La Tierra vista como exoplaneta
Keywords: Astrobiology, exoplanets, habitable planets, biosignatures
AbstractEarth, as the only example of a habitable world, offers the first elements to characterize the spectra of terrestrial planets around other stars. Those planets may be detected in the next decade by missions like CoRoT and Kepler, and characterized by Terrestrial Planet finder and Darwin. In this paper, I reviewed the research that uses Earth to determine the possible characteristics of habitable worlds around other stars. Comparing Earth’s characteristics with those of the terrestrial planets in the Solar System, the main properties of a habitable world have been determined. A habitable planet must have atmosphere, liquid water and the right size to keep that atmosphere and to maintain tectonic activity for long geologic periods. A habitable world could be recognized as such by the detection of biosignatures on its spectrum. Simulations of past and present Earth-like atmospheres and the knowledge of the geological evolution of our planet indicate that oxygen (O2) is an excellent signature of life, in particular if it comes along with compounds like methane and nitrous oxide. Also, the pigments used by photosynthetic organisms could generate a signature in a planet’s spectrum. This signature may be similar to the chlorophyll absorption on Earth. Earthshine observations help to analyze the disk average spectrum of our planet and to determine the changes of the biosignatures given certain conditions of illumination and geometry. From such observations and models that generate disk averaged spectra of Earth it has been found that clouds are the biggest challenge to identify biosignatures and characteristics of the planetary surface in general. The atmospheric abundance of the compounds produced by life depends on the amount of ultraviolet radiation received by the planet as it drives most of the atmospheric chemistry. This radiation depends on the stellar type of the planet’s parent star. The characterization of terrestrial planets requires the knowledge of the target star properties (age, effective temperature, radiation emitted from the ultraviolet to the infrared), as well as to build spectra libraries that allow recognizing habitable worlds from those that are not.