Exoplanet habitability

Exoplanet habitability concerns whether planetary conditions could sustain liquid water and life, while recognizing that observability and habitability are distinct questions.

The circumstellar habitable zone is the orbital range in which a specified planet and atmosphere can maintain surface liquid water. Its boundaries depend on stellar spectrum, albedo, greenhouse physics, clouds, planetary mass, and atmospheric inventory.

A runaway greenhouse occurs when outgoing longwave radiation reaches a limiting value while absorbed stellar energy is larger, causing ocean evaporation and rapid surface warming.

At the maximum-greenhouse limit, adding carbon dioxide no longer warms efficiently because condensation and Rayleigh scattering offset its greenhouse effect. This defines a common conservative outer habitable-zone boundary.

A Hycean planet is a proposed sub-Neptune with a hydrogen-rich atmosphere above a liquid-water ocean. The category is a model hypothesis rather than evidence of habitation.

This is the remote planetary specialization of a biosignature.

An exoplanet biosignature is an observable gas, surface feature, temporal variation, or chemical disequilibrium whose biological explanation survives plausible abiotic alternatives and environmental context.

An atmospheric gas or gas combination whose abundance and environmental context can support a biological interpretation. Detectability, a plausible production flux and exclusion of significant abiotic alternatives matter together. Modern-Earth molecular oxygen and nitrous oxide have predominantly biological sources, but an oxygen biosignature false positive illustrates why extrapolation to another planet requires context.

Nonbiological molecular oxygen can accumulate when photochemistry and planetary redox sinks allow it, for example after water photolysis and hydrogen loss. Ozone produced from that oxygen is not independent proof of a biological source. The stellar spectrum, water inventory, atmospheric escape and other gases constrain these interpretations.

For transmission through $N_H$ scale heights, a useful estimate isSmall host stars, low gravity, high temperature, and low mean molecular mass make atmospheric features larger.

For a thin hydrostatic isothermal atmosphere, $\Delta D\simeq 2R_p{N}_{H}H/R_{\star }^2$ and $H=k_{B}T/(\mu m_{u}g)$. Hence $T\simeq \mu m_{u}g\Delta D{R}_{\star }^2/(2k_B{R}_p{N}_H)$. A measured exoplanet transmission spectrum amplitude constrains $N_{H}H$, so gravity, mean molecular mass and the number $N_H$ of sampled atmospheric scale heights are additional inputs. In a clear fixed-composition model, $N_H\simeq \log ({\kappa }_{\mathrm{band}}/{\kappa }_{\mathrm{cont}})$; clouds or abundance gradients can change it.