THE SOLAR SYSTEMSOME 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???

Planet Main Gases in Atmosphere

MERCURY Almost none

VENUS Carbon dioxide

EARTH Nitrogen, Oxygen

MARS Carbon dioxide

JUPITER Hydrogen, Helium

SATURN Hydrogen, Helium

URANUSHydrogen, Helium,

Methane

NEPTUNEHydrogen, Helium,

Methane

Terrestrial Planets

Gas Giants

Gases in the atmospheres of

the planets

Again – Why so different???

“Escape Velocity” from planets

matters

So does temperature

Average velocityof gas molecules (km/s)

Hydrogen

Oxygen

Gas molecules move slower as temperature decreases

Average velocityonly 11 km/s at 40K

Average velocity of100 km/s at 1000K

1341oF 260oF -100oF -280 oF -387 oF

Graph showing escape velocity vs temperature for planets and moons

Lines show averagevelocity of gas moleculesat 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.