thick hydrogen atmospheres or as water-rich worlds dominated by oceans and ice. However, comprehensive simulations developed by an international team of scientists from the University of Oxford, Groningen, Leeds, and ETH Zurich reveal that L 98-59 d does not conform to either paradigm. Instead, it represents a novel planetary archetype defined by sulphur-rich chemistry and a molten, magma-dominated interior.

Using advanced physical and chemical modeling techniques, the research team meticulously reconstructed the evolutionary history of L 98-59 d over nearly five billion years. These simulations, integrating observational data with detailed planetary interior and atmospheric models, unveiled a planet with a mantle composed predominantly of molten silicate rock—akin to terrestrial lava—wrapped in a global magma ocean extending thousands of kilometers in depth. This magma ocean serves as a massive reservoir capable of sequestering enormous quantities of sulphur, influencing the planet’s atmospheric composition over geological timescales.

One of the key insights from this study lies in the dynamic exchange of volatile compounds between the molten interior and the overlying hydrogen-rich atmosphere. Despite the intense X-ray radiation emitted by the host star, which typically strips light atmospheres from low-mass planets, L 98-59 d has retained a dense envelope containing gases such as hydrogen sulfide (H₂S). The magma ocean acts as a stabilizing buffer, gradually releasing sulphurous volatiles into the atmosphere, maintaining chemical equilibrium across eons.

The implications of these findings suggest the existence of an entire class of sulphur-laden, magma-ocean-bearing exoplanets, challenging the simplicity of planetary classification and hinting at a far richer diversity of planetary types than previously recognized. If L 98-59 d is representative, then the Galaxy may host numerous planets where internal geochemical processes fundamentally shape atmospheric characteristics in ways not witnessed in our own Solar System.

Dr. Harrison Nicholls, the leading astrophysicist on this research, expressed enthusiasm about the ramifications of this discovery. “Our findings imply that the current frameworks astronomers use to categorize small exoplanets may be overly simplistic,” he said. “While a molten, sulphur-rich planet like L 98-59 d is unlikely to harbor life, it underscores the extraordinary variety of planetary environments awaiting discovery. This prompts the profound question: what other exotic worlds remain hidden in the cosmos?”

Central to the planet’s atmospheric composition is the interaction of ultraviolet radiation from the red dwarf star with sulphur-based gases high in the atmosphere. JWST data from 2024 revealed spectral signatures consistent with sulphur dioxide and other sulphur compounds. The research models elucidate how photochemical reactions driven by stellar UV flux produce these gases. Simultaneously, the vast magma ocean below continuously supplies these volatiles, ensuring a steady-state atmospheric composition that aligns with observational data.

From a planetary formation standpoint, simulations suggest that L 98-59 d originally possessed an abundance of volatile compounds, possibly resembling a sub-Neptune in its youth. Over billions of years, the planet cooled and contracted, shedding portions of its primordial atmosphere yet retaining its characteristic magma ocean and sulphur-rich atmosphere. This evolutionary pathway provides a new lens to interpret how rocky planets, including Earth and Mars, may have cooled from molten beginnings, but with the added complexity of volatile sulphur chemistry.

Professor Raymond Pierrehumbert, a prominent co-author, emphasized the power of computer modeling to illuminate the concealed interiors of distant worlds. “Despite our observational constraints—we can only assess a planet’s size, mass, and atmosphere remotely—these models provide a means to reconstruct a planet’s hidden past and interior processes,” he noted. This approach heralds a new era where theoretical frameworks and observational data interlace to reveal the intricate tapestries of alien planets unseen by human eyes.

Looking forward, the study’s authors highlight the promise of forthcoming missions such as Ariel and PLATO, which will yield more detailed atmospheric measurements of exoplanets. By applying machine learning algorithms to integrate these data with sophisticated interior-atmosphere models, researchers aim to map the vast diversity of worlds populating our galaxy. This pursuit not only deepens our understanding of planetary formation mechanics but also informs the search for potentially habitable environments beyond Earth.

Dr. Richard Chatterjee from the University of Leeds and Oxford remarked on the intriguing role of hydrogen sulfide—a gas familiar to us as the source of the scent of rotten eggs—in shaping planetary atmospheres in unforeseen ways. “Our simulations enable us to essentially rewind the evolutionary clock, offering insights into how this peculiar class of rocky exoplanets developed complex atmospheres dominated by sulphur compounds,” he explained. “Further observational efforts will be critical to discern whether such ‘pungent’ planets are anomalies or a common occurrence in the Universe.”

This pioneering research opens an exciting chapter in exoplanetary science by unveiling a heretofore unrecognized planetary type, defined by deep magma oceans and sulphur-rich atmospheres. As humanity’s astronomical toolkit advances, revealing ever more detailed portraits of distant planets, the cosmic menagerie of worlds continues to expand, reminding us that the Universe is more diverse and stranger than our most imaginative speculations.

Subject of Research: Evolution and characterization of a sulphur-rich molten exoplanet, L 98-59 d, including its interior magma ocean and atmospheric chemistry.

Article Title: Volatile-rich evolution of molten super-Earth L 98-59 d

News Publication Date: 16 March 2026

Web References:
DOI Link – http://dx.doi.org/10.1038/s41550-026-02815-8

Image Credits: Mark A. Garlick / markgarlick.com

Keywords

Exoplanet, L 98-59 d, sulphur-rich atmosphere, magma ocean, hydrogen sulfide, James Webb Space Telescope, planetary evolution, molten super-Earth, red dwarf star, photochemistry, planetary interior, volatile compounds