FORMATION OF THE SOLAR SYSTEM

• Sun
• Planets (9)
• Natural satellites (63 presently known; 4 discovered by Galileo)
• Asteroids
• Comets
• Rings
• Dust

PLANETS

• inner planets (Mercury, Venus, Earth, Mars)--small, dense, rocky (terrestrial);
• outer planets (Jupiter, Saturn, Uranus, Neptune)--large, gaseous, low density (Jovian)
• Pluto--icy?

Bode's law: the difference between the size of a planet's orbit and that of Mercury is twice that of the preceding planet

Atmospheres of the planets

• lighter gases (hydrogen, helium) retained by high gravity of Jovian planets
• lower gravity terrestrial planets have atmospheres composed of heavier gases or none

ASTEROIDS (also called "planetoids")--small rocky bodies

• largest, Ceres, has diameter 940 km
• Size can be estimated from brightness (assuming reflectivity is known), or from occultation of stars
• most have orbits in asteroid belt between Mars and Jupiter
  • but some have elongated orbits that cross orbits of Mars and Earth: some occasionally collide with Earth
  • some are found locked into Jupiter's orbit (Trojan asteroids)

COMETS--Small icy bodies (Whipple's dirty snowball theory)

• Long period comets, mostly in orbit far from the Sun, far beyond Pluto: Oort cloud. Orbits are nearly parabolic, and not confined to plane of solar system.
• Short period comets have orbits that take them into the inner solar system. The orbits are usually close to the plane of the solar system. Source in Kuiper belt lies beyond the orbit of Neptune.

Parts of a comet:

• nucleus: icy core
• coma: sphere of gas evaporated from nucleus when it approaches the sun
• tail: ice and gas that streams away from comet.

NOTE: direction of tail is opposite to sun, not opposite to motion of comet; streaming is due to solar wind

Halley's comet

• orbit stretches from Earth's orbit to Neptune's; period 76 years
• return predicted by Edmund Halley in 1705.
• during 1986, Halley was photographed by spacecraft from USSR, western European consortium and Japan. Pictures show that surface of Halley is dark (covered with dirt) and that gas escapes from cracks in the surface

METEORS

small particles that strike the atmosphere. Usually burn up on entry, but sometimes they reach the ground: they are then called meteorites:

• extraterrestrial origin not established until 1803 (Biot)
• origin: come from fragments of asteroids (this explains why there are two types of meteorites, stony and iron)
• some apparently come from the Moon, and some from Mars
• some 4.5 billion years old, give clues to origin of solar system

• 60 m body strikes Tunguska, Siberia in 1908: equals 10 Megaton bomb
• very large impact may have caused extinction of the dinosaurs: "nuclear winter" evidence: iridium layer

Meteor showers occur when Earth passes through former comet's orbit. The meteors appear to come from a radiant: the direction of the original comet

KEY FEATURES TO EXPLAIN

1. planets are far apart, not bunched together
2. orbits of planets are nearly circular
3. orbits of planets lie mostly in a single plane
4. directions of revolution of planets about Sun is the same, and is the same as the direction of the Sun's rotation
5. directions of rotation of planets about their axes is also mostly in the same direction as the Sun's (exceptions: Venus, Uranus, Pluto)
6. most moons revolve around their planets in the same direction as the rotation of the planets
7. differentiation between inner (terrestrial) and outer (Jovian) planets
8. existence and properties of the asteroids
9. existence and properties of the comets

FORMATION OF THE SOLAR SYSTEM

• Condenses from a rotating cloud of gas and dust--the solar nebula.
• As cloud condenses, it rotates faster (conservation of angular momentum).
• This makes the equator bulge--result: solar system is disk-shaped.

Thus far, this is the theory of Rene Descartes, 1644, Immanuel Kant, 18th C, Pierre-Simon de Laplace, 1796. To this we must add dust, which helps cool the nebula (it takes heat and radiates it as electromagnetic radiation) and act as seeds (nuclei) for the coalescence of matter.

• orbiting dust particles collide and stick together (accretion) to form planitesimals.
• Planitesimals collide and stick together to form planets and moons but coalescence of asteroids is prevented by influence of Jupiter's gravity.
• In the inner solar system, high temperatures mean that only metallic and rocky materials can condense; lighter gases are blown away by solar wind to outer regions, where they are swept up by Jovian planets. Terrestrial planets lose whatever hydrogen and helium they have.
• The planets sweep up a lot of the junk; period of cratering on terrestrial planets, about a billion years.
• Much of the remaining junk is given "gravitational assists" and thrown out of the solar system into Oort cloud.
• Solar wind blows away remaining gases.

Some problems that still need to be worked out.

• Angular momentum of sun is too low to agree with this theory. (But there are several explanations of how the sun could lose angular momentum.)
• retrograde motion of Venus; and of Uranus (perhaps due to a collision when the planet was forming?)
• Pluto, the oddity: tilted orbit; large eccentricity, wrong composition. Problem appears to be solved by the theory that Pluto belongs to Kuiper belt.

ALTERNATIVE HYPOTHESES

Chamberlin-Moulton hypothesis:

planets pulled out of sun by gravitational force of passing star.

• Hypothesis offered in order to solve angular momentum problem; but it doesn't actually solve it.
• No explanation of how planets condense.
• Also doesn't explain difference between Terrestrial and Jovian planets.

This is an example of a "catastrophic theory" (as opposed to an "evolutionary theory").

If this theory is true, there probably aren't very many other planetary systems around....

DO OTHER STARS HAVE PLANETS?

• Other stars are too far away for us to be able to see their planets directly.
• There is some evidence that planet formation is a common feature of star formation, such as observation of disk around Beta Pictoris.
• Observations of Eagle Nebula suggest planets may be rare.
• There is indirect evidence for planets around neutron stars, and since 1995, around normal stars. Many of these new observations don't fit our theory of the formation of the Solar System. Theory may need to be revised.

Previous section

Next section

your instructor