Euhabitability: A New Term to Aid in the Search for Life in Addition to the Habitable Zone Framework.

The search for life on exoplanets is difficult due to observational limitations stemming from vast separating distances. The Habitable Zone (HZ) is a simplifying framework for identifying exoplanets that may be suitable for life, given our observational limitations. The HZ framework is built on many simplifying assumptions and only attempts to bound the circumstellar regions where surface oceans are not precluded. In other words, HZ exoplanets are where surface oceans are possible, not necessarily where life is possible. Water is essential for life as we know it, however, life requires additional factors such as nutrients and a magnetosphere. Therefore, the criteria of the HZ framework is one step removed from assessing the suitability for life. This nuance is easily lost in translation and has caused confusion among researchers and the general public; as the term "Habitable Zone" implies that planets in the HZ are habitable and those outside are uninhabitable. Here we define a new term, euhabitable (suitable for life), that addresses this confusion. In brief, a euhabitable exoplanet can exist outside the HZ (e.g., a Europa-like planet), and an uninhabitable exoplanet can exist within the HZ (e.g., a desiccated planet). The "euhabitable" term can be used to discuss these cases explicitly, avoiding confusion between an exoplanet's suitability for life and it's position relative to the HZ. The euhabitable framework is informed by the advancements in astrobiology over the past three decades, and primes exoplanet science for the future by moving the habitability discussion beyond "follow the water."