Sven-Erik (177541) writes "The solar system once had five giant gaseous planets rather than the four it has today. That's the conclusion from a computer simulation of the solar system's evolution, which suggests the fifth giant was hurled into interstellar space some 4 billion years ago, after a violent encounter with Jupiter.
Astronomers have struggled for decades to explain the solar system's current structure. In particular, Uranus and Neptune couldn't have formed where they are today – the disc of gas that congealed into the planets would have been too thin at the edge of the solar system to account for the ice giants' bulk.
A more likely scenario is that the planets were packed close together when they formed, and only spread out when the disc of gas and dust from which they formed was used up. The tighter orbits of extrasolar planet systems support this idea."