The perfect planet

Astronomers are at odds over what makes exoplanets “habitable.” These days, astronomers barely bat an eyelid when they find a new exoplanet: there have been nearly 2,000 discovered in the last 10 years, up from roughly 100 in the previous 10 years. Nonetheless, astronomers were intrigued by a distant world discovered in July of 2015 (1). The planet, named Kepler-452b, is among the most Earth-like exoplanets ever discovered. It is only about 60% larger than Earth and takes 385 days to orbit a star slightly older and larger than our Sun. The question on everyone’s mind: could Kepler-452b harbor life?To directly detect exoplanets, a starshade, such as in this concept drawing, would fly in formation tens of thousands of kilometers in front of a telescope. At ∼30 meters in diameter, the starshade would block starlight, creating a shadow and allowing only planet light to enter the telescope. Image courtesy of NASA/JPL-Caltech.The answer depends on whether a planet is in the habitable zone, often described as the narrow range of distances that a planet can be from its parent star, which would allow liquid water to exist on the planet’s surface. Kepler-452b’s host star shines a bit brighter than our Sun, so the standard account would place the exoplanet just on the edge of what’s considered a likely place to find lakes and oceans. But prospects for life on Kepler-452b improved when the researchers adopted a more expansive version of the habitable zone, acknowledging that it is an “evolving concept.” Their conceptual flexibility belies an ongoing debate among astronomers and astrophysicists about which planets could potentially harbor life.Traditional models of what’s inhabitable assume a planet not too different from our own: small, rocky, full of water, and with a thin atmosphere similar to ours. But are we being …