This search takes many forms, including the exoplanet searches, SETI and exobiology.
L is the length of time such a civilisation remains detectable.
However, it is not known what fraction of these stars would also form planetary systems, and to answer this question is one of the aims of current exoplanet searches. These are described in more detail here. Bioastronomy is a term which is applied to the investigation of the term n e , which is itself closely linked to exobiology. In order to estimate what range of planetary conditions could be hospitable, we must discover what conditions known lifeforms can adapt to and thrive in, and what conditions are too extreme. Some exobiologists look at the problem in another way.