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/LiquidH2O 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 Beta-Pictoris 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 AsteroidsThe 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

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Redshift / Blueshift

Planet Hunting 3: Gravitational Lensing

Planet Hunting 4: Transit Light Curves