Planets are often unstable after their creation. They will frequently change orbitsbecause their orbits are continuously altered by the gravitational pull of other planets. In 2005, three groups of researchers used this theory to propose the Nice Model of the formation of the solar system.
Right after the big bang, the gravitational pull of other planets made Uranus and Neptune swap orbits and sent Jupiter and Saturn farther away from the Sun. Jupiter also supposedly moved closer to the Sun before returning to the outer solar system.
The Nice Model was accepted as true until it was partly disproved in 2011. At that time, some scientists said there had to be a fifth planet between Mars and Jupiter for it to be true. However, they added that the planet was probably flung out of the solar system by the gravitational pull of either Saturn or Jupiter.
In 2015, other scientists disproved the Nice Model because it did not explain the creation of the inner planets (Mercury, Venus, Earth, and Mars). They said that Jupiter would have cleared the inner planets, particularly Mercury and Mars, if it had ever strayed into the inner solar system.
The four inner planets would have formed long after the four outer planets if the Nice Model were true. Or they could be the survivors of Jupiter’s apocalypse. This means that the other inner planets were flung farther into the solar system along with one or two planets from the outer solar system