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.

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A Model of Solar System Formation

• 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 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.

Condensation and the Formation of the Planets

• 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

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.

Atmospheres

• 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.

• 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.

Finding Young Planets in Their Formative Years

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.

• 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

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.

• 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.

Jupiter-Sized Worlds