Exoplanet cloud

An exoplanet cloud is suspended condensed material in an exoplanet atmosphere. Condensation makes particles when local composition and temperature permit; mixing can keep them aloft while atmospheric condensate rainout removes them by settling. The resulting distribution depends on transport as well as on local chemical equilibrium.

Particles absorb and scatter radiation, changing the reflected brightness, thermal spectrum and exoplanet transmission spectrum. An opaque exoplanet cloud deck hides deeper molecular absorption, while a thinner cloud can leave wavelength-dependent structure. Particle size, composition and coverage therefore matter as much as the statement that a cloud is present. A weak gas band can indicate obscuration rather than low molecular abundance, making clouds a central uncertainty in atmospheric retrieval.

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 that scatter or absorb radiation, often formed through atmospheric photochemistry rather than direct condensation of a locally abundant vapour. In an exoplanet atmosphere, transport, growth and settling determine the particles' altitude and size distribution.

A haze can add a wavelength-dependent continuum to an exoplanet transmission spectrum and conceal molecular bands. The scattering slope of a transmission spectrum connects an idealized extinction law with the atmospheric scale height, but a measured slope alone does not establish a particular haze composition. Stellar contamination of a transmission spectrum can also alter the slope. Distinguishing these explanations requires spectral coverage and a joint model of the star and planet.