SDSU Astr 310 "Astrobiology"				Prof. W. Welsh

               The Doppler Effect and Finding Exoplanets
________________________________________________________________________

Q: What is the connection between atoms and light and finding exoplanets?

The sequence goes something like this (this looks much worse than it is!):


Part I: Light, Atoms, and Spectroscopy
- different colors of light have different energies.
- we can split light up into its colors using a prism or equivalent.
- atoms have electons around them in specific orbits.
- electrons can only gain or lose specific amounts of energy.
- the change in energy creates or absorbs light at specific colors.
- each element has its own "fingerprint" pattern of colors.


Part II: The Doppler Effect
- the motion of an object causes its light to very subtly change color.
- this is called the Doppler effect.
- when an object moves toward us, it is a tiny, tiny bit bluer.
- when an object moves away from us, it is a tiny, tiny bit redder.


Part III: Stars and their Motion
- when we split starlight into its colors, we can see the fingerprints
    of all the different elements in that star.
- if the star is moving, the fingerprint pattern shifts color.
- motion towards us shifts the pattern bluer; away from us is redder.
- if the star is moving back and forth, the fingerprint pattern moves
    back and forth (red then blue then red then blue...).
- a star doesn't normally move back and forth in space; if it does, it
    means something else is there causing the star to move - maybe 
    another star or perhaps a planet.


Part IV: Finding Planets via the Doppler Technique
- a star's gravity makes the planet orbit the star.
- a planet has gravity too, and it causes the star to orbit around too.
- even though stars are much too far away to see them wobble, we can
    detect the star's motion by measuring its fingerprint move back and
    forth in color.
- the amount of back and forth shifting in color tell us how much mass
    the planet has: a bigger planet creates bigger shifts.
- the amount of time it takes to shift back and forth tell us how long
    it takes the planet to orbit the star.

________________________________________________________________________


Q: Why don't we see color changes due to the Doppler effect?

The Doppler effect was first worked out for sound waves. We notice
the Doppler effect for sound all the time - it is the change in pitch 
when an object is moving fast and goes past us. The classic example was 
the sound of a train going by, but a car's horn is a more modern example. 
As a car zooms by, its horn drops from high to low pitch.

Sound is a wave, and the waves get bunched up when the object is coming
towards us, and the waves get spread out when it is moving away. The
compression of the waves means higher pitch or higher frequency.
Moving away creates longer waves and lower pitch.

Light is also a wave, so the exact same thing happens. But instead of
pitch, it is color. Moving towards us make the object bluer, and moving
away makes it redder.

We don't notice this color change in everyday life because the effect is 
so small.

For sound, the speed of sound is about 600 mph. So moving at 60 mph is
10% of the speed of sound, and the effect is easily noticeable. Even a 
few percent is noticeable.

But light waves travel at 670 million mph (!!), and nothing in our daily 
lives goes even a tiny fraction of that fast. So we don't pick up  the 
color changes with our eyes. But special instruments, called 
high-resolution spectrographs designed to measure Doppler shifts,
can measure this. Why? Because the speeds of astronomical objects are
increadibly fast! Even the motion of the Earth around the Sun is a
whopping 67,000 miles per hour (30 km/s). While this is still only about 
0.01% of the speed of light, it produces a measureably Doppler shift.
But nothing in our ordinary experience goes anywhere near this fast.
________________________________________________________________________


Q: Why can't we use the Doppler effect to find Earth-like planets?

Suppose we measure the Doppler shift for a star, and we notice that 
the star's speed changes back and forth, from postive to negative.
This tells us the star coming and going, and is almost certainly
orbiting something. That something might be another star (stars
frequently come in pairs), or perhaps a planet.

Massive objects like stars will cause a big change in velocity, while 
small objects (meaning "low-mass" to be technically correct) will cause 
a small change. So if we want to find planets, we need to find *small* 
back-and-forth changes in velocity.

Now if the planet is real small, like Earth-mass instead of Jupiter-mass, 
the back-and-forth change in velocity is practically too small to measure! 
So the Doppler method really, really struggles find Earth-mass planets 
around Sun-like stars. But with cutting edge spectrographs, and tons of 
observations, this endeavor is right at the edge of feasibility. Next 
generation instruments and telescopes will have a much better chance.


BUT... suppose the star itself is small (low mass). Then it will be 
easier to move around. So it will show a larger change in velocity, 
even if the planet is small. So we could find an Earth-mass planet 
around a low-mass star. Low-mass stars are called "M" stars, and they 
are very common: most stars are M stars. But, they are also very faint. 
And the Doppler shifts are hard to measure, due to technical reasons 
(the absorption lines are very blended). So carrying our a search and 
measuring the Doppler shifts is hard to do for large numbers of M-type 
stars - but it is possible. 
________________________________________________________________________ 
________________________________________________________________________