Evolution of the Solar System 
and the Planets

 

09/21/08

Format for printing

In this lecture period we discuss:

  • How was the solar system formed and how does it evolve?
  • How and of what materials were the terrestrial planets formed?
  • What differentiates the outer and inner planets?
  • What classification schemes are used to study planets?

09/21/2008


Evolution of the Solar System

The planets are byproducts of the formation of the Sun.


Relative sizes and sequence of the planets in our solar system 
(positions are not to scale)

Star birth

Click here for a gallery of famous stars.  Stars form from dense interstellar clouds of gas and dust. These clouds are typically ~30 light years in dimension and ~10,000 times larger than our Sun. When mutual gravitational attraction dominates, regions of the cloud start to condense into stars. 

Fusion reactions start when energy provided by further gravitational collapse is large enough to heat the core to ~10,000,000 K (1E7K).

 

How were the Planets formed?

The Nebula hypothesis. Probable sequence of steps in the formation of the solar system. (a) Gravitational contraction of a rotating gas cloud leads to a dense central region (eventually forming the Sun) and a more diffuse, flattened nebula. (b) Dust particles from the nebula settle onto a disc. (c) Accretion of dust into numerous small planetesimals, each a few kilometers in diameter. Collisions between planetesimals lead to capture, disintegration, or deflection of their orbits. (d) Eventually larger bodies capture the smaller ones. Uncondensed gas is blown away by the "solar wind"; this process may begin in earlier stages.

The Nebula Hypothesis

The planets of our Solar System formed due to two properties of interstellar clouds: rotation and turbulence.

 

Galileo image of asteroid Gaspra; 10/29/91 (NASA)The role of dust grains

Planets such as our own could not have formed from gas alone, but need matter in the solid phase, such as dust grains. To This process is known as accretion.  The dust grains continue to accrete slowly, eventually forming clumpy "protoplanets" or "planetesimals" of a few kilometers in dimension, like the asteroid shown. Collisions between the planetesimals eventually lead to a few larger bodies that capture smaller ones. This process is chaotic, with collisions sometimes leading to break-up of the planetesimals, changes in orbits, and often forming craters on the larger bodies. 

Summary of Planetary Evolution

  • Spin forms a fragmented disk about the protosun (core) of the shrinking nebula.
  • Chemistry, acting on elements in interstellar Nebula, allows the formation of dust grains, up to a few mm in size.
  • Gravity allows the dust grains to collide and coalesce (starts "accretion").
  • Accretion, over tens of millions of years, builds planets.

Chemical Composition of the planets.

The outer planets (Jupiter, Saturn, Uranus, Neptune and Pluto) have compositions different from the Earth and more consistent with the composition of the solar system - lots of hydrogen and helium. The table below compares the properties of the inner and outer planets and the planet contain inks to images.

Some Properties of the Planets

Planet

Diameter (km)

Distance from Sun 
(x106 km)

Surface temperature
(C)

Density
(g/cm3)

Main atmospheric constituents

Sun

1,392,000

-

5,800

 

-

Mercury

4,880

58

260

5.4 (rocky)

-

Venus

12,100

108

480

5.3 (rocky)

CO2

Earth

12,750

150

15

5.5 (rocky)

N2, O2

Mars

6,800

228

-60

3.9 (rocky)

CO2

Jupiter

143,000

778

-150

1.3 (icy)

H2, He

Saturn

121,000

1,427

-170

0.7 (icy)

H2, He

Uranus

52,800

2,869

-200

1.3 (icy)

H2, CH4

Neptune

49,500

4,498

-210

1.7 (icy)

H2, CH4

Pluto

2,300

5,900

-220

2.0

CH4

 

 

 

 

 

 

The reason for the difference between the rocky dense inner planets and the icy/gaseous outer planets is : the composition of each planet is determined by the type of material that can survive in the solid form given the temperature of the particular part of the Nebula: Condensation theory.

 

Sequence of condensation of minerals in the nebula as a function of temperature. At temperatures above about 1300K, metals and silicates can condense and become solid dust grains. At lower temperatures more volatile minerals become solids, and at temperatures of less than ~400K, hydrogen-bearing gases such as methane and ammonium become solids. Hydrogen and helium remain gases. For the inner planets, at high temperatures, the planet-building dust grains were made up of rocky materials (silicates, iron, etc.). The hydrogen and helium could have been blown away by the solar wind. For the outer planets, the hydrogen and helium was retained by a combination of the larger gravity for these massive bodies and the formation of ice.

Structure of Earth

Apart from the thin outer regions of atmosphere, ocean and crust, the Earth is composed of three main compositional layers. The mantle is about 2900km thick and makes up around 65% of Earth's total mass. Below the mantle is a dense core, which has an outer liquid region and an inner solid region. The table summarizes the properties of these various regions. 

Constituents of Earth

Component

Average Thickness (km)

Average Density (x103 kg/m3)

Fraction of Total (%)

Principal Constituents

Atmosphere

-

-

0.00009

N2, O2

Oceans

4

1.03

0.024

H2O

Crust

45

2.8

0.5

Silicates and other oxides

Mantle

2900

4.5

67

Mg silicates

Core

3400

11.0

30

2

Fe, +/-S (liquid)

Fe-Ni (solid)

 

The layered structure of the Earth (with density increasing with depth) can only be interpreted as being the product of differentiation. Differentiation is the gravitational separation of materials according to their specific gravities in a liquid mixture that was originally homogeneous throughout.

 

 

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