Here are a couple of (obsolete) theories on how the Solar System was formed which I think would be interesting if they were represented in Celestia:
Encounter Hypothesis
The planets were formed when a star passed by the Sun around 5 billion years ago. Hot gas was stripped off both stars, which cooled and contracted to form the planets. The other star’s material became Jupiter, Saturn, Uranus, and Neptune while the material from the Sun became Mercury, Venus, Earth, and Mars. The earliest hypothesis on how the Solar System was formed, it has the advantage of explaining why the planets orbit and rotate (except for Venus) in the same direction (derived from the geometry of the encounter) and explains why the terrestrial planets are denser than the gas and ice giants.
Conflicting evidence: this scenario is wildly unlikely
Band structure model
Swedish astrophysicist Hannes Alfven proposed the band structure model in which he proposed the existance of four molecular clouds by the Sun. There was an A cloud, which contained mostly helium and iron, a B cloud, mostly hydrogen, a C cloud, mostly carbon, and a D cloud, made of silicon and iron. The A cloud condenses to form Mars and the Moon, the B cloud condenses to form Jupiter, Saturn, Uranus, and Neptune, the C cloud condenses to form Mercury, Venus, and the Earth (or the B cloud forms Mercury, Venus, and Earth while the C cloud forms the outer planets), while the D cloud collapses to form Pluto and Triton.
Capture theory
The Sun’s gravity completely ripped apart a nearby forming low-density protostar with a diameter 4,000 times greater than the Sun today. The Sun’s gravity ripped off the protostar’s diffuse material, which condensed to form the planets. A later version added a collision concept:
1. As the protostar is ripped apart by the Sun, its material originally forms six planets, in order: “A”, “B”, Jupiter, Saturn, Uranus, and Neptune.
2. “A” and “B” collide.
3. “A”, with a mass twice that of Neptune, is ejected out of the Solar System.
4. “B”, which is a third of the mass of Uranus, is shattered.
5. The two major fragments of B become the Earth and Venus.
6. The other fragments become the asteroid belt and comets.
7. Mars, the Moon, and Triton are the ex-satellites of body A.
8. Mercury is either a minor fragment of B or a satellite of A.
Conflicting evidence: this theory suggests that the Sun is older than the planets; the Sun and planets are estimated to be the same age.