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The orbits of the planets and most minor bodies are co-planar, roughly
in the plane of the sun's rotation.
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The orbits are prograde---the rotation is in the same sense as the sun's
rotation. Planetary obliquities (the angle between the rotation axis and
the orbital axis) are generally less than 30 degrees (earth's obliquity
is about 23 degrees). The exceptions are Uranus (about 90 degrees), and
Venus and Pluto (which rotate "backwards").
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The orbits of planets in our solar system are nearly circular. However,
the orbits of planets around other stars are often highly eccentric.
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Orbital spacing increases with distance from the sun. There are no massive
planets beyond 30 AU (an AU, or astronomical unit, is the mean distance
between the earth and the sun). The orbital paths of the eight major planets
do not cross or even approach each other closely. Pluto's orbit crosses
that of Neptune. Aside from the asteroid belt, interplanetary space is
remarkably debris-free.
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Planetary rotation: six of the eight major planets rotate in the same sense
as they revolve around the sun. Venus, Uranus (and Pluto) rotate in the
retrograde sense.
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Satellite systems: All but two planets have natural satellites. Most close-in
satellites travel on low inclination, low eccentricity prograde orbits,
with significant exceptions, such as Triton.
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Bulk compositions: The terrestrial planets (out to Mars) have high densities,
and are believed to be composed of refractory (high condensation temperature)
rocky material. Jupiter and Saturn have very low densities, suggesting
that they are composed primarily of H and He. Uranus and Neptune are denser
than Jupiter or Saturn, suggesting the presence of either icy or rocky
cores.
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Elemental Abundances: The atmospheres of the giant planets and the bulk
material of the earth and meteorites contain trace elements in amounts
similar to those seen in the sun.
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Mass distribution: the planets account for less than 1/500 of the mass
of the solar system, more than half of this small fraction is tied up in
Jupiter.
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Angular Momentum: Over 98% of the angular momentum of the solar system
is contained in the Jovian planets.
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Ages: Radioisotope dating of primitive meteorites indicates ages of 4.56
billion years (4,560,000,000).
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Cratering record: Most solid planetary and satellite surfaces are heavily
cratered. The heaviest cratering occurred within the first 800,000,000
years of solar system history.
Solar System Formation
Star Formation
Stars form in the condensed cores of molecular clouds (Click
here to see the star forming region M16). These cores are much larger
in extent than the resulting stars, so they must collapse. The collapse
may be triggered by nearby supernovae, or it may be a natural consequence
of the cooling of gas in the cloud.
Difficulties
-
Too much pressure in the gas to allow collapse: The interstellar medium
(ISM) is supported by thermal pressure. The gas has to cool to be able
to collapse. It does so by radiating away energy in the infrared band.
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Too much angular momentum: In order for the collapse to proceed, large
amounts of angular momentum must be lost. The mechanism for this is unknown.
However, we observe jets of high velocity material ejected from protostars.
(Click
here to see a jet and disk in Herbig-Haro 30, a young star). These
jets may carry away large amounts of angular momentum, but the physics
of jet formation are very uncertain.
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Too much magnetic field: The gas in the ISM is partially ionized, so that
magnetic fields should be tied to the gas. However, the neutral component
of the gas can slip past the field. This is called ambipolar diffusion.
Planet Formation
Do planets form
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When the sun forms, from the same material? If so,
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Do they form directly from the gas in a gravitational collapse?
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Or do they form from solids that have condensed by chemical interactions
out of the gas?
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After the sun forms, when a passing star pulls material out of the sun?
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In the interstellar medium, and then are subsequently captured by the sun?
Next:Lecture Four
Previous: Lecture Two
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