Key Concepts:

1. What are the key characteristics of our Solar System?
2. What do they tell us about the origin of our Solar System?
3. How do astronomers discover other solar systems?

A Quick Tour of the Solar System

Four Major Characteristics of the Solar System

• Large bodies in the solar system have orderly motions.


• Planets fall into two main categories: Terrestrial and Jovian planets



• All planets orbit the Sun in a counter-clockwise direction.
• Most planets also rotate around their own axis in a counter-clockswise fashion.
• Asteroids: made up of rock, mostly in the inner solar system, between Mars and Jupiter, orbit in the same direction and in nearly the same plane as the planets.
• Kuiper Belt comets: made up of mostly ice. Orbit between Neptune and about 100AU from the Sun. Orbit in the same direction and nearly the same plane as the planets.
• Oort Cloud comets: made up of mostly ice. Orbit at great distance as far as one fourth of the distance to nearby stars ie. 1 light year. Orbits inclined at every posible angle and going in all directions.

Terrestrial Planets smaller size and mass higher density (rocks and metal) solid surface few moons and rings closer to the Sun

Jovian Planets larger size and mass lower density (hydrogen, helium) no solid surface rings and many moons farther from the Sun

• Swarms of asteroids and comets populate the solar system.
• Notable exceptions to these trends stand out (unusual rotation axis tilts, large moons, unusual orbits).

Formation of Solar Systems

	Stars are born in the dusty gas clouds. Driven by gravity, denser parts of the cloud collapses further to form "cores". The picture shown on the left is a false color HST image of a Galactic nebula called M16.
	As the gas core collapses on itself due to its own weight, a disk of gas and dust is formed. Gas and dust falls in so quickly with so much energy that sometimes a jet of material is seen being shot out. (Pictures shown on the left are false color HST images of nearby young stellar systems.)
	Sometimes strong winds and shocks from the nearby young stars can "blow out" the dusty cover of the young solar systems and reveal the remnants of the forming solar system. (Pictures shown on the left are false color HST images of young stellar systems being revealed by the ionizing radiation of the luminous neighboring young stars in Orion Nebula.)
	When the solar system is finally formed and dust has cleared, remnants of the formation process and outlying material in the solar systems (such as "Kuiper belt objects" and "Oort clouds") can be found surrounding the systems. (Picture shown on the left is a false color image of a dusty disk seen around a nearby star named Fomalhaut, imaged in submm/infrared light at James Clark Maxwell Telescope on Mauna Kea, Hawaii.)
	Remnants of the solar system formation can also be seen in visible or infrared light around some of the nearest stars when the bright light from the central star can be blocked out. (Picture shown on the left is a false color image of a dusty disk (rocks and ice chunks?) seen around a nearby star named beta Pictoris, imaged at European Southern Observatory.)

Solar Nebular Theory

Quiz 9a: Dating Earth...

Quiz 9b: Killer Asteroids

Detecting Planets in Other Solar Systems ("Extrasolar Planets")

• Doppler Effect:
• Measure the wobble of the stars due to the gravitational pull of the planets using Doppler effect.
• Over 100 planetary systems are known and still counting. This technique is most effective for finding only massive planets, within a few AU of the stars.
• Transit Search:
• Planets can eclipse the central star along the line of sight in some cases, and one can measure a brief dimming of the star and infer the presence of a planet.
• HD 209458 is such an example.
• Direct Imaging:
• First image of a planet outside our Solar system?
• NASA SIM project
• Atacama Large Millimeter Array project

Reading assignment for next lecture: Chapter 15 (p.496-p.517)