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This Set of Slides. This set of slides covers age and formation of solar system, exoplanets. Units covered: 33, 34. A number of naturally occurring atoms undergo radioactive decay. The atom splits apart into lower-mass atoms (fission.)

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This set of slides
This Set of Slides

  • This set of slides covers age and formation of solar system, exoplanets.

  • Units covered: 33, 34

Radioactive dating

A number of naturally occurring atoms undergo radioactive decay.

The atom splits apart into lower-mass atoms (fission.)

The time it takes for half of the atoms in a given sample to decay is called the material’s half-life.

After a number n of half-lives, the fraction of original material left is:

We can then use radioactive dating to tell how old a rock is.

The oldest rocks on Earth are around 4 billion years old.

Even older samples have been found on the Moon and in meteorites.

All bodies in the Solar System whose ages have so far been determined are consistent with having formed about 4.5 billion years ago.

Radioactive Dating

A model of solar system formation
A Model of Solar System Formation decay.

  • In Unit 32, it was pointed out that any model of the Solar System’s formation must account for all observations:

    • Planets revolve around the Sun more or less in the same plane.

    • Planets rotate about their axes in the same direction as they revolve around the Sun.

    • Rocky, dense planets are found close to the Sun, and gaseous bodies are farther from the Sun.

Solar nebula theory

The most successful model of Solar System formation is the decay.Solar Nebula Theory:

The Solar System originated from a rotating, disk-shaped cloud of gas and dust, with the outer part of the disk becoming the planets, and the inner part becoming the Sun.

4.5 billion years ago, the immense cloud of gas and dust that would become our Solar System began to contract (due to gravity).

As it contracted, it flattened into a disk and began to spin faster (Conservation of Angular Momentum).

Most of the material in the cloud moved to the center to become the Sun.

Solar Nebula Theory

Condensation and the formation of the planets
Condensation and the Formation of the Planets decay.

  • As the material in the center gathered, its temperature increased.

  • The rest of the disk began to cool, and the gasses present began to condense.

    • Near the center, where the temperature was highest, only silicates and metals could condense.

    • Farther out, volatile gasses could condense.

    • The layout of our current solar system takes shape.

Planetesimal formation

In the inner solar system, silicate (rocky) and metal grains accreted (stuck together) over time, to form rocky planetesimals. These would become the terrestrial planets.

In the outer solar system, icy planetesimals formed.

These planetesimals collided and gathered mass over millions of years to form the planets

Planetesimal Formation

Protoplanets and differentiation

Planetesimals grew through accretion into protoplanets, which were heated by collisions and by radioactive decay.

Denser material sank toward the center of the bodies, and lighter material floated toward the surface.

This separation process is called differentiation.

Protoplanets and differentiation


  • The atmospheres of the terrestrial planets formed last, by either (or both):

    • Outgassing

      • Volatiles trapped inside the planet escape through volcanoes or other processes.

    • Collisions

      • Volatiles could have been freed from the planet’s crust by collisions, or via direct delivery by comets.

Finding young planets in their formative years
Finding Young Planets in Their Formative Years

  • We cannot watch a planetary system evolve – it takes too long – millions of years.

  • We can, however, find other stellar systems in various stages of development.

  • In the gas and dust of the Orion Nebula, we find many protoplanetary disks, disks of dark, dusty material orbiting young stars.

  • The one shown here is only around 10 million years old, a stellar-system baby picture.

Young systems
Young Systems

  • We can view the disk directly by blocking out the light from the young star at the center.

  • These images lend credibility to the solar nebula theory.

Detecting exoplanets
Detecting Exoplanets

  • We can detect planets around other stars by using the Doppler Shift method.

    • A planet and its star revolve around a common center of mass.

    • We cannot detect the planet directly, but we can detect the resulting “wobble” in the star.

    • As the star approaches us in its orbit, its spectrum will be blue-shifted.

    • As it recedes, the spectrum will be red-shifted.

Detecting exoplanets continued
Detecting Exoplanets continued

  • Astronomers can also use the transit method.

    • We look for dimming of light from the central star as the planet eclipses the star (passes between us and the star.)

  • Both the transit method and the Doppler Shift method requires the distant planetary system to lie in a plane that is parallel to our point of view.

  • If the distant planet is large enough, or our telescopes powerful enough, we can detect distant plants by directly viewing them.

Jupiter sized worlds
Jupiter-Sized Worlds

  • Most planets we have detected are very large.

    • Several Jupiter-masses.

    • The planets we detect must be large in order to create a large enough Doppler wobble.

    • Some objects detected are not planets, but brown dwarfs.

      • Stars too low in mass to fuse hydrogen.

  • We have detected some smaller planets.

  • Could be (likely to be?) millions (billions?) of planets even in our own galaxy.