Review on habitability and biomarkers

The detection of O₂ or its product O₃ is our most reliable biomarker so far. The existence of H₂O in liquid state on the surface of a planet is considered essential for the development of life; even so, it is not a bioindicator. CO₂ indicates an atmosphere, and abundant CH₄ can indicate biological sources, although depending on the degree of oxidation of a planet's crust and upper mantle non-biological sources could also produce large amounts of CH₄. In the thermal part of the spectrum, the shape gives a measure of the temperature of the object examined. The mid-IR spectra can determine the planet's albedo, the temperature of the observable emitting regions and thus the planet's size. Visible to near-IR spectra offer higher spatial resolution for the same collecting area, are minimally affected by temperature and therefore able to determine the abundance of atmospheric species. However the visible/near-IR continuum does not give direct indication of the planet size because of the possible albedo range.