Solar System Observed Properties
• Solar system is flat – all planets orbit in same direction
• Two types of planets
– Inner: rocky; small, more dense, less massive
• Chemical composition is sun-like, excluding Hydrogen and Helium
– Outer: gaseous or liquid; large, less dense, more massive.
• Chemical composition is sun-like
• Maximum age of measured (by radioactivity) rocks (Earth,
Moon, meteorites) is 4.6 billion years; matches estimates
for age of the sun from stellar evolution theory.
The Solar System is Flat
Most of the solar system mass (the planets) is confined to a plane.
Inner Planets are Rocky
•Thin crust
•Generally thick, rocky (silicate) mantle
•Iron/Nickle core
•Chemical composition similar to Sun for
elements heavier than Hydrogen or
Helium
Outer Planets are Gaseous/Liquid
H2O ice
•Gaseous with gradual phase transition to liquid
•Rocky core deduced from spacecraft flybys
•Chemical composition similar to that of the Sun
Age From Radiometric Dating
• Oldest measured age of
any rock sample(from
Earth/Moon/meteorite):
4.6 billion years.
• Determined by
– Knowledge of radioactive
decay of isotopes
– Relative concentrations of
isotope in sample
– Technique gives minimum
age.
• Matches estimates for age
of the Sun from stellar
evolution theory.
Example: Canyon Diablo Meteorite
from Barringer Crater, AZ
Solar System Formation
• Our Milky Way Galaxy is
filled with cold, dark clouds
of gas and dust.
• These clouds are mostly
hydrogen and helium with
dust containing mostly iron,
rock, and ice.
• The Solar System is thought
to have formed from a huge,
slowly rotating cloud about
4.5 billion years ago
• A nearby passing star or
stellar explosion may have
caused the cloud to collapse
Collapsing Gas Clouds
• As the cloud collapsed the
original slow spin began to
speed up. This caused the cloud
to flatten into a disk shape.
– Ordered rotation leads to
flattening
• The gravitational pull of the
cloud caused it to shrink further
and caused most of the material
to fall towards the core forming
a large bulge.
– Infall
Collapsing Gas Clouds?
• In the Great Nebula of
the constellation Orion
are huge clouds of gas
and dust.
• Among these clouds
the Hubble Space
Telescope observed
lumps and knots that
appear to be new stars
and planets being
formed.
Planets in Formation?
• Around the star Beta
Pictoris a large disk of
dust and gas has been
observed.
• The light from the star
is much brighter than
the disk so it had to be
blocked for the disk to
appear clearly.
• Disks have been seen
around other stars too
including Vega.
Zodiacal Light
• Dust in the plane of
the solar system
scatters light from the
Sun
– This figure shows the
dust-scattered sunlight
and three planets
• Compare with BetaPictoris figure in
previous slide
Birth of the Sun
• As material falls into towards
the disk it collides with other
material and heats up and
melts.
– Infall / friction
• The increasing mass of the
core also increases the
gravitational pull and causes
more material to be pulled in.
– Accretion
• When the mass is large
enough and temperatures
high enough nuclear fusion
reactions begin in the core
and a star is born!
– Critical mass triggers
fusion
Condensation: Transition from
Gas to Liquid or Solid
• Iron and silicates
condense at temperatures
over 1000K
• Water condenses at
temperatures under 500K
• Water (and other
molecules made from H
and He) will not condense
into solid form unless the
temperature drops below
500K.
Heating and Condensation of the
Solar Nebula
•
•
•
•
The heat from the Sun prevents
ices from reforming on the dust
grains in the region near the Sun.
Ices condensed only in the outer
parts of the Solar nebula.
In the inner portion of the disk
only materials like iron and
silicates (rock) can condense into
solids. Slowly they form clumps
of material.
In the outer portion of the disk
much more material can
condense as solids including ice.
This extra material allows
clumps to grow larger and faster.
Gravity does the job
• Within the disk, material is
constantly colliding with one
another. If the collisions are not
too violent material
(planetesimals) may stick
together.
• In the outer parts of the Solar
Nebula the planets become large
enough to have a significant
gravitational pull and collect gas
around them.
– Ice is ten times more abundant
than silicates and iron
compounds, therefore there is
more planet building material
in the outer solar system.
• Planets in the inner nebula can not
grow enough to collect much gas.
• Eventually most but not all of the
material was swept up by the
planets.
The Last of the Planetesimals
The remaining material
exists today as
– comets which were flung
out to a region far beyond
Pluto called the Oort
cloud and
– asteroids mostly between
Mars and Jupiter (the
Asteroid Belt) and beyond
Pluto (the Kuiper Belt)
Comets and Asteroids
The Kuiper Belt and Oort Cloud
The Last of the Planetesimals
Note that the composition
of planetesimals
approximates that of the
planets
– comets – mostly in the
outer solar system: ice +
rock
– Those asteroids between
Mars and Jupiter: rock
In general, there are no
permanent icy residents
in the inner solar system.
Collisions
• A Mars-sized body impacted the Earth, the
Moon forms from the debris
• A massive impact likely blasted away
Mercury’s crust
• Are planetesimal impacts responsible for
the peculiar rotation axis tilts of Venus and
Uranus?
Outer Planet Satellites
• Planetesimal capture
• Planetesimals then collide and form larger
objects (moons).
Atmospheres
• Outer planets
– Captured Hydrogen from solar nebula
• Inner planets
– Volcanic activity
– Captured gas from comets that vaporized on
impact
Planet Hunting 1:
Direct Imaging
Planet Hunting 2:
Doppler Shift
Redshift / Blueshift
23
Planet Hunting 3:
Gravitational Lensing
Planet Hunting 4:
Transit Light Curves