Formation of the Formation of the Solar SystemSolar System

The Age of the Solar SystemWe can estimate the age of the Solar System by looking at radioactive isotopes. These are unstable forms of elements that produce energy by splitting apart (i.e., fission).

The radioactivity of an isotope is characterized by its half-life – the time it takes for half of the parent to decay into its daughter element. By measuring the ratio of the parent to daughter, one can estimate how long the material has been around.

Radioactive Elements

Isotope#

protons#

neutrons Daughter Half-life (years)

Rubidium-87 37 50 Strontium-87 47,000,000,000

Uranium-238 92 146 Lead-206 4,510,000,000

Uranium-235 92 143 Lead-207 710,000,000

Potassium-40 19 21 Argon-40 1,280,000,000

Aluminum-26 13 13 Magnesium-26 730,000

Carbon-14 6 8 Nitrogen-14 5,730

Each of these isotopes spontaneously decays into its daughter. In each case, the daughter weighs less than the parent – energy is produced.

Age of the Solar System

When rocks are molten, heavier elements (such as uranium) will separate out from other elements. (In liquids, dense things sink, light things rise.) Once the rocks solidify, the material can no longer differentiate. Lighter elements (made from radioactive decay) stay in the same location as they form.

• On Earth, most old rocks have ages of 3 billion years

• The oldest asteroids have ages of 4.5 billion years

• Rocks from the “plains” on the Moon have ages of about 3 billion years. The oldest Moon rocks have ages of 4.5 billion years.

The solar system is therefore 4.5 billion years old.

Keys to Solar System FormationAny theory for the formation of the Solar System must explain

The flatness of the Solar System, and orbital similarities The separation of Terrestrial and Jovian planets The decrease in planet densities with distance from the Sun Bode’s Law

Star/Planet Formation

The story of planet formation is in large part, the story of star formation. Inside dense interstellar clouds of gas and dust, the temperature is just a few degrees above absolute zero. Since the temperature is so low, there is no gas pressure to resist gravity. The cloud collapses.

Initial Collapse

Each fragment is a protostar.

Also, since the clouds are lumpy to begin with, the collapse process causes the clouds to fragment.

Dark clouds are much denser in their center than on the outside, so their inner regions collapse first.

Formation of the Solar Nebula

Self gravity begins to collapse the cloud

As the cloud gets smaller, it begins to rotate faster, due to conservation of angular momentum.

Centripetal force prevents gas from collapsing in the plane of rotation

Gas falling from the top collides with gas falling from the bottom and sticks together in the ecliptic plane

In a large, slowly rotating cloud of cold gas

Formation of the Solar Nebula

The densest region (the center) becomes the Sun. Friction in the disk causes the Sun to accrete matter and grow in mass. Eventually, fusion occurs.

Atoms orbiting in the disk bump together and form molecules, such as water. Droplets of these molecules stick together to form planetesimals.

In the flat solar nebula

Formation of the Solar Nebula

Over time, the planetesimals grow as more molecules condense out of the nebula

Differential rotation (due to Kepler’s laws) cause particles in similar orbits to meet up. They stick together forming a bigger body.

The bigger the body, the greater its gravity, and the more attraction it has for other bodies. Protoplanets form.

Planetesimals grow …

Formation of the Solar Nebula

While protoplanets are forming, the Sun’s luminosity is growing, first due to gravitational contraction, then due to nuclear ignition.

Regions of the nebula close to the Sun will get hot; the outer regions will stay cool. In the hot regions, light elements will evaporate; only heavy elements will condense out of the nebula

Material begins to evaporate

Temperature of the Solar Nebula

Inside the orbit of the Earth, only metals can condense out of the solar nebula. Rocky (silicates) can condense near Mars. In the outer solar system, water and ammonia ice can survive.

Radiation Pressure and the Solar Wind

Two other processes are also important for driving light gases from the inner part of the solar system.

Radiation pressure: Photons act like particles and push whatever particles and dust they run into.

Solar wind: The Sun constantly ejects (a little) hydrogen and helium into space. This solar wind pushes whatever gas and dust it runs into.

The Pre-Main Sequence Sun

As the Sun formed, it generated its energy via gravitational contraction. During this time, it was a lot brighter than it is today. The radiation pressure in the inner solar system was greater.

In addition, due to conservation of angular momentum, the young Sun was also spinning faster than it is today. This caused the solar wind to be stronger.

The Pre-Main Sequence Sun

As the Sun formed, it generated its energy via gravitational contraction. During this time, it was a lot brighter than it is today. The radiation pressure in the inner solar system was greater.

In addition, due to conservation of angular momentum, the young Sun was also spinning faster than it is today. This caused the solar wind to be stronger.

Radiation pressure and the solar wind blew out the light material from the inner part of the solar system.

The Protoplanetary Disk

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Accretion Once the major bodies of the solar system were formed, most of the remaining debris was either ejected out of the solar system or accreted onto other bodies by gravitational encounters.

Accretion Once the major bodies of the solar system were formed, most of the remaining debris was either ejected out of the solar system or accreted onto other bodies by gravitational encounters.

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Accretion Once the major bodies of the solar system were formed, most of the remaining debris was either ejected out of the solar system or accreted onto other bodies by gravitational encounters.

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Accretion Once the major bodies of the solar system were formed, most of the remaining debris was either ejected out of the solar system or accreted onto other bodies by gravitational encounters.

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Unless a body is well-separated from everything else, or its orbit is in a resonance, its orbit will be chaotic. Eventually, it will either crash into something, or leave the solar system completely.

Accretion Once the major bodies of the solar system were formed, most of the remaining debris was either ejected out of the solar system or accreted onto other bodies by gravitational encounters.

Formation of the Solar System

From interstellar cloud to planetary system

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Observations of Protostellar Disks

Our technology is just beginning to be able to resolve the proto-planetary disks around stars.

Observations of Protostellar Disks

Our technology is just beginning to be able to resolve the proto-planetary disks around stars.

Evolution of Terrestrial Planets

After the condensation and accretion phases of planet formation, terrestrial bodies can go through 4 different stages of evolution. (The rates of evolution can vary greatly.)

Differentiation – in a molten planet, heavy materials sink

Differentiation

Early in the history of the solar system, planets would be molten due to

Continuous accretion of left over material from the solar system formation.

Energy from the fission of radioactive isotopes.

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Evolution of Terrestrial Planets

After the condensation and accretion phases of planet formation, terrestrial bodies can go through 4 different stages of evolution. (The rates of evolution can vary greatly.)

Differentiation – in a molten planet, heavy materials sink Cratering – left over bodies impact the planet’s surface

Evolution of Terrestrial Planets

After the condensation and accretion phases of planet formation, terrestrial bodies can go through 4 different stages of evolution. (The rates of evolution can vary greatly.)

Differentiation – in a molten planet, heavy materials sink Cratering – left over bodies impact the planet’s surface Flooding – water, lava, and gases trapped inside the planet

come to the surface and cover the terrain.

Evolution of Terrestrial Planets

Erosion – surface features are destroyed due to running water, atmosphere, plate tectonics, and geologic motions

After the condensation and accretion phases of planet formation, terrestrial bodies can go through 4 different stages of evolution. (The rates of evolution can vary greatly.)

Differentiation – in a molten planet, heavy materials sink Cratering – left over bodies impact the planet’s surface Flooding – water, lava, and gases trapped inside the planet

come to the surface and cover the terrain.