Inner system architectures: The role of gas giant planets in system evolution. Blue wavy lines represent the habitable or “liquid water” zone; the gray wavy line represents the ice line. The location of these boundaries varies according to the central star's luminosity. Top to bottom:

1. Based on a simulation by Levison & Agnor (2003), representing a system without any gas giants. The outer planets have extremely eccentric orbits.
2. An alternative outcome for the Solar System, based on Run 26 by Thommes et al. (2008). With Jupiter orbiting as usual at 5.2 AU, 5 planets instead of 4 form inside the ice line, and the innermost is considerably more massive than Mercury.
3. Based on Simulation JD-5 by Mandell et al. (2007), in which a gas giant migrates to 0.2 AU, a rocky Super Earth forms at about 0.1 AU, and a chain of rocky and icy planets on highly eccentric orbits extends beyond the ice line.
4. Based on the actual configuration of the Mu Arae system, with 3 gas giants and 1 ice giant orbiting a hot Sun-like star. The configuration of this system remains poorly constrained. Some investigators suggest highly inclined orbits for the outer planets (Short et al. 2008). Although terrestrial planets are impossible in the inner system, massive moons around the third planet (Mu Arae b) might sustain Earthlike conditions.

Inner system architectures: Three potential Solar Systems and one “anti-Solar System.” Blue wavy lines represent the habitable or “liquid water” zone; the gray wavy line represents the ice line. The location of these boundaries varies according to the central star's luminosity. Top to bottom:

1. The actual Solar System from Mercury to the Asteroids.
2. An alternative outcome for the Solar System, based on Run 15 by Thommes et al. (2008), in which two Earth-mass planets form in the habitable zone.
3. Another alternative outcome for the Solar System, based on Simulation 10 by Raymond et al. (2004).
4. Based on the actual configuration of the HD 74156 system, with 3 gas giants orbiting a bright, hot G0 star. The moons orbiting the outer planet are hypothetical. Evidently the gas giants' migration prevented the formation of terrestrial planets inside the ice line.