How do you Find Worlds beyond the Solar System?
- Just look in a telescope?
- If Jupiter were in orbit around a nearby star the Hubble
Space Telescope is sensitive enough to detect the small amount of light it
- HOWEVER the glare from the bright star a
hair's-breadth away would completely wash out the feeble light from
- Planets can be revealed by detecting the influence they have upon their star.
- As a planet orbits its star, Newton's third law
(action/reaction) requires that the star also orbit the planet.
- Two objects in orbit about one another orbit about
their common "center-of-mass".
- This point is called the barycenter of the
- If you could put both objects on a seesaw, the
barycenter is the place where you locate the pivot
so that they would balance.
- When Jupiter orbits the Sun, the Sun is displaced by
nearly its diameter (once every 11 years).
- The motion of the Sun induced by Jupiter has two observable
- The Sun wiggles back and forth in space a distance equal
to its diameter every 11 years.
- If viewed from a nearby star the amount of motion is
equal to the diameter of a dime as viewed at a distance of 200
- The Sun moves toward and away from a distant observer
with a velocity of about 20 meters/second.
- This motion is observable by measuring the "Doppler Shift"
of light coming from the Sun.
- The effect is tiny -- if we were measuring sound waves to
the same precision you would be able to detect the change in pitch of
something moving toward you at a velocity of one inch per hour.
- In principle, both of these effects can be detected by modern
- In practice, it's much easier to measure the Doppler effect.
- To date several hundred other planets have been detected using
the Doppler method and thousands of candidates have been detected by the Kepler mission via the transit method.