The Solar System today is a very stable place. Every planet has a well-defined, nearly circular orbit (except Pluto, which possibly is a planetoid anyway), maintaining a respectful distance from the neighboring planets. But according to a new computer simulation, it was not always like this.
The new simulation uses Chaos Theory to solve certain nagging problems about the formation of the Solar System. The research traces three seemingly unrelated phenomena - the giant planets' (Jupiter, Saturn, Uranus and Neptune) orbits, craters on the Moon, and the behaviour of certain asteroids - to the motions of these giants nearly four billion years ago.

Solar System Salad:).. How many can you identify? (Courtesy: NASA)The work answers the following important questions.

• How did the intense bombardment of the Earth and the Moon start (3.9 billion years ago), that filled the Moon with large lava basins, and delayed the beginning of life on Earth?
  
• Why did Jupiter and Saturn leave their circular orbits and take on the more oval paths seen today, and how did their orbits became so tilted compared to other planets?
  
• Why does Jupiter share its orbit with thousands of asteroids that precede and follow it around the sun?
  

According to the researchers, who are publishing their work in three papers in Nature, the four gas giants originally formed in 10 million years within the current orbit of Uranus. Surrounding them in a ring were several thousand rocky objects called planetesimals, left over from the formation of the planets. Due to gravity, after 700 million years, Saturn had migrated outward and Jupiter inward to the extent that they reached a resonance point. This means they began to march in lockstep with each other, with Jupiter completing two orbits around the Sun for every one of Saturn's. The resonance allowed the pair to greatly disturb the orbits of the other planets :).
In the model, Jupiter and Saturn hurl Uranus and Neptune outwards like bowling balls into a sea of planetesimals, which scatter like pins. Asteroids are hurled inwards towards the smaller planets including the Earth, causing the so-called Late-Heavy-Bombardment (LHB) that rains meteorites on Earth and Moon. Samples of lunar rocks collected by astronauts had dated the impacts at about 650 million years after the formation of the Solar System. Some planetesimals bounce off Saturn's gravity, and get trapped in Jupiter's gravity well, and these (so-called) Trojans are still preceding and following Jupiter in its orbit :D.
This research is elegant, in that it ties together three seemingly disparate phenomena. More studies are needed, but it does seem that three problems have been solved in one master stroke :):).