Formation of Protoplanet Systems and Diversity of Planetary Systems

Abstract

We investigate the formation of protoplanet systems from planetesimal disks by global ( N = 5000 and 10,000 and 0.5 AU < a < 1.5 AU, where N is the number of bodies and a is the distance from a central star) N -body simulations of planetary accretion. For application to extrasolar planetary systems, we study the wide variety of planetesimal disks of the surface mass density Σ solid = Σ 1 ( a /1 AU) -α g cm -2 with Σ 1 = 1, 10, 100 and α = 1/2, 3/2, 5/2. The results are all consistent with the prediction from the "oligarchic growth" model. We derive how the growth timescale, the isolation (final) mass, and the orbital separation of protoplanets depend on the initial disk mass (Σ 1 ) and the initial disk profile (α). The isolation mass increases in proportion to Σ , while the number of protoplanets decreases in proportion to Σ . The isolation mass depends on a as a (3/2)(2-α) , which means it increases with a for α < 2 while it decreases with a for α > 2. The growth timescale increases with a but decreases with Σ 1 . Based on the oligarchic growth model and the conventional Jovian planet formation scenario, we discuss the diversity of planetary systems. Jovian planets can form in the disk range where the contraction timescale of planetary atmosphere and the growth timescale of protoplanets (cores) are shorter than the lifetime of the gas disk. We find that for the disk lifetime ~10 8 yr, several Jovian planets would form from massive disks with Σ 1 ≳ 30 with Uranian planets outside the Jovian planets. Only terrestrial and Uranian planets would form from light disks with Σ 1 ≲ 3. Solar system-like planetary systems would form from medium disks with Σ 1 ≃ 10.