Figure 1 : The inner Solar System and outcomes of simulations of its formation.

From: How the Solar System didn't form

Figure 1

In these cartoons, distances are roughly to scale, but the sizes of the planets relative to the Sun have been increased. a, The inner Solar System contains the terrestrial planets (Mercury, Venus, Earth and Mars) and the asteroid belt. Asteroid orbits can be inclined by up to 30° to the ecliptic (Earth's mean orbital plane). b, Simulations of the Solar System's formation in which material in the nascent system has a 'shallow' density profile (that is, planetary embryos can be found within 4 AU of the Sun; 1 AU is the distance from the Sun to Earth) typically result in an Earth-sized 'Mars' and asteroid orbits 'excited' at sizeable inclinations; Mercury is often absent. Embryos may get stuck in the asteroid belt for long times, eventually leading to a different belt from that now observed. c, Izidoro and colleagues2 report simulations of steep initial density profiles (embryos within 2 AU of the Sun). The size and orbital distributions of the terrestrial planets are well reproduced in these simulations, but the asteroid belt remains nearly flat.