The last several years have made it clear we are living in a golden era of extrasolar planet studies. Less than 20 years ago, astronomers discovered the first planets orbiting Sun-like stars. Now, the number of exoplanets is in the thousands, including a growing number of “Earth-like” planets, a designation based on some combination of the planets’ mass, radius, and orbit around their stars. Just this past Friday, for example, astronomers reported detecting three planets between 1.5 and 2.4 times the radius of the Earth orbiting a single star, one of which lies in the star’s habitable zone, where liquid water could exist on its surface.
We don’t know, however, if these or other Earth-like planets are really like the Earth in the characteristics that really count: whether they have atmospheres like the Earth, oceans of liquid water like the Earth, and life like the Earth. Those determinations are largely beyond the capabilities of ground- and space-based observatories in operation today. A new generation—arguably, generations—of instruments and telescopes will be needed to determine just how Earth-like these Earth-like worlds really are. And the ability to develop those instruments will depend, at least in part, on the ability of exoplanet scientists to come into agreement on what’s needed to enable that next round of discoveries.