A team of astronomers using the James Webb Space Telescope has found what appears to be methane in the atmosphere of a giant exoplanet called HATS-75 b, but the discovery comes tangled in a problem that is fast becoming one of the biggest headaches in exoplanet science: the host star itself may be faking part of the signal.
HATS-75 b orbits an M-dwarf, the most common type of star in the Milky Way and a favorite target for planet hunters because small, cool stars make transiting planets easier to detect. The planet is a hot, inflated gas giant with roughly the mass of Jupiter and a radius about once that of Jupiter, completing a full orbit around its dim host in approximately 2.5 days. When it passes in front of the star, a sliver of starlight filters through its atmosphere, and JWST can read the chemical fingerprints embedded in that light. In this case, the fingerprints suggest methane is present. But dark, cooler patches on the star’s surface, known as starspots, may be stamping their own chemistry onto the signal, making it genuinely unclear how much of what JWST measured belongs to the planet and how much belongs to the star.
The findings, posted to the arXiv in April 2026, emerged from the GEMS transmission spectroscopy survey, a Cycle 2 program that uses JWST’s NIRSpec/PRISM instrument to study exoplanet atmospheres across wavelengths from 0.6 to 5.3 micrometers.
What the data actually show
The GEMS team observed three separate transits of HATS-75 b. The combined transmission spectrum reveals a slope at shorter wavelengths that could point to atmospheric haze, a common feature in giant planet atmospheres, or to contamination from the Transit Light Source (TLS) effect. The TLS effect occurs when a transiting planet blocks a portion of the stellar disk that looks different from the rest of the star. If the unblocked regions are peppered with starspots at different temperatures, the resulting spectrum picks up distortions that can mimic or mask planetary chemistry.
Independent indicators in the same dataset favor the starspot explanation. The researchers found wavelength-dependent variations in transit depth that line up with the temperatures expected for spots on M-dwarf surfaces, suggesting the star’s patchy photosphere is a major source of spectral interference.
HATS-75 b is not an isolated case within the GEMS survey. Observations of TOI-5293 A b, another giant planet orbiting a variable M-dwarf, revealed a heterogeneous stellar photosphere that materially altered the planet’s measured transmission spectrum. A separate GEMS study of TOI-5205 b found significant stellar contamination and indications that the planet’s atmosphere may be metal-poor, though that result has not yet been independently confirmed. Taken together, the pattern is striking: active M-dwarfs routinely imprint spectral signatures that can be mistaken for, or blended with, planetary atmospheric features.
Why the methane question is so hard to settle
Methane detection in exoplanet atmospheres has a fraught track record. A Nature overview documented how earlier claims from pre-JWST instruments repeatedly failed to survive re-analysis with better data. JWST raised the evidentiary bar with far sharper spectral resolution, but the HATS-75 b case shows that sharper instruments do not automatically produce clearer answers when the astrophysical environment is messy.
The core problem is that atmospheric haze and starspot contamination can produce similar spectral slopes across the wavelength range NIRSpec/PRISM covers. Retrieval models that assume a smooth, uniform stellar surface may overcount methane or misattribute spectral features that actually originate in the star. Models that aggressively correct for stellar heterogeneity risk scrubbing out a genuine planetary signal if the correction overshoots. Peer-reviewed work on HAT-P-18 b, led by Marylou Fournier-Tondreau and colleagues using JWST’s NIRISS instrument, demonstrated exactly this tension: when the team allowed for multiple stellar surface components at different temperatures and covering fractions, the inferred atmospheric properties shifted compared with simpler one-component models.
No standardized framework for handling stellar contamination has emerged across the field. That means two teams analyzing the same HATS-75 b dataset could reach meaningfully different conclusions depending on their modeling choices, and both could be internally consistent.
For comparison, consider WASP-80 b, a warm gas giant where JWST detected methane at greater than 6-sigma significance across the 2.4 to 4.0 micrometer range using NIRCam, as reported by Taylor Bell and colleagues in Nature in 2023. WASP-80 b’s host star is far more quiescent, and the dominant uncertainties in that detection stem from instrumental systematics and modeling details rather than a roiling stellar surface. The contrast is instructive: when the star cooperates, JWST delivers molecular identifications that stand on firm ground. When the star is an active M-dwarf, the same telescope produces data that demands much more cautious reading.
How M-dwarf starspots are reshaping JWST observation strategy
M-dwarfs host a disproportionate share of known exoplanets, including many of the rocky, potentially habitable worlds that drive public excitement about JWST. If stellar surface heterogeneity routinely distorts transmission spectra around these stars, then a significant fraction of JWST atmospheric characterizations, particularly for smaller planets, may need careful re-examination as modeling techniques improve.
Researchers are already discussing practical fixes. “The community is realizing we cannot treat the star as a featureless lamp,” said Nestor Espinoza, an astronomer at the Space Telescope Science Institute who works on transit spectroscopy methods, in a recent conference presentation. Future observations may require synchronized, multi-wavelength stellar monitoring during transits to disentangle planetary signals from stellar noise. Photometric campaigns before, during, and after JWST observations can map starspot coverage and track how it evolves. Ground-based spectroscopic monitoring can flag chromospheric activity indicators that correlate with surface features. Layering these datasets on top of JWST spectra could tighten constraints on the TLS effect and narrow the range of plausible interpretations.
For now, HATS-75 b sits in an uncomfortable but scientifically productive limbo. The JWST data are real, reproducible, and publicly available for re-analysis. The methane signal is plausible. But so is the possibility that a restless, spot-covered star is masquerading as planetary chemistry. Resolving which explanation holds will likely require additional transits, better stellar models, and the kind of painstaking cross-checking that rarely makes headlines but ultimately determines what we actually know about worlds beyond our own.
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*This article was researched with the help of AI, with human editors creating the final content.
