The direct detection of extant life has not been attempted by NASA since the Viking Missions in the late 1970s. The Ladder of Life Detection was generated to stimulate and support discussions among scientists and engineers about how one would detect extant life beyond Earth within the practical constraints of robotic space missions. The Ladder draws from lessons learned from previous attempts at detecting life. These past attempts have provided criteria for measurements to constitute convincing evidence for life (columns with blue and orange headers). The Ladder summarizes features of life as we know it, how specific they are to life, and how they can be measured. These features (rows) are sorted in a general sense based on their likelihood of indicating life. More information about the rationale behind the Ladder is provided in a peer-reviewed publication freely available at https://www.liebertpub.com/doi/full/10.1089/ast.2017.1773.

The Ladder of Life Detection is not intended to endorse specific biosignatures or instruments for life-detection measurements, and is by no means a definitive, final product. It is intended as a starting point to stimulate community discussion, debate, and further research on the characteristics of life, what constitutes a biosignature, and the means to measure them. For example, there is room for debate regarding the specific order of each feature in the Ladder, which is highly dependent on the environment in which the measurements would be made.