The Exoplanet Population Observation Simulator. I. The Inner Edges of Planetary Systems

The Kepler survey provides a statistical census of planetary systems out to the habitable zone. Because most planets are non-transiting, orbital architectures are best estimated using simulated observations of ensemble populations. Here, we introduce EPOS, the Exoplanet Population Observation Simulator, to estimate the prevalence and orbital architectures of multi-planet systems based on the latest Kepler data release, DR25. We estimate that at least 42% of Sun-like stars have nearly coplanar planetary systems with seven or more exoplanets. The fraction of stars with at least one planet within 1 au could be as high as 100% depending on assumptions about the distribution of single transiting planets. We estimate an occurrence rate of planets in the habitable zone around Sun-like stars of η _⊕ = 36 ± 14%. The innermost planets in multi-planet systems are clustered around an orbital period of 10 days (0.1 au), reminiscent of the protoplanetary disk inner edge, or which could be explained by a planet trap at that location. Only a small fraction of planetary systems have the innermost planet at long orbital periods, with fewer than ≈8% and ≈3% having no planet interior to the orbit of Mercury and Venus, respectively. These results reinforce the view that the solar system is not a typical planetary system, but an outlier among the distribution of known exoplanetary systems. We predict that at least half of the habitable zone exoplanets are accompanied by (non-transiting) planets at shorter orbital periods, hence knowledge of a close-in exoplanet could be used as a way to optimize the search for Earth-size planets in the Habitable Zone with future direct imaging missions.