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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
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Hydrogen Absorption Spectrum
Courtesy of
Dan
Bruton's Color Page
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Moving away -->
if shifted this way
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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
!
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What are these planets like?
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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.
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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.
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Geoffrey Marcy, is a Professor at San Francisco State University.
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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
!
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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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