Structure & Formation of the Solar System What is the Solar System? The Sun and everything gravitationally bound to it. There is a certain order to the Solar System. This gives us information on its formation. Build a Solar System http://www.exploratorium.edu/ronh/solar_system/index.html The planets to scale with a portion of the Sun visible in the background.

Part 1: Structure of the Solar System All the planets orbit the Sun in the same direction. All the planets orbit within nearly the same plane. Like a disk. Two type of planets Solid, rocky, small planets close to the Sun (like Earth) Gaseous, large planets far from the Sun (like Jupiter)

The Sun The Sun is a star. It is completely gaseous. It emits light and heat through nuclear fusion in its core. It is by far the largest object in the Solar System. 700 times more massive than all of the other objects in the Solar System put together. It is composed mostly of Hydrogen and Helium gas and traces of many other elements. The Sun spins on its axis counter-clockwise.

The Planets In order of increasing distance from the Sun: Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

The Planets All the planets orbit in the same direction counter-clockwise as seen from above Earth’s North Pole. All the planets spin counter-clockwise too except for Venus, Uranus and Pluto.

The Inner or Terrestrial Planets Mercury, Venus, Earth and Mars share certain characteristics: All are rocky bodies. All have solid surfaces. Except for Mercury all have at least a thin atmosphere They are called Terrestrial planets because of their resemblance to Earth. Pluto is going to be dealt with separately

The Outer or Jovian Planets Jupiter, Saturn, Uranus and Neptune share certain characteristics: All are large, gaseous bodies. All have very thick atmospheres, with possibly liquid interiors and solid cores All have rings They are called Jovian planets because of their resemblance to Jupiter.

Pluto Pluto is unlike any of the terrestrial or jovian planets. much farther from the Sun than the terrestrial planets. much smaller than any jovian planet. composition is thought to be a mixture of ice and rock It is similar to some of the satellites of the jovian planets and similar to some asteroids. There has been some discussion among astronomers whether Pluto should be considered a planet at all.

Satellites (Moons) Most of the planets have satellites. Most of the satellites orbit in the equatorial plane of the planet. Most satellites orbit counter-clockwise. The jovian planets have more than a dozen satellites each. Ganymede, Callisto, Io, and Europa. Four of Jupiter’s largest satellites. These were discovered by Galileo Galilei and together are called the Galilean satellites of Jupiter.

Comets and Asteroids The Solar System is filled with millions of smaller bodies. Comets - composed of ice and rock Asteroids - composed of rock and/or metal There is also dust in space which can be seen in meteor showers

Part 2: Solar System Formation Our Milky Way Galaxy is filled with cold, dark clouds of gas and dust. These clouds are mostly hydrogen and helium with dust containing mostly iron, rock, and ice. The Solar System is thought to have formed from a huge, slowly rotating cloud about 4.5 billion years ago A nearby passing star or stellar explosion may have caused the cloud to collapse

Collapsing Gas Clouds As the cloud collapsed the original slow spin began to speed up. This caused the cloud to flatten into a disk shape. The gravitational pull of the cloud caused it to shrink further and caused most of the material to fall towards the core forming a large bulge.

Collapsing Gas Clouds? In the Great Nebula of the constellation Orion are huge clouds of gas and dust. Among these clouds the Hubble Space Telescope observed lumps and knots that appear to be new stars and planets being formed.

Planets in Formation? Around the star Beta Pictoris a large disk of dust and gas has been observed. The light from the star is much brighter than the disk so it had to be blocked for the disk to appear clearly. Disks have been seen around other stars too including Vega. see http://exoplanets.org

Birth of the Sun As material falls into towards the disk it collides with other material and heats up and melts. The increasing mass of the core also increases the gravitational pull and causes more material to be pulled in. When the mass is large enough and temperatures high enough nuclear fusion reactions begin in the core and a star is born!

Heating and Condensation of the Solar Nebula The heat from the Sun prevents ices from reforming on the dust grains in the region near the Sun. Ices condensed only in the outer parts of the Solar nebula. In the inner portion of the disk only materials like iron and silicates (rock) can condense into solids. Slowly they form clumps of material. In the outer portion of the disk much more material can condense as solids including ice. This extra material allows clumps to grow larger and faster.

Gravity does the job Within the disk, material is constantly colliding with one another. If the collisions are not too violent material may stick together. In the outer parts of the Solar Nebula the planets become large enough to have a significant gravitational pull and collect gas around them. Planets in the inner nebula can not grow enough to collect much gas. Eventually most but not all of the material was swept up by the planets.

The Last of the Planetesimals The remaining material exists today as comets which were flung out to a region far beyond Pluto called the Oort cloud and asteroids mostly between Mars and Jupiter (the Asteroid Belt) and beyond Pluto (the Kuiper Belt)