Transmission spectroscopy of sub-Neptunes was expected to reveal their compositions and hence origins, yet many show flat near- to mid-infrared #spectra.
Such spectra can be explained either by metal dominated #atmospheres or by high-altitude, grey #aerosols.
Observations of escaping hydrogen and helium from several of these #planets rule out metal dominated atmospheres, while homogeneous distributions of small aerosols cannot produce flat spectra and large particles require unphysically high production rates.
Alternatively, modestly optically thick clumps at high altitudes can produce flat spectra even with small particles and physically realistic production rates.
Clumping increases the effective photon mean-free path while reducing wavelength dependence, allowing the #aerosol distribution to behave as an effective grey absorber.
Clumpy aerosol distributions naturally resolve the tension between flat spectra and low-metallicity atmospheres and may be a common feature of sub-Neptune #exoplanets.
#astronomy
https://astrobiology.com/2025/11/resolving-the-flat-spectrum-conundrum-clumpy-aerosol-distributions-in-sub-neptune-atmospheres.html