The Solar System

How did the solar system form? The nebular theory (or accretion hypothesis) is the most widely accepted explanation. NEBULAR THEORY A nebula is an extremely large cloud of dust and gas. The nebula is unstable, and begins to collapse around different regions because of an unequal distribution of matter.

Nebular Theory As the nebula collapses around different areas of high mass, the gravitational attraction of these regions grows, which attracts more and more material. These points where the matter collects become stars. For this reason, nebulae are often called stellar nurseries.

Nebular Theory As matter collects unevenly around the center, it begins the whole mass spinning. (PROTOPLANETARY DISK) Most of the mass (99%) collects in the center forming the Sun. The rest of the mass (1%) flattens into a spinning disk of dust and gas around the Sun.

Nebular Theory Within the disk, areas of higher mass attract more and more material (accretion). This accretion forms “planetessimals”, or the beginnings of planets.

Nebular Theory When accretion is finished, we are left with our solar system.

The Sun The Sun is the star at the center of our solar system. Made mostly of hydrogen (92.1%) and helium (7.8%) Nuclear fusion produces the Sun’s energy About 1.3 million Earth’s could fit inside the Sun 1.4 million km in diameter (109 times larger than Earth) 5500 °C (9900 °F)

Gravity Law of Universal Gravitation All objects with mass exert a gravitational force on the objects around them. Strength of force depends on the masses of the objects and the distance between them. Because of the Sun’s great mass, all other objects in the solar system orbit around it.

Planets A planet: There are 8 planets in our solar system Orbits the Sun Has a spherical shape Is significantly more massive than the objects around it There are 8 planets in our solar system Inner planets Outer planets

Inner Planets The four planets closest to the sun (inside the asteroid belt) Mercury, Venus, Earth, Mars Smaller size (than the outer planets) Rocky (Terrestrial) Mostly iron core Solid outer crust

Outer Planets The four planets farthest from the Sun (outside the asteroid belt) Jupiter, Saturn, Uranus, Neptune Larger size (than the inner planets) Called “gas giants” Ice and gasses

Dwarf Planets Like a planet… But not a planet! Made of rock and ice Spherical shape Orbits the Sun But not a planet! Orbits near other similarly sized objects.

Asteroids Small, rocky objects that orbit the Sun Millions found in the asteroid belt between the orbits of Mars and Jupiter Between one meter and several hundred thousand meters in length Usually not spherical

Comets Relatively small ball of gas, dust, and ice Orbits the Sun Usually an extremely elliptical orbit Come from outer parts of the solar system Sometimes called a “shooting star” NOT A STAR!!!

Measuring Distances Because of the great distances between objects in our solar system, we use a larger than usual measurement… AU – Astronomical Unit The average distance between Earth and the Sun 150 million kilometers (150,000,000 km)

Geocentric Model? It was once believed that Earth was the center of the universe and all other objects revolved around it. Geo = Earth Centric = Center

Heliocentric Model Helio = Sun Centric = Center In the 1500’s, Copernicus proposed that Earth and other planets revolve around the Sun Though the Sun was later observed NOT to be the center of the universe! Supported by evidence collected as technology allowed further investigation.

Planetary Orbits Kepler discovered that planetary orbits are elliptical rather than circular Planets move faster when they are closer to the Sun, and slower when they are farther from the Sun. Sun at one focus.