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.