The Demographics of Rocky Free-floating Planets and their Detectability by WFIRST
Abstract
Planets are thought to form via accretion from a remnant disk of gas and solids around a newly formed star. During this process, material in the disk either remains bound to the star as part of either a planet, a smaller celestial body, or makes up part of the the interplanetary medium; falls into the star; or is ejected from the system. Herein we use dynamical models to probe the abundance and properties of ejected material during late-stage planet formation and estimate their contribution to the free-floating planet population. We present 300 N-body simulations of terrestrial planet formation around a solar-type star, with and without giant planets present, using a model that accounts for collisional fragmentation. In simulations with Jupiter and Saturn analogs, about one-third of the initial (∼5 M ⊕) disk mass is ejected, about half in planets more massive than Mercury but with a mass lower than 0.3 M ⊕, and the remainder in smaller bodies. Most ejections occur within 25 Myr, which is shorter than the timescale typically required for Earth-mass planets to grow (30-100 Myr). When giant planets are omitted from our simulations, almost no material is ejected within 200 Myr and only about 1% of the initial disk is ejected by 2 Gyr. We show that about 2.5 terrestrial-mass planets are ejected per star in the Galaxy. We predict that the space-borne microlensing search for free-floating planets from the Wide-Field Infra-Red Space Telescope will discover up to 15 Mars-mass planets, but few free-floating Earth-mass planets.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2017
- DOI:
- 10.3847/1538-4357/aa705b
- arXiv:
- arXiv:1704.08749
- Bibcode:
- 2017ApJ...841...86B
- Keywords:
-
- gravitational lensing: micro;
- methods: numerical;
- planets and satellites: dynamical evolution and stability;
- planets and satellites: formation;
- planets and satellites: terrestrial planets;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- Accepted for publication in ApJ, v2 fixes a few typos