THE SOLAR SYSTEM SOME BASIC FACTS 1) All planets orbit the sun in the same direction.
Dec 26, 2015
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.