THE SOLAR SYSTEM SOME BASIC FACTS 1) All planets orbit the sun in the same direction.

2) All planetary orbits lie nearly in the same plane. (but not Pluto’s).

3) Planets travel in nearly circular orbits (they are not very eccentric - except Pluto’s). 4) The distance between the planets increases as distance from the sun increases.

5)Most planets rotate in the same direction that they orbit. 6) Almost all moons orbit their planet in the same direction as the planet rotates, and orbit nearly above the planet’s equator. 7) The sun rotates in the same direction that the planets orbit.

SO, in general, motion in the solar system is pretty regular and somewhat predictable. It agrees with the idea that everything started as a spinning cloud of dust and gas. But how about the SIZES of the planets??

Relative sizes of the 8 planets “Gas Giants” “Terrestrial Planets” Why so different???

Gases in the atmospheres of the planets Again – Why so different???

“Escape Velocity” from planets matters So does temperature

Average velocity of gas molecules (km/s) Hydrogen Oxygen Gas molecules move slower as temperature decreases Average velocity only 11 km/s at 40K Average velocity of 100 km/s at 1000K

Graph showing escape velocity vs temperature for planets and moons Lines show average velocity of gas molecules at a given temperature.

So – The outer planets are “gas giants” because: They are very COLD – gas molecules are moving slower. AND They are very BIG – gas molecules must be moving very fast to escape their large gravity.