Physics 101- Introduction to Mechanics
April 8- Center of Mass & Conservation of Momentum






  Extrasolar Planets

So, how do we "see" distant planets orbitting other stars?

The planets are too far away to see with ordinary optical telescopes.  (Think about it- this would be like us trying to look for a "firefly next to a nuclear blast".  The relative brightness between the two objects is many orders of magnitude.)

The center of mass for the planet- star system is fixed (there are no external forces).  As the two orbit the center of mass (not the planet orbitting the star!), the star's position will be perturbed or wobble. By measuring this perturbation over several years, one can calcualte how big and how far away a planet would have to be to generate such a wobble.

The below animated GIF illustrates the planet's orbital affect on the position of its parent star.

(Note: affect is greatly exagerated and the green cross is the center of mass of the system)

Okay so how do we actually "see" this perturbation?  Again, the motion would be too small to notice in a telescope.

The perturbation can be measured by measuring its parent star's spectrum. The light from a star can be broken down into its constituent colors and analyzed. This spectrum contains dark and light lines which are specific to the elements, ions and molecules which make up the star. Essentially this is a chemical fingerprint of the star and it can be accurately measured.

The Doppler Effect is the physical effect of waves created by a moving source that causes them to be compressed when approaching an observer and spread out when the wave has past the observer (like the change in the pitch of a train's whistle as it comes into and then leaves the station).  This Doppler Shift applies to light also.  Light that is captured when the star wobbles toward the earth looks more blue and redder when it is moves away from the earth.  By measuring the movement of the spectral lines over a period of time the orbital period of the planet can be determined. The resulting size of the displacement then allows the mass of the planet to be calculated.

The below animated GIFs illustrate the Doppler Shift of starlight caused by a perturbing planet.

The grey lines are the hypothetical spectra from a stationary source.

The red lines are due to the star wobbling away from the earth.

The blue lines are due to the star wobbling toward the earth.

This method has shown that planets do exist outside our solar system! 

Geoff Marcy also has a site that talks about extrasolar planets in greater depth.  There you will find a constantly updated list of the discovered planets, readable articles and much more!



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Jeff Phillips
Loyola Marymount University
Spring 2002