With five kids, I often make sure to count and recount them whenever I take my family to a public place. I could make them all hold hands but they prefer that I take a regular inventory. So far, every inventory has resulted in the correct number. It seems the early solar system did not fare so well.
Looking at the solar system today, we see eight planets (sorry, Pluto). Four rocky planets—Mercury, Venus, Earth, and Mars—orbit closer to the Sun. Four gas giants—Jupiter, Saturn, Uranus, and Neptune—orbit much farther out. Scientists have always figured that this inventory of planets represents the basic components of the solar system at any point in its history. However, recent research indicates we may have started out with five gas giants, but as they migrated to their current positions, one of them was ejected into space.
In fact, such a scenario solves a long-standing problem in understanding our solar system formation. Astronomers know that the gas giants started out in different orbits and migrated to their current positions (image 1). Typically, this migration is thought to have occurred over millions of years with the gas giants generally moving from closer to more distant orbits. However, such a migration would have significantly disturbed the rocky planets—unless it happened more quickly.
Image 1. Orbits of gas giants before (left), during (middle), and after (right) migration. Image credit: en:User:AstroMark.
If the giants’ orbits moved on normal timescales, they would have passed through resonances that transferred angular momentum to the inner, rocky planets. Generally, these resonances destabilize the orbits of smaller planets. A different picture emerges if Jupiter experienced a strong interaction with another gas giant. In this latter case, the “migration” happens almost instantly, ejecting the other gas giant from the solar system, and leaving the inner planets undisturbed (image 2).1 This scenario is known as “Jupiter-jumping.”
Image 2. Simulation of a sudden ejection of a gas giant (at t=0 Myr) as the remaining gas giants quickly migrate to their current positions.
Gas giant migration most certainly happened early in the solar system’s history. Remarkably, this dynamic process played a critical role in ensuring Earth’s habitability. Yet it could have destroyed Earth’s ability to host life. The migration could have happened slower or the “Jupiter-jumping” scenario could have sent the ejected planet through the inner solar system, thus, disrupting the rocky planets. The fine-tuning of the gas giants’ migration represents another example of powerful processes designed to enhance, rather than destroy, Earth’s capacity to support life.