Chapter 19: Origin of the Solar System
Any GOOD hypothesis about the origin of the solar system
must explain most - if not all - of its characteristics:
1. All of the planets orbit the sun in the same direction,
and in the same plane
2. The planets closest to the sun are small and rocky,
have few moons
3. The planets further from the sun are large and
contain more gas and icy materials
4. Most of the Moons orbit their planets in the same
direction as the planets orbit the sun
5. Oldest meteorites are about 4.566 billion years old
6. Planetary surfaces are all younger than the oldest
meteorites
Relative sizes of the planets
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Sun-planet distance (relative to Earth: AU)
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
0.4 AU
0.7
1.0
1.5
5.2
9.5
19
30
1 AU = 150 million km
Other residents of the solar system:
1. Dwarf planets
diameter = 1000-3000 km, smaller than Moon, orbit the sun
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Other residents of the solar system
2. Asteroids - rocky, d < 1000 km, orbit the sun
Other residents of the solar system
3. Comets - rock & ice, wide
range of sizes (~10 m to
100 km)
Other residents of the solar system
4. Moons - orbit planets, some are larger than Mercury
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Other residents of the solar system
5. Meteoroids - small fragments of asteroids that enter
earth’s atmosphere (dust to boulder sized)
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How many of the planets orbit the Sun in the same direction?
1. About half
2. All except 1 or 2
3. All of them, no exceptions
What fraction of moons orbit their planet in the same
direction as the orbit around the Sun?
1.
2.
3.
4.
None
About half
Most - a few exceptions
All of them, no exceptions
Any GOOD hypothesis about the origin of the solar system
must explain most - if not all - of its characteristics:
1. All of the planets orbit the sun in the same direction,
and in the same plane
2. The planets closest to the sun are small and rocky,
have few moons
3. The planets further from the sun are large and
contain more gas and icy materials
4. Most of the Moons orbit their planets in the same
direction as the planets orbit the sun
5. Oldest meteorites are about 4.566 billion years old
6. Planetary surfaces are all younger than the oldest
meteorites
•
•
•
•
Interstellar cloud of gas &
dust collapsed under its own
gravity
Predicted that protoplanetary
nebulae would be observed
Explains all of the major
features of solar system, and
also the exceptions
Observations continue to
support this theory
Protoplanetary disks last for only about 1-10 million years
What are the planets made of?
Element
how many atoms gas or solid at
(total)
Earth
Jupiter
________________________________________________
Hydrogen
705,700
gas
gas
Helium
275,200
gas
gas
Carbon
3,032
gas
soot (solid)
Nitrogen
1,105
gas
ice
Oxygen
5,920
H2O gas
H2O ice
Silicon
653
rock
rock
Iron
1,169
metal
metal
Planet formation: Terrerstrial vs. giant planets
Giant (“jovian”)
1. Lots of solids in the
disk (cold > 5 AU)
2. Cores form from
ice, rock and metal
3. Grow large, quickly
(~1 million years)
4. Big enough to trap
H and He gas from
disk
Terrestrial (“earth like”)
1. Very little solid material in
disk at 1 AU
2. Form from rock and metal
only
3. Grow slowly (~100 million
years)
4. Too small to trap any gas
from disk
The next billion years: Debris disks
•
•
•
•
•
Gas and fine dust blows
away after ~ 10 million years
Older stars have ‘debris
disks’ around them
Need a supply of larger
objects to regenerate the
dust that gets blown away
evidence of planets forming
around other stars
Debris disks are analogous
to the Oort cloud and Kuiper
belt of comets, and the
asteroid belt
Asteroid belt
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The Nebular theory predicts
most other sun-like stars
should have planets
Do they?
358 planets have been found around other stars!!!
http://www.exoplanets.org
Detecting planets around other
stars: Doppler method
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Transit method (Kepler Mission)
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