JWST's transmission spectroscopy works by watching a planet cross in front of its star and measuring how the starlight filters through the planet's atmosphere. Different molecules absorb at different wavelengths: water, methane, carbon dioxide, ammonia each leave distinct fingerprints. A sub-Neptune with a hydrogen-rich atmosphere should show spectral features — dips and peaks where atmospheric constituents absorb the starlight at characteristic wavelengths.
Wallack, Gao, Greklek-McKeon, and the COMPASS team (arXiv 2602.22327, February 2026) point JWST's NIRSpec G395H grating at HD 15337 c — a sub-Neptune 2.5 times Earth's radius, 6.8 Earth masses, orbiting a K dwarf at an equilibrium temperature of 656 K — and find nothing. The transmission spectrum is featureless. A flat line, consistent with a step function from detector systematics. No water. No methane. No CO₂. Nothing.
The flat spectrum rules out low-metallicity atmospheres — compositions below 300-600 times solar metallicity would produce detectable features at this precision. What remains: either a very high-metallicity atmosphere (so metal-rich that the mean molecular weight is high and the scale height is small, compressing any features below detectability), or aerosols. Clouds and hazes at high altitudes can act as an opaque deck that blocks the starlight before it penetrates deep enough to encounter the molecular absorbers. The atmosphere is there, but the window is opaque.
The COMPASS survey is finding a pattern across its target list. The aerosol opacity appears to peak for planets with equilibrium temperatures between 500 and 700 K. Below this range, the aerosols condense out (the atmosphere is too cold to maintain them as a high-altitude haze). Above this range, the aerosols may dissociate (the atmosphere is too hot for the haze precursors to survive). The peak is a condensation-driven maximum: the 500-700 K range is where the conditions for photochemical haze production and lofting are optimal.
HD 15337 c sits right in this maximum. Its featureless spectrum may not indicate an unusual atmosphere. It may indicate a typical one — at the temperature where nature is most effective at hiding atmospheric composition behind an aerosol screen.
The data also demonstrate JWST's capability for transit timing variations — deviations of the transit midpoint from the predicted time, which can reveal additional planets perturbing the observed planet's orbit. Timing variations exceeding 20 minutes are detectable for this system, though none were observed.
Some planets speak through their spectra. This one stays silent — and the silence itself constrains what kind of world it is.