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Our Solar System
Brief Overview
of Stellar Evolution
• Pre-Main Sequence (really short time):
The phase in which a protostar forms out of a cloud of gas that is slowly
contracting under gravity
• Main Sequence (long time):
The phase in which a star-wannabe becomes hot enough to initiate and
maintain nuclear fusion of hydrogen in its core to become a true star.
• Post-Main Sequence (sorta short time):
H-burning ceases, and other kinds of burning may occur, but the star is
destined to become a White Dwarf, Neutron Star, or Black Hole
Formation of Stars and Planets
Observational Clues from the
Solar System:
1.
2.
3.
4.
5.
Orbits of planets lie nearly in
ecliptic plane
The Sun’s equator lies nearly in
the ecliptic
Inner planets are rocky and outer
ones gaseous
All planets orbit prograde
Sun rotates prograde
6.
7.
8.
9.
Planet orbits are nearly circular
Big moons orbit planets in a
prograde sense, with orbits in
equatorial plane of the planet
Rings of Jovians in equatorial
planes
S.S. mass in Sun, but angular
momentum in planet orbits
Trending the
Planets
• Substances exist in 3 states: solid, liquid, and gas.
• Condensation refers to how a gas changes to liquid.
• Shown are temperatures at which different substances
condense. Also shown is the temperature in a disk
and the current location of planets.
Accretion and Sub-Accretion
Process of
Planetesimals to
Planets
Solar Nebula Theory
Immanuel Kant (German): 1775, suggested that a rotating cloud that contracts under
gravity could explain planetary orbit characteristics
Basic Modern View –
1.
2.
3.
4.
Oldest lunar rocks ~4.6 Gyr
Planets formed over brief period of 10-100 Myr
Gas collects into “disk”, and cools leading to formation of condensates
Growth of planetesimals by collisions
a) Build up minor bodies and small rocky worlds
b) Build up Jovian cores that sweep up outer gases
Share Question
If an interstellar cloud contracts to become a star, it does so because of what force?
a) electromagnetic
b) nuclear
c) gravitational
d) centrifugal
The Chaotic Early Solar System
• Recent computer models are challenging
earlier views that planets formed in an
orderly way at their current locations
• These models suggest that the jovian planets
changed their orbits substantially, and that
Uranus and Neptune could have changed
places
• These chaotic motions could also explain a
‘spike’ in the number of impacts in the
inner solar system ~3.8 billion years ago
The Moon and terrestrial planets were bombarded by
planetesimals early in solar system history.
Cosmic Billiards
100 Myr
20 AU
880 Myr
• The model predicts:
1. After formation, giant planet orbits were
affected by gravitational ‘nudges’ from
surrounding planetesimals
2. Jupiter and Saturn crossed a 1:2 orbital
resonance (the ratio of orbital periods), which
made their orbits more elliptical. This suddenly
enlarged and tilted the orbits of Uranus and
Neptune
3. Uranus / Neptune cleared away the
planetesimals, sending some to the inner solar
system causing a spike in impact rates
planetesimals
883 Myr
~1200 Myr
N
J
S
U
The early layout of the solar system may have changed dramatically
due to gravitational interactions between the giant planets. Note how
the orbits of Uranus and Neptune moved outwards, switched places,
and scattered the planetesimal population.
The Big Picture
• The current layout of our solar system may bear
little resemblance to its original form
• This view is more in line with the “planetary
migration” thought to occur even more
dramatically in many extrasolar planet systems
• It may be difficult to prove or disprove these
models of our early solar system. The many
unexplained properties of the nature and orbits of
planets, comets and asteroids may provide clues.
Artist’s depiction of Neptune orbiting close to
Jupiter (courtesy Michael Carroll)
4 + {0,3,6,12,24,...}
d(AU) =
10
Bode’s Law
Planet
Bode’s
0.4
Actual
Error
0.4
<1%
Venus
0.7
0.7
<1%
Earth
1.0
1.0
Perfect
Mars
1.6
1.5
7%
Asteroids
2.8
2.8
<1%
Jupiter
5.2
5.2
<1%
Saturn
10.0
9.5
5%
Uranus
19.6
19.2
2%
Neptune
---
30.0
Miserable
Pluto
38.8
39.4
2%
??
77.2
---
---
Mercury
Radiative Equilibrium
Global Temperatures of Planets
Planet
Predicted
(K)
Actual
(K)
Error
(%)
Mercury
Venus
Earth
440
230
250
400
730
280
10
68
11
Mars
Jupiter
Saturn
220
105
80
210
125
95
5
16
16
Uranus
Neptune
Pluto
60
45
40
60
60
40
<1
25
<1
As the Sun, changes how will we?
Isothermal Atmosphere
One can construct a
simple model of an
atmosphere by
assuming the gas has
constant temperature.
In this case both the
pressure and density
of the gas decline with
altitude following an
exponential, like
P = P0 e-z/H
H = the scale height
For Earth H = 8 km
Atmospheric Retention/Loss
Whether or not a
planet keeps an
atmosphere arises
from a competition
between gravity and
the tendency of a gas
to expand into
vacuum (meaning
space!).
To determine if an
atmosphere is kept or
lost, compare the gas
thermal speed with
the gravitational
escape speed.
The break is vth ~ 0.1vesc
Density and Composition
<r>
(kg/m3)
<r>
(kg/m3)
Water
1000
Ices
1000
Rock
3000
2800 - 3900
Air
1.3
Brass
8600
Volcanic rock and
and stony meteorites
meteorites
5000 - 6000
Steel
7830
Iron rich minerals
minerals
Gold
19300
iron
~7900
Ex:
Moon: r(surf) ~ 2800 and <r> ~ 3300
Earth: r(surf) ~ 2800 but <r> ~ 5500