The Invisible Majority? Evolution and Detection of Outer Planetary Systems without Gas Giants

TL;DR

This study models the formation of outer planetary systems without gas giants, predicting the existence and detectability of such planets through various observational methods and their potential prevalence.

Contribution

It introduces a comprehensive numerical model of planet formation without gas giants, exploring system stability, planet characteristics, and detection prospects.

Findings

A 5-9 Earth mass planet forms near the ice line.

Predicted planets have low eccentricities, unlike giant exoplanets.

Inner two-planet systems are typically unstable or do not form.

Abstract

We present 230 realizations of a numerical model of planet formation in systems without gas giants. These represent a scenario in which protoplanets grow in a region of a circumstellar disk where water ice condenses (the "ice line''), but fail to accrete massive gas envelopes before the gaseous disk is dispersed. Each simulation consists of a small number of gravitationally interacting oligarchs and a much larger number of small bodies that represent the natal disk of planetesimals. We investigate systems with varying initial number of oligarchs, oligarch spacing, location of the ice line, total mass in the ice line, and oligarch mean density. Systems become chaotic in ~1 Myr but settle into stable configurations in 10-100 Myr. We find: (1) runs consistently produce a 5-9 Earth mass planet at a semimajor axis of 0.25-0.6 times the position of the ice line, (2) the distribution of planets' orbital eccentricities is distinct from, and skewed toward lower values than the observed distribution of (giant) exoplanet orbits, (3) inner systems of two dominant planets (e.g., Earth and Venus) are not stable or do not form because of the gravitational influence of the innermost icy planet. The planets predicted by our model are unlikely to be detected by current Doppler observations. Microlensing is currently sensitive to the most massive planets found in our simulations. A scenario where up to 60% of stars host systems such as those we simulate is consistent with all the available data. We predict that, if this scenario holds, the NASA Kepler spacecraft will detect about 120 planets by two or more transits over the course of its 3.5 yr mission. Future microlensing surveys will detect ~130 analogs over a 5 yr survey. Finally, the Space Interferometry Mission (SIM-Lite) should be capable of detecting 96% of the innermost icy planets over the course of a 5 yr mission.