Exoplanet cloud

An exoplanet cloud is a suspended condensate aerosol that scatters or absorbs radiation. Its particle sizes, composition, vertical extent, and horizontal coverage shape transmission, reflection, and thermal-emission spectra.

Condensation followed by gravitational settling removes chemical elements from an atmospheric layer. The remaining gas may be in local chemical equilibrium with a depleted elemental inventory, while the column is not a closed equilibrium system.

A sufficiently cold layer condenses a volatile or refractory carrier and limits its transport to higher layers. Water depletion above the terrestrial tropopause and titanium-bearing condensates in some hot Jupiter models illustrate this process.

An exoplanet cloud deck is an optically thick cloud layer whose top pressure fixes the deepest level visible over a wavelength interval. A high deck can truncate a transmission spectrum and mute molecular features.

With constant gravity and mean molecular weight, hydrostatic equilibrium gives $P_c=P_s\exp (-H_c/h)$, where $h$ is the atmospheric scale height. A flat exoplanet transmission spectrum alone does not determine $H_c$ or a unique cloud-top pressure.

With constant atmospheric scale height $H$, a molecular band at height $\Delta z$ above the measured continuum satisfies $P_{\mathrm{band}}=P_{\mathrm{cont}}{e}^{-\Delta z/H}$. If an opaque exoplanet cloud deck sets that continuum, then $P_c\simeq P_{\mathrm{cont}}$; the band pressure is not the cloud pressure. If the clear band height would be $NH$ above a clear continuum at $P_0$ and the cloud reduces the visible feature to $mH$, thenWithout the clear-atmosphere band opacity or $N$, the measured feature amplitude alone cannot locate the cloud top. This follows directly by subtracting the two hydrostatic height relations.

Atmospheric haze consists of small suspended particles, often produced photochemically. A haze can flatten molecular features or produce a wavelength-dependent scattering slope.