1. Introduction to the Solar System
Chapter 6

2. The Solar System
Ingredients?

3. The Solar System Ingredients? 8 Planets + a few “minor planets”
1 Star: the Sun
8 Planets + a few “minor planets”
126 moons around these planets
Asteroids, meteoroids, comets
A lot of nearly empty space

4. Questions
What percentage of the total mass of the solar system does the Sun contribute?
How is the solar system laid out in space? Spacing between planets? Orbital directions?

5. Mass in Solar System Sun 99.8% Jupiter 0.1% Comets 0.05%
All Other Planets
0.04%
Earth
0.0003%

6. Sun, Planets and Moon to scale
Sun accounts for 99.9% of solar system mass!

7. Solar System Temperatures
Planet
Distance
Temperature (top of atmosphere)
Mercury
0.38 AU
450 K
Venus
0.72 AU
330 K
Earth
1.00 AU
280 K
Mars
1.52 AU
230 K
Jupiter
5.20 AU
120 K
Saturn
9.54 AU
90 K
Uranus
19.22 AU
60 K
Neptune
30.06 AU
50 K
Pluto AU
40 K
350 F
45 F
-390 F

8. Comparative Planetology
Categorize planets by properties
Compare similarities and differences
Ask: What physical processes can explain these properties?

9. 6.2 Planetary Properties 63 60 13

10. 6.2 Planetary Properties Distance from Sun known by Kepler’s laws
Orbital period can be observed
Radius known from angular size
Masses from Newton’s laws
Rotation period from observations
Density can be calculated knowing radius and mass

11. 6.3 The Overall Layout of the Solar System
All orbits paths are close to the ecliptic plane
Pluto’s orbit does not (17° tilt)

12. Planet Orbits

13. Planet Orbits Orbits aligned in same plane (the ecliptic)
Explains why planets always found in Zodiac
Pluto’s orbit tipped the most (17 degrees)
All planets orbit Sun counter-clockwise
Planets rotate counter-clockwise
except Venus
Rotation axis roughly perpendicular to orbit
except Uranus and Pluto

14. The Terrestrial Planets

15. Terrestrial Planets Terrestrial = Earth-like Small, low mass
Mercury
Venus
Earth (and Moon)
Mars
Small, low mass
No large moons (except Earth)
Mars has two small ones…
Close to Sun

16. Terrestrial Planets Rocky Surface
High density (3-5 gm/cm3) (water = 1 gm/cm3)
Geologic Activity (volcanoes, continental drift)
Present on larger planets (Earth and Venus)
Absent on smaller planets (Moon, Mercury, and Mars)
Atmosphere
Little hydrogen and helium
Mostly carbon dioxide (Venus and Mars) or nitrogen (Earth)
Smaller planets have no atmosphere (Mercury, Moon)

17. The Kuiper Belt Objects
Origin of Pluto
Large member of a class of objects in the outer reaches of the Solar System:
The Kuiper Belt Objects
100's found since 1992.
Orbits tend to be more tilted, like Pluto's.
Leftover planetesimals from Solar System formation?

18. Asteroids
Mars
The Asteroid Belt

19. Asteroids Small rocky bodies Asteroid Belt High density (3-5 gm/cm3)
Usually not round
Primitive composition
(oldest bodies in solar system)
Asteroid Belt
Found between Mars and Jupiter
Probably a failed planet?

20. Asteroid Belt
Perhaps a planet was going to form there. But Jupiter's strong gravity disrupted the planetesimals' orbits, ejecting them out of Solar System. The Belt is the few left behind.
And Finally . . .
Remaining gas swept out by intense period of solar wind activity.

21. The Jovian Planets

22. Jovian Planets Jovian = Jupiter-like Large, massive Many moons
Saturn
Uranus
Neptune
Large, massive
Many moons
Far from Sun

23. Jovian Planets Low density (1 gm/cm3) No obvious surface Atmosphere
Mostly hydrogen and helium
Other gases (methane, ammonia)
may form ices

24. The Outer Solar System
Comets
Kuiper Belt and Oort Cloud

25. The Outer Solar System Pluto Kuiper Belt Oort Cloud Small, icy
Low density
Little or no atmosphere
Kuiper Belt
Small icy bodies
Is Pluto really the largest Kuiper Belt object?
Distributed in disk-like geometry (in plane of solar system)
Distance: ,000 AU
Oort Cloud
Distributed spherically around solar system
Distance: 10, ,000 AU

26. Let’s consider a scale model
of the Solar System!

27. 6.4 Terrestrial and Jovian Planets
Relative sizes of the Sun & Planets
It would take 109
Earths to span the
Sun!

