Looking for Planets in all the Right Places (Continued)

A large planet orbiting a star is kept in orbit by the gravitational pull between the two objects. If you swing a tennis ball on a string around you, you must pull in on the string to keep it moving in a circle. Now make the tennis ball heavier and you'll notice that you too are moving in a little circle. If the ball has the same mass as you, then you both move in circles around a common "center of mass." Now if our planet is heavy enough, its parent star is also pulled around in a little circle. This little circular motion is what Marcy and Butler are detecting, and here's how they do it:

Any object that emits waves will show a shift in frequency (pitch) if that object is moving towards or away from you. You can hear this when a car with the horn blowing passes you. This change in pitch is called the "doppler effect." Click here to hear the doppler effect of a car horn moving past you at 30 miles per hour.

Light waves show this same shift in frequency if the source of light is moving towards or away from the observer. When you look at the light from a star and pass that light through a prism, the light spreads out into a spectrum. This spectrum is crossed by many dark lines where the light is being absorbed by cool elements in the outer atmosphere of the star. It is these lines that will shift in position if the star is in motion. The lines shift towards the red end of the spectrum if the star is moving away from us, and towards the blue end of the spectrum if the star is moving towards us.
 <-- Moving toward
if shifted this way

Hydrogen Absorption Spectrum
Courtesy of Dan Bruton's Color Page
 Moving away -->
if shifted this way

Because Marcy and Butler are looking for a star moving in a small circle, they look for a shift that first moves one way and then the other. The length of time this back-and-forth motion of the spectral lines takes gives them the orbital period of the planet and star. The magnitude of the shift (how fast the star is moving) tells them something about the mass of the planet.

Now, I'm making this sound easier than it really is. The measurement must be very, very precise to actually find the planet. The accuracy of measurement is plus or minus 3 meters per second. This is a vanishingly small shift in the spectral lines that is masked by all sorts of other motions. You must remember that the earth is moving in its orbit around the sun at a velocity of about 30,000 meters per second (10,000 times the velocity they are looking for!) Add to this the earth's rotation at about 380 meters per second (at the telescope), plus the pulls from the moon and other planets in the solar system, and you have quite a mess to untangle! Using sophisticated computer analysis and comparison with spectra produced right at the telescope, they are able to tease out the accurate values they need. Absolutely remarkable!

  What are these planets like?
All of the planets discovered thus far are probably large gas giants, much like Jupiter in our solar system. The reason for this has to do with the method used for detecting the planets. First, a small, earth-sized planet would pull so weakly on a sun-like star that the velocity of the star would be too small to detect with the Marcy/Butler method. This means that only large planets will be seen. The closer that a planet is to the parent star, the stronger the pull, and the larger the effect. Hence it's easier to detect close-in, big planets. The farther a planet from its star, the longer it takes to orbit. Jupiter takes 12 years to orbit the sun. Since Marcy and Butler have only been observing since 1987 (a little under 10 years,) they haven't had the time to see the farther out planets yet.
This artists rendering of what the planet orbiting Pegasi 51 may look like. This image comes courtesy of Robert Casey from the 51 Peg web site.
Geoffrey Marcy, is a Professor at San Francisco State University.

 So, most of the planets discovered so far are very large and very close in. Some of these planets orbit frighteningly close to their parent star. A few orbit their star in 4 days or less! Our closest planet, Mercury, takes 88 days to orbit the sun. These planets must be real scorchers! There is much debate over the origin of these bodies and how they can hold on to an atmosphere at such a close distance. A couple of these new planets orbit a little farther out than the earth's distance from the sun, about at the distance of Mars. Although these bodies would be rather cold, there is still a possibility that liquid water, necessary for life as we understand it, can exist.

What does the future hold?

We've only seen the beginning. As Marcy, Butler, and other astronomers refine their techniques and observe for longer periods, and as other methods begin being employed, we will certainly be treated to many more discoveries, each more spectacular than the one before. It's certainly nice to know that Nature is busy out there trying to make more worlds, each with different characteristics and possibilities. Hopefully we'll find some new friends someday!

Related Sites:

Searching for Extrasolar Planets You've listened to the interview, you've read the story, now visit Geoff's homepage. The latest on extrasolar discoveries.

Other 'Solar' Systems? An excellent resource for further exploration into planets outside our solar system. This site has a great collection of links.

Peg 51 A great page to learn about the first discovery on an extra-solar planet.

Known Planetary Systems This site contains a listing of all known planetary systems, along with additional information about each find.

NPR's Talk of the Nation Science Friday RealAudio interviews with Geoffrey Marcy, and others. Originally aired October 27, 1995.

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