Planetology - A Review of the Facts:

Orbital Characteristics:

Planetary Types

Terrestrial (Inner) Planets consist of Mercury, Venus, Earth-Moon, Mars hard, rocky planets thin or little atmosphere demonstrate differentiated structure with heavier elements settled in cores, lighter elements in surface and atmospheres (were applicable) densities typically 3 - 5 g/cc
Jovian (Gaseous) Planets consist of Jupiter, Saturn, Uranus, Neptune very large with little element differentiation primarily gaseous fluid with possible solid cores atmospheres extensive and consist of hydrogen, helium, carbon, nitrogen, oxygen and numerous molecular complexes. densities very low, 0.7 - 1.6 g/cc.

Note: Pluto does not fit either classification. As well, between Mars and Jupiter lies the asteroid belt - the asteroids constitute another class of solar system object.

Planetary Formation

Kant's Nebular Hypothesis

• The modern theory extends and refines Kant's hypothesis. The difference is that we now believe that collision processes were a key factor in the formation of planets.
• The condensing solar nebula had an element distribution consistent with an intermediate Pop I star. As it collapsed it first assumed a spherical center with a large accretion disk . The accretion disk would itself be dense enough that instabilities would lead to the production of small scale local collapse around dust grains present in the nebula. Small clumps measuring from a few cm to as much as a km would form. As these small clumps began to form frequent collisions caused them to stick together to form larger bodies called plantesimals (small bodies approx 100 km diameter) to form. The formation of planetesimals increased the clumpiness of the accretion disk and gentle collisions between the planetesimals lead to a steady accumulation to form protoplanets .
• Another important process that was ongoing was the process of element differentiation. As the protosun began to shine and the cloud began to heat up the lighter elements were "driven off" the inner region of the nebula. The inner part of the accretion disk was swept clean of the very light elements. Hence the terrestrial planets formed from chemically differentiated material. The Jovian planets formed in the cooler, outerregions of the solar nebula where light volatile elements could form simple molecules and ices.

• Eventually the sun became a main sequence star. As this was occurring it passed through the T Tauri phase during which time it was bright with a very strong solar wind and strong radiation pressure which helped sweep remaining matter out of the solar system as illustrated in the FLASH movie shown below. It is not unlike a butterfly sheading its cocoon. The solar nebula was further cleared away by the "vacuum cleaner" action of the planets slowly accreting matter and lastly by the "slingshot" affect that planetary interaction sometimes had on solar system debris.

Evidence for the Solar Nebula Theory

• Correctly predicts the gross orbital properties for the solar system
• Predicts (roughly) the transition from the inner (Terrestrial) planets to the outer (Jovian) planets and predicts (with reasonable success) the chemical make-up of the planets assuming formation from a Pop I composition nebula.
• Slow rotation rate of the sun: If the sun collapsed from a large cloud it should have a much higher rotational velocity than we observe (conservation of angular momentum). On the other hand, the drag created by a solar system as well as the interaction between the sun's magnetic field and the accretion disk leads to a process called magnetic braking .
• Accretion disks in other star systems: Accretion disks are common in close binary star systems and also in a number of rather special systems. A good example of these is the beta Pictoris system. Special masking techniques have been applied to images made of this star sytem. The images reveal the presence of a large disk surrounding the star.
• The best extra-solar planetary system, however, is likely the newly discovered planet around the star 51 Pegasi In October 1995 Michel Mayor and Didier Queloz discovered a Jupiter-like planet orbitting this star. The planet, however, orbits 51 Pegasi in a mere 4 days! As of March 2002, approximately 80 extra-solar planets have been discovered! Go to the most current info here...
• If you would like to learn more about either 51 Pegasi or the detection of planets around distant suns follow these links...
• more on 51 Pegasi
• Other Planets? - an excellent and comprehensive discussion from SEDS
• link to Astronomy 200 notes on detecting stellar motion

Some Interesting Web Sites to Explore ...

• Views Of The Solar System
• The Nine Planets

Time to take a quiz! This link will take you to Quiz 5

Seeds: Chp 19