How Planetary Systems Evolve 

Our solar system might well have begun with more than nine planets. If so, what happened to those other worlds? It appears that some may have been evicted. Somewhere, deep in interstellar space there may drift lonely planets which once basked in the Sun’s light. Other young planets may have collided with their siblings. How does all this change occur? It’s a matter of gravity. 

Planets can collide or get ejected from a planetary system through gravitational interactions with sibling worlds. Every planet feels the gravitational force of all the other planets as well as that of its parent star. In the early years of a solar system’s evolution there may be enough planets to make these interactions very strong. These multiple gravitational pulls can drastically change the shape and even the nature of a planet’s orbit. As the planets tug on each other circular orbits can be distorted into elliptical ones. If three or more bodies interact strongly, enough energy can be transferred to one of the dance partners to send it hurtling out of the system. The laws of physics tell us that the energy and angular momentum (a quantity related to a rotation) of an entire system of planets must always stay the same. That means that worlds that do not get ejected must end up on very different orbits from the ones they started on. The change in orbits can also send one planet crashing into another. Our own moon may have formed from a collision between Earth and a Mars sized body. The asteroid belt between Mars and Jupiter may have been a planet at one time. Together all this change means that a solar system at the beginning of its life may look very different from its incarnation a few billion years later after the chaos has all but ended.  

There is one point that is really key to remember. At this point in its life, our solar system is amazingly stable. If you use this interactive to watch our solar system evolve you will see that the planets move happily in their orbits for as long as you care to sit there.