The Doppler Technique for finding Sub-Stellar Objects
Doppler Shift
What happens when a train or ambulance passes us? Moving towards us,
the pitch is high (blue-shifted), after it passes the pitch is low (red-shifted).
The speed of sound is 740 miles/hour ---fast, but not so fast that our ears can't
detect the shift. We do not normally see a
Doppler shift in light, however. Light travels 300,000 km/second = 675
million miles/hour.
The Unseen Planet
When one object is a dark, yet massive planet, then we
see only the spectrum of the star. The star is seen to wobble or
shift, red, blue, red, blue, indefinitely, with the period of the star-planet
orbit.
The more massive the planet the bigger the wobble.
The closer the planet, the shorter the period.
If the planet were Pluto-Sun distance, we'd have to
wait 250 years for a full velocity shift!
Typical velocity shifts are tens of meters/second --one part
in 300 million.
The short period of this system shown below (just a few days) makes it easy to find.
See other orbits here
(then click on the Star Name to see the radial velocity graph).
