It is foreseen that in the near future we will be able to measure the light from extrasolar planets similar to the Earth. When these data become available, a truly interdisciplinary approach to their analysis will be necessary in order to understand the physical properties of these worlds based on globally-averaged measurements. In this task, observation of the Earth (as the only inhabited planet that we know of) and the rest of the planets of the solar system will be our guide to interpret the observations. Here I make a brief review of the existing observations of the Earth seen as a planet.