Star date: 12:14:98
All Good Things...
Jupiter delighted amateur astronomers throughout the
summer. Saturn came up a few months later, and dazzled people worldwide
with it's majesty and the grace of its intricate ring system. Jupiter is
now only visible in the southwestern sky early in the evening, and Saturn
is slowly slipping towards the western horizon. Before they are gone, we
will want to visit these majestic planets one more time in our dance across
the sky.
Jupiter is by far the brighter of the two, and can be found in the southeastern
sky just after sunset. If you go out then and look to the southeast, Jupiter
will be the brightest object by far in that area. There is little else
in the sky other than the Sun and Moon which can compare with the brightness
of Jupiter, and none of them are in that region of the sky. Jupiter will
shine with a bright white color.
Nearly 40 degrees from Jupiter lies the second largest planet in the
solar system, Saturn. It looks yellow, and can be found to the east and
above Jupiter. Jupiter contains more mass than all the other planets in
the solar system combined. In fact, Jupiter contains 99% of the planetary
mass in the solar system. Saturn contains more mass than any other planet
in the solar system other than Jupiter. Saturn, however, is over 80% as
large as Jupiter. This results in Saturn possesing a very low density.
In fact, the density of Saturn is so low, that if you could find a bathtub
large enough, Saturn would float in it!
The density of Jupiter, on the other hand, is only about 1/3 more than
the density of water. Compare this with the Earth, whose density is about
5 times that of water. Why is this? Why should such large planets have
such a low density? The answer lies in the materials which compose these
planets. Jupiter is composed nearly entirely from hydrogen and helium gas,
which fill most of the bulk of this king of the planets. Under this thick
atmosphere lies the largest ocean of any sort in the solar system. This
is an ocean of pure liquid hydrogen. This ocean of liquid hydrogen, however,
does not have a surface. It forms slowly from the atmosphere above.
Beneath this frigid ocean may lie a rocky core of some sort, possibly
about the size of the Earth. This core may be surronded by a layer of an
exotic substance known as metallic liquid hydrogen. Although postulated
for several years, this substance was never seen on Earth until recently,
when it was first produced in a laboratory. We can see from this that although
Jupiters rocky core and possible layer of metallic hydrogen may add something
to the overall density of this planet, the enormous amounts of liquid hydrogen
and gas keep the overall density fairly low.
Saturn is even more dramatic in this regard. The outside
layers of this planet are similar to Jupiter, being composed of hydrogen
and helium gas. Unlike Jupiter, however, Saturn is unlikely to have a rocky
core. The presence of a metallic hydrogen core is certainly well within
the bounds of possibility, however. This is what is responsible for the
extraordinarily low density of Saturn. Why should it be this way, however?
If we look at the composition of Uranus and Neptune, we find a similar
situation to that which exists at Jupiter and Saturn. Until recently, the
answer for this seemed simple enough. The thought was that when the solar
wind from the Sun first began during the formation of our solar system,
the light gases such as hydrogen and helium were blown away from the Sun,
out towards the developing outer planets. The thought then was that when
planets were discovered outside the solar system, that they would obey
a similar relation, and large planets would always or nearly always be
found away from its parent star.
In the last 10 years, however, several planets have been found circling
stars other than the Sun, and several of those have been found to be large
planets orbiting near their parent star. Of course, one possible reason
for this could be that we cannot see the planets directly, and the discoveries
of these planets is accomplished by measuring the "shaking" of
a star side to side as the planet revolves around it. The systems in which
there are large planets near their local star would thus be the easiest
to detect. It is therefore possible that the original theory is correct,
and that the reason we are seeing systems with a large planet near the
star is simply because they are the easiest to see.
Clear skies, and good viewing.
Jim Maynard is the head of the astronomy department
at Earth Treasures. In addition, he has been an amateur astronomer for
nearly 25 years, and is a physics major at Keene State college. His articles
may also be accessed on the world wide web at http://www.geocities.com/Athens/Styx/2842/index.html