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Wobble
Planet hunters have a tough task ahead of them. Planets around
distant stars are not visible because a star's light is far
brighter than any orbiting planet - typically by about a billion
times - and there's too much glare to discern a new extrasolar
planet. So astronomers fall back on an indirect method for
detecting planets. They look at wobble.
Stars
aren't stationary, they rotate as they revolve through their
galaxies. A star on its own rotates around its axis, which
is its center of gravity. But stars and orbiting planets exert
a gravitational pull on each other. When a planet is present
a new center of gravity is established for the star system.
Instead of rotating around its axis, the star will rotate
around a point a tiny bit closer to the planet's orbit. With
every planetary revolution, the star moves a tiny bit further
away from us observing on earth and then a tiny bit closer.
Even though astronomers can't see the planet itself, they
can detect this "wobble" in the star's movement and infer
the planet's presence. To figure out what's going on, they
rely on differences in the type of light that reaches us.
The
technique is based on the Doppler Effect that light waves
traveling to earth from a distant star experience. If a star
is tugged a tiny bit away from earth by an orbiting planet,
the light it emits will appear a bit redder because the light
waves are stretched out. On the other hand, if an orbiting
planet pulls the star toward earth, the light it emits will
appear a bit bluer because the light waves are condensed.
Astronomers collect this data from a star and then compare
the color of its light over time. More than 100 planets orbiting
other suns have been discovered so far.
Gravity
Gravity is the force of attraction between any two things
that have mass. That force is related to the mass of the objects.
You probably have heard the apocryphal story about how Isaac
Newton hit on the laws of gravity when an apple fell on his
head. He extrapolated the idea that there was an attractive
force that pulls objects toward the Earth's surface to say
that this attractive force must also hold the cosmos in order,
keeping the planets in orbit around the sun and so on.
 But
why doesn't gravity pull the planets into the sun? Each planet
also has velocity in a direction perpendicular to that gravitational
force. If there were no sun, each would be traveling on a
straight line through space. That 'forward' velocity is left
over from when our solar system first formed from a cloud
of interstellar debris.
The
strength of the force of gravity is related to the mass of
the objects involved, as well as the distance between them.
Planets revolve more slowly around the sun the further they
are from it. So Mercury's velocity is greater than that of
Saturn.
The
same phenomenon was expected to be seen in rotating galaxies,
with matter on the periphery moving more slowly. But it was
the discovery that objects on the outer edges of these galaxies
is going just as fast as those toward the center that got
scientists thinking seriously about dark matter. For the laws
of physics to hold true, there needs to be more mass out at
the edges of these galaxies to counteract the vast distance
in to the gravitational center. So astronomers believe there
must be some matter out there that we can't see - dark matter
- that is causing this gravitational acceleration. 
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