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09/05/2000
Other Worlds
As I start this column, Brian Trumbore is enjoying the challenges
inherent in what, for a golfer, is another world, golfing at
Lahinch in Ireland. Astronomers have long been hoping to find
other worlds, planets outside our own solar system. Until 6 years
ago, the possibility of detecting any so-called "exoplanets"
seemed remote. However, advances in astronomy, and in the
cleverness of astronomers themselves, have led to today''s
bonanza of about 50 known exoplanets. These planets are
generally giant bodies of the size of Jupiter, or much larger.
More recently, however, some planets about the size of Saturn
have been found.
"Wobbles" are the key to the detection of these exoplanets. These
wobbles involve changes in velocity of a star due to its planet''s
gravity. It''s much like what would happen if I watched you
while you were playing with a child running around you while
tugging on a rope that you were holding. When the kid was
between you and me, you would be tugged toward me; when the
kid was on the other side you''d be tugged away from me. From
my perspective, you would wobble. As the exoplanet orbits its
star, the star will wobble. Actually, it''s a bit more complicated.
Let''s say that the star is moving away from the earth at a certain
speed. Now the wobble shows up as slight changes in that speed
as the star gets tugged toward and away from the earth by the
planet. Amazingly, astronomers can measure changes of only
36 feet per second in the velocity of a star moving toward and
then away from the earth. This is just a little faster than a
sprinter running the hundred-yard dash - and the stars are zillions
of miles away!
How do they do that? It''s the Doppler effect, exemplified by
the changing wavelength (pitch) of the sound of an auto horn as
it speeds by you. Instead of listening to sound waves, the
astronomers look at the light coming from the star. As the star is
tugged toward earth by its planet, the wavelength (color) of the
light will shift towards the blue region of the spectrum. When
the planet is tugged away from earth, the light will shift toward
the red. (The famous redshift led astronomers to discover the
expansion of the universe.) Very precise spectrometers are used
to measure the changing color of the light as the planet orbits the
star. To gather enough light to feed the spectrometers,
astronomers use powerful telescopes such as the ones on Mauna
Kea in Hawaii or at the Geneva Observatory in Switzerland.
With sophisticated computer software, it is also possible to
combine the light gathered by two or more telescopes. Not only
that, but with flexible mirrors and tricky computer control
techniques, the fuzziness or distortions introduced by our
atmosphere can be canceled out to give much sharper images.
As I said, astronomers are very clever!
But there are limitations. You can''t see the exoplanet - it''s too far
away. Because of that, you don''t know whether you''re looking at
the planet-star pair head-on, perpendicular to or at an in-between
angle to the planet''s orbit. If you plug in the math, this means
that you can only calculate a minimum value for the mass of the
planet. However, with more and more planets being found, you
can be reasonably sure statistically that you have a good idea of
the range of probable masses. To date, over half the known
exoplanets have minimum masses ranging from a fraction of a
Jupiter to 2 Jupiters. A little less than half are in the 2 to 6
Jupiters range and a handful of real giants are in the 6 to 10
Jupiters range. Jupiter, incidentally, is 318 Earth masses.
Naturally, the bigger the planet the bigger the pull on its star and
the bigger the wobble. It''s no surprise that the big guys were the
first planets detected. The challenge is to increase the sensitivity
and gather enough light to detect the wobble caused by little
biddy planets like the earth. In the works are projects to detect
velocity shifts as low as 3 feet per sec. This should be sufficient
to detect exoplanets of only 10 times the mass of the earth.
Meanwhile, there''s a lot to occupy us in our own solar
neighborhood. For centuries, Mars was the planet thought most
likely to harbor life. Much of the early speculation was brought
about by a mistranslation of the Italian word for ''channel'' as
''canal''. Canals, of course, implied intelligent life. Today, the
most exciting possibility of finding life on a solar body is not on
Mars, but on Europa, one of the moons of Jupiter. The Galileo
spacecraft''s observations of Europa have yielded spectacular
images of a surface of fractured water ice, generating immediate
speculation that underneath the frozen ice is an ocean. Just
looking at the pictures of the surface leads you to think there''s a
lot of breaking up going on. It''s not surprising. The gravitational
effects due to Jupiter''s mass should produce strong tidal effects
on Europa. While the surface features strongly suggest that
liquid water exists on Europa, they could arise from local melting
of the ice or flow of soft ice. Not that much is known about the
behavior of ice under such an environment.