28. 6.4 Terrestrial and Jovian Planets
Terrestrial planets:
Mercury, Venus, Earth, Mars
Jovian planets:
Jupiter, Saturn, Uranus, Neptune
Pluto is neither but a new class called the
Dwarf planets

29. 6.4 Terrestrial and Jovian Planets
Differences (Comparative Planetology) between the terrestrial planets:
Atmospheres and surface conditions are very dissimilar
Only Earth has oxygen in atmosphere and liquid water on surface
Earth and Mars rotate at about the same rate; Venus and Mercury are much slower, and Venus rotates in the opposite direction
Earth and Mars have moons; Mercury and Venus don’t
Earth and Mercury have magnetic fields; Venus and Mars don’t

30. The image at right shows a picture of the Sun. The dark
spots located on this image
are sunspots. How does the
size of Earth compare to the
size of the sunspot that is
identified on the right side of
the image of Sun?
A) Earth and the sunspot
are about the same size.
B) The sunspot is much
larger than Earth.
C) The sunspot is much
smaller than Earth.
Sunspot

31. If you were constructing a scale model of the solar system that used a Sun that was the size of a basketball (approximately 12 inches in diameter), which of the following lengths would most closely approximate the scaled distance between Earth and the Sun?
A) 3 feet (length of an outstretched arm)
B) 10 feet (height of a basketball goal)
C) 100 feet (height of an 10 story building)
D) 300 feet (length of a football field)

32. Questions
What are some of the smaller objects (or debris) found in the solar system?
What information do they contain that the planets and moons do not?
(Hint: What effects do erosion, geological activity, vulcanism, etc. have on a planet?)

33. Questions
What are some of the smaller objects (or debris) found in the solar system?
Comets, asteroids, meteoroids
What information do they contain that the planets and moons do not?
Solar system debris is unevolved => gives direct evidence of conditions during solar system formation!

34. Solar System Debris Comets Short Period Comets Long Period Comets
Comet Halley (1986)
Comet Hale-Bopp (1997)
Short Period Comets
Long Period Comets
year orbits
Orbits prograde, close to plane of Solar System
Originate in Kuiper Belt
Few times 105 or 106 year orbits
Orbits have random orientations and large ellipticities
Originate in Oort Cloud

35. Oort Cloud is a huge, roughly spherical reservoir of comets surrounding the Solar System. ~108 objects?
A passing star may redirect Oort cloud objects, creating long period comets.
Kuiper Belt object can be redirected by Neptune, creating a short-period comet.

36. Question What causes the tail of a comet?
(Hint: The tail always points directly away from the sun.)

37. Comet Structure
Nucleus: ~10 km ball of ice, dust
Coma: cloud of gas and dust around nucleus (~106 km across)
Tail: Always points away from Sun.
Coma and tail due to gas and dust removed from nucleus by the Solar Wind.
Far from Sun, comet is a nucleus only.

38. Comet Trajectory

39. Meteor Showers
Comets break up when near Sun solar wind, evaporation, tidal
force.
e.g. Halley loses 10 tons/sec when near Sun. Will be destroyed in 40,000 years.
Debris spreads out along comet orbit.
Intersection of orbits => meteor shower

40. How did the Solar System Form?
What must be explained?
Solar system is very flat.
Planetary orbits are nearly circular.
Almost all moons and planets (and Sun) rotate and revolve in the same direction.
Planets are isolated in space.
Terrestrial - Jovian planet distinction.
Leftover junk (comets and asteroids).

41. Solar Nebula Start with rotating cloud of gas and dust
Collapses because of gravity
spins faster
flattens into disk-shape
gets hotter
Sun forms in center
Temperature decreases outward
As nebula cools, gas condenses
Forms solid particles (dust grains)

42. Nebular Theory
Nebula: Cloud of interstellar dust and gas about a light- year across
Condensing cloud heats up - star forms at center
But why is solar system flat?
Conservation of Angular Momentum!
Ang. Mom. = mass x rotation speed x “size”

43. Conservation of angular momentum (Demo)

44. So, as nebula contracted it rotated faster.
It became a flattened disk, like a pizza crust. (Centrifugal hoops demo)

45. But, clumps in rotating gas tend to disperse. Need modified theory.
Solar Nebula:
98% of mass is gas
2% in dust grains
Condensation theory:
1) Dust grains act as "condensation nuclei. Also radiate heat => help to cool gas => faster gravitational collapse.
2) Accretion: Clumps collide and stick
3) Gravity-enhanced accretion: objects now have significant gravity => faster growth

46. Forming Planets Dust grains stick together Grow into planetesimals
form rocks
Grow into planetesimals
some still survive today
asteroids
comets
Larger planetesimals attract smaller ones (gravity)
Planetesimals accrete
form protoplanets / planet cores
initially cold
Collisions become violent
heating melts protoplanet
differentiation occurs

47. Forming Jovian Planets
Snow line
Location beyond which ices form
Building blocks (solids)
both silicates and ices
Protoplanets / planet core
grew larger
gravity captured hydrogen & helium
composition similar to Sun
gaseous accretion disk forms around planet
Moons form in disk around planet

48. Evolution of the Solar System
Collisions dominate early-on
produces early heavy bombardment
comets collide with terrestrial planets
Deposit volatiles that form atmosphere (water, carbon dioxide, etc.)
Planets sweep up / throw out remaining planetesimals
Ones thrown out:
Oort cloud
Ones that remain:
Comets (Kuiper belt)
Asteroids (asteroid belt)

49. Planetary Ejection

50. Planetary Evolution - Geological
Internal heating leads to geologic activity
volcanism, tectonics
active worlds
As core cools & solidifies, activity slows, eventually stops
e.g. Moon
Earth, Venus large enough to still be active

52. Planetary Evolution - Atmosphere
Atmosphere formed by
gases escaping from interior
impacts of comets (volatile-rich debris)
Fate of water depended on temperature (distance from Sun)
Atmospheres changed chemically over time
Life on Earth substantially changed the atmosphere