A threshold was crossed in the late 20th century with the discovery of planets around other stars (e.g., Mayor & Queloz 1995). At this key point in history, humanity knows that exoplanets are both abundant and diverse — and the possible discovery of life-bearing worlds is within our grasp. This monumental objective demands powerful and flexible new tools, as well as application of multi-disciplinary scientific skills.
The next frontier is to extend our characterization capabilities to rocky exoplanets, including finding the “pale blue dots” in the solar neighborhood. With the right tools, we can determine whether those worlds have Earth-like surface conditions and probe them for signs of life. Focusing on the planetary systems most like the solar system, those with Earth-size exoplanets orbiting in the habitable zones of Sun-like stars, increases the chances of finding and recognizing biosignatures. Concurrently, we will nurture a new discipline — comparative exoplanetology — by studying a huge range of exoplanets and comparing them with the vastly better studied Solar System planets.
As preparation for the 2020 Astrophysics Decadal Survey (Astro2020), in 2016, NASA initiated studies of four large space telescope concepts. Two of these concepts — HabEx and LUVOIR — have the driving goal of finding and studying potentially habitable exoplanets around Sun-like stars, as well as enabling a wide range of revolutionary astrophysics and Solar System studies. Here we will outline the observational requirements for achieving these goals, describe the technical solutions proposed by the two concepts, and compare their capabilities and challenges. The results of these detailed mission concept studies, which spanned nearly four years, were presented to Astro2020 last year and are currently being considered for prioritization as NASA’s next Great Observatory.