Download Formation of the Solar System

Lecture 37
The Formation of the Solar
System
How did the solar system come about?
Condensation sequence
Are there others?
Apr 24, 2006
Astro 100 Lecture 37
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The Solar System
Brief History of Time (assumes T = 13.7 Gyr)
• 3 minutes: Formation of He, D
• 1-2 billion yrs: Milky Way Earliest Stars
• 2-8 Gyr: MW Disk supernovae: C,N,O,Fe..
• 9 Gyr: Collapse of solar gas cloud
• 13.699 Gyr: People evolve (1 million BP)
• 13.699999 Gyr: Astronomy (1000 BP)
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Some important features of Solar System
• 1) Radioactive dating of old SS rocks gives same
age:
Earth: 3.9 Gyr; Moon 4.5 Gyr; Meteorites 4.6 Gyr
• 2) Most orbital and rotation planes confined to
ecliptic plane; motions counterclockwise
• 3) Extensive satellite and ring systems around
Jovian planets
• 4) Planets have more heavy elements than Sun;
divided into two classes based on density
(composition):
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Planet Classes
Type
Planet
Mass
Terrestrial
Mercury
0.06
5.4
Venus
0.82
5.2
Earth
1.00
5.5
Mars
0.11
3.9
Jupiter
318
1.3
Saturn
95
0.7
Uranus
14
1.3
Neptune
17
1.7
Jovian
Density
gm/cm3
Chief Material
Rock, Iron
Hydrogen, Helium
Ices
• Note: terrestrial planets all closer to the Sun
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Models
• (1) suggests planets formed at same time as sun: 5 Gyr
consistent with sun being ~2x luminosity of zero-age main
sequence G2 star.
• (2,3) suggests we associate planetary and satellite/ring
systems with remnants of dusty disks seen around forming
stars (eg T Tauri stars): The Nebular Model.
• (4) suggests that what planets condensed out of depended
on how far they were away from sun: The Condensation
Sequence
The different densities of the planets is now explained by
condensation temperatures in Nebular Model:
• Nebular Dust temperature should increase to center of
nebula:
– More of remnant heat from collapse retained at center (more
opaque there)
– After sun turns on, dust reaches "equilibrium temperature":
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Equilibrium Blackbody Temperature
• Energy absorbed/area from sun (flux) = Energy emitted as Blackbody
– Solar Energy Absorbed/ Area = Lum(Sun)/(4π Dist2)
– Energy Emitted/ Area = constant x T(equil)4
=> T4 (equil) = constant × Lum(Sun) / Dist2
• Condensation Sequence: what materials are solid at each position in
nebula based on freezing point:
Atom
Molecule
Freeze
Dist
Hydrogen (H)
Helium (He)
H2
10 K
> 100 AU
Oxygen (O)
Carbon (C)
Nitrogen (N)
H2O
CH4
NH3
200- 300
> 10 AU
Iron (Fe)
Silicon (Si)
Magnesium(Mg)
FeS
700
> 1 AU
1000
> 0.5 AU
Apr 24, 2006
(Mg,Fe)SiO4
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Formation Scenario
Formation stages:
• I. Solid particles collide, stick together, sink toward disk
("planetesimals"):
– Ices don't condense inside orbit of Mars:
– Terrestrial planetesimals mostly rock, metals; not very massive
– Jovian planetesimals mostly Icy materials + rock; much more
massive
• II. Coolest, most massive (J,S) collect H,He by gravity
• III. More Collisions => heating, differentiation of
interiors
• IV. Flushing of remnants by solar radiation, wind;
planetary collisions, ejection
• V. Evolution of atmospheres
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Other Solar Systems
Until 14 years ago, our solar system was the only one known.
Since then, 97 more (163 planets) found around nearby
solar-type stars by doppler effect: planet- star system is
like spectroscopic binary, with acceleration of planet on
star seen as cyclic variation of positions of absorption lines
in stellar spectrum (like "single-line spectroscopic
binary"). Need very high accuracy: doppler effects of like
10 meters/sec. Check out this website:
http://planetquest.jpl.nasa.gov/index.html
• Example: 47 Ursa Majoris (follow Big Dipper pointers
backwards to 5th mag star)
–
–
–
–
Distance: 17 pc (55 ly)
Spectral Type: G0V
Luminosity: 1.3 Lsun
Planet (47 UMa b): M > 2.4 M(Jup)
Period: 1103 days
Velocity amplitude: 45 m/s
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Comparative Solar Systems
• A problem: this method most sensitive to massive
planets close in, which shouldn't exist according to
simple formation theory!
• Latest fix to theory suggests that it is possible for
planet orbits to spiral in towards star after
formation, if the nebula is very thick, or the TTauri flushing occurs late.
• But: why didn't this happen to our SS?
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Our Solar System
Figure 14.2, p440, Arny
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Interiors of the Planets
Figure 14.6, p445, Arny
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ProtoPlanetary Systems in Orion
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Nebular Model
Figure 14.11, p450, Arny
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Some Solar Systems
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