In the August 25 issue of Science, Margaret Kivelson and her
colleagues at UCLA reported on magnetic measurements taken
from Galileo. These measurements were made on a number of
passes near Europa as Galileo orbited Jupiter. I found it
interesting that in the acknowledgments the authors thanked their
programmers for working overtime and on holidays to acquire
and process the data. They also thanked workers at the Jet
Propulsion Laboratory for designing a pass by Europa that
provided the data critical to their paper. This critical pass
occurred on January 3 of this year (no Xmas holidays?) and
provided the clincher, recently picked up by the media.
The clincher to which I refer is that the magnetic data are
consistent with the presence of a global "current-carrying outer
shell" beneath the icy surface of Europa. The data are consistent
with this current-carrying shell being a liquid ocean of salty
water, similar to our own, possibly 100 kilometers (around 60
miles) deep. With temperatures at the surface of Europa down at
levels that make a Minnesota cold snap look like a heat wave,
you might wonder how liquid water could exist. The most
plausible explanation is that the tremendous mass of Jupiter pulls
and stretches Europa hither and yon and all this deformation
generates enough heat to sustain an ocean. To confirm that the
current-carrying shell is indeed an ocean will require a spacecraft
to orbit Europa and monitor the tidal variations.
Jupiter itself is quite a piece of work. In the core of Jupiter, the
pressure is so immense that theorists believe that the hydrogen in
the core is solid metallic hydrogen. At the Lawrence Livermore
National Laboratory, they''ve succeeded in making liquid metallic
hydrogen. However, it only lasts for about a millionth of a
second! Amazingly, that''s enough time to determine its electrical
conductivity and prove it is metallic. Solid metallic hydrogen is
unknown to us earthlings and it''s highly improbable that we''re
going to dive into the center of Jupiter to mine the stuff.
Let''s turn to something more down to earth. Or rather, up to the
planet Neptune. Neptune is about 17 times heavier than the earth
and is a nice blue color with white clouds floating around in a
weather pattern where winds may blow at a thousand miles per
hour! The blue color isn''t due to water, but to methane gas. The
pressure is so great in the lower regions of the atmosphere that
we may have metallic liquid hydrogen the consistency of
pudding, according to an article in the September Discover
magazine. Sounds a little yucky to me!
But what has gotten Neptune more play in the media this past
year is some work by a graduate student, Robin Benedetti, at the
University of California at Berkeley. She put methane gas in a
pressure chamber and then shot in a laser beam to create the
temperature and pressure conditions expected about a third of the
way down towards Neptune''s center. Chemically, methane is a
carbon atom bonded to 4 hydrogen atoms. In Benedetti''s work,
the hydrogen atoms broke away, leaving behind the carbon
atoms. These carbon atoms, having nothing better to do, decided
to socialize with each other and form particles of dust. Not just
ordinary dust, but diamond dust! This spawned speculation that
it may be raining diamonds on Neptune! I don''t think DeBeers
has anything to worry about, however. Diamond mining on
Neptune is as likely as diving into the center of Jupiter.
More interesting to astronomers than diamonds is the influence
Neptune apparently had on the orbits of other members of our
solar system. It used to be thought that the solar system formed
from this big bunch of swirling gas and that the planets formed
and stayed pretty much in place in their original orbits around the
sun. Now it seems that Neptune has wandered some 30 percent
farther out from its original orbit and that it has also influenced
the orbits of other planets, notably Pluto. Pluto has this strange
orbit in which it most of the time is much farther out from the
sun than Neptune. However, some of the time it actually swings
in closer to the sun than Neptune. This odd orbit actually fits in
with the thousands of objects comprising the so-called Kuiper
belt. Indeed, some astronomers want to strip Pluto of its status as
a planet. Others recoil in horror at the suggestion, saying it
would be unfair to renege on our treatment of that body.
It''s thought that Neptune steered Pluto and these outer objects
into their eccentric orbits. The result was that the inner planets
would pick up or reject some of the objects as they whizzed by.
As a result, some planets moved out while others moved in
toward the sun. It seems that Jupiter moved in while Saturn,
Uranus and Neptune moved out.
This concept of wandering planets is a comfort to the discoverers
the new exoplanets. Some exoplanets are huge. Yet they''re in
orbits closer to their sun than Mercury is to ours. If such a large
planet had formed that close to its star, it would have been
sucked into the star. However, a wandering planet a la Neptune
or its "steered" companions could explain this anomaly.
All in all, we''re lucky to be living on this primate-friendly planet
of ours. Think of having to breathe diamond dust all day!
Allen F. Bortrum
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