1. The Planets
Chapter 5

2. Characteristics of the Planets
Planets – non luminous objects
Stars – luminous objects
Visible: Mercury, Venus, Mars, Jupiter, Saturn
Telescopic: Uranus, Neptune, Pluto
Order from Sun:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto

3. Planetary Orbits
All planets have similar orbits: ecliptic path similar to the sun
Most planets travel in a elliptical (oval) path
Ellipses have two focus points; one being the sun

4. Kepler’s laws of planetary motion
Planets move in ellipses with the sun at one focus.
An imaginary line from the center of the sun to the center of a planet always sweeps over an equal area in equal time.
The square of time it takes to make one orbit of a planet is proportional to the cube of its average distance from the sun.

5. Kepler’s laws made simple
Planets move in ellipses with the sun at one focus.
A planet closer to the sun will be traveling faster along its orbit; slower as it moves farther from the sun.
A distant planet will take longer to orbit than a planet that is closer to the sun.

6. Planetary Orbits
Perihelion – the distance when it is the closest to the sun
Aphelion – the distance when it is the farthest from the sun
Tables contain the average distance from the sun

7. Brightness Planets only have relative brightness Distance
Albedo – the ability for the planets surface or atmosphere to reflect light; always a decimal 1.0 = 100% reflection; 0.65 = 65% reflection
Size – the larger the planet the brighter it will appear

8. Mass Mass – the measure of the amount of matter the planet contains
Weight – the amount of force that gravity is applying to the mass
Surface gravity: the downward pull the planet exerts on objects at its surface
Earth – 100 pounds
Pluto – low surface gravity – 6% (6 lbs)
Jupiter – high surface gravity – 2.5 times more (253 lbs)

9. Classifying the Planets
Position
Inferior: Mercury and Venus
orbits are between the earth’s orbit and the sun
Superior: Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto
orbits lie outside the earth’s orbit

10. Classifying the Planets
Size and Density
Terrestrial: Mercury, Venus, Earth, Mars, Pluto (?)
Have the same density and/or are smaller than earth
Difference: Earth is the only terrestrial that has oxygen and liquid water
Jovian: Jupiter, Saturn, Uranus, and Neptune
Larger than the earth and therefore have a lower density

11. Close-Up of the Planets
Mercury:
Observation: difficult to observe with eye; too close to sun most of the time
Size: second smallest planet
Atmosphere: very thin; helium, hydrogen, and trace amounts of carbon dioxide
Moons: none

12. Close-Up of the Planets
Venus:
Observation: somewhat easy to observe; 15 times brighter than Sirius
Size: most similar to the Earth
Atmosphere: clouds containing almost 97% carbon dioxide, 3% nitrogen and traces of sulfuric acid; reflects 65% of light
Moons: none

13. Close-Up of the Planets
Mars:
Observation: appears red in the sky; at its best is brighter than all the other planets and stars
Size: ½ the size of the earth; 85% less mass
Atmosphere: very thin atmosphere
Moons: Phobos and Deimos

14. Close-Up of the Planets
Jupiter:
Observation: brighter than the brightest star, but never brighter than Venus; no human has ever seen
Size: Largest of the planets; 11 times that of earth’s diameter; 318 times as massive
Atmosphere: extensive; turbulent clouds; magnetosphere
Moons: Many (>63); Galilean moons – Io, Europa, Callisto, and Ganymede (larger than Mercury and Pluto);

15. Close-Up of the Planets
Saturn:
Observation: Saturn and the rings are seen through a telescope;
Size: 2/3 the size of Jupiter
Atmosphere: bands of clouds parallel to equator; rings are particles in orbit; magnetosphere
Moons: Many (> 31); Titan – the largest has an atmosphere of mostly nitrogen with some methane

16. Close-Up of the Planets
Uranus:
Observation: Can be seen without a telescope if conditions are perfect.
Size: 4 times larger than earth; ~ Neptune
Atmosphere:cloud layer of helium, hydrogen, methane, and traces of ammonia; strong magnetic field
Moons: 27 known; orbit almost perpendicular to the planets orbit

17. Close-Up of the Planets
Neptune:
Observation: Similar to Uranus through a telescope;
Size: ~ Uranus; 4 times larger than earth
Atmosphere: cloudy containing helium and hydrogen; 11 rings
Moons: 13 but only two are observable from earth; Triton slightly larger than the earth’s moon and Nereid has the most eccentric orbit of any known moon

18. Close-Up of the Planets
Pluto:
Observation: very difficult to see
Size: smallest planet; smaller than the earth’s moon
Atmosphere: thin atmosphere of mixed gases
Moons: Charon – half the size of Pluto