Chapter 8: The Solar System

Strategy to Observe Other Solar Systems

  • Our solar system extends to about 100,000 AU == 20 X 1012 from the Sun.
  • Any further, we would be closer to another star.
  • Our solar system contains planets, dwarf planets, many moons, asteroids, and comets.
  • However, there are systematics to our solar system that help us understand how it and other solar systems formed.

Components of the Solar System

  • Sun is the heart of the solar system
  • It is much more massive than the sum of all the other members of the solar system
  • Its radiation warms the members of the system.
  • Its gravity holds the solar system together.
  • However, despite its importance, we don’t study the Sun in this course.
  • Planets orbiting the Sun.
  • Inner planets - “terrestrial” planets that are close to the Sun and similar to Earth - “Terra”: Mercury, Venus, Earth, and Mars. They share a lot of common properties.
  • Outer == “Jovian” planets that are farther from the Sun and similar to Jupiter - “Jove”: Jupiter, Saturn, Uranus, and Neptune. (named after Jupiter because we recognize it is the largest in that family)
  • Dwarf planets that are tiny and very far from the Sun: Pluto (first found), Eris,…

Asteroids and Comets

  • Asteroids are rocky objects orbiting the Sun, that are much smaller than planets so they look like stars - asters. (also called minor planets)
  • Most asteroids orbit between Mars and Jupiter in the “asteroid belt”.
  • Comets are tiny ice objects that orbit the Sun mostly beyond Neptune in the Kuiper belt (named after an astronomer) and the Oort (astronomer who figured out comets are at an even more distance then thought of) cloud.
  • (When comet is close to the sun, the ice is heated, vaporized and develops a tail)

Orbits and Spins

  • All the major (terrestrial and Jovian) planets orbit in the same direction and in nearly the same plane aligned above the Sun’s equator.
  • Most of the major planets spin in the same direction as they orbit- except for Venus and Uranus
  • Pluto's orbit is tilted, and eris is tilted even more


  • We have already learned how we can measure a planet’s radius (Ch 2) and mass (Ch 3).
  • Using these we can find the planet’s
  • Avg density == mass/ volume
  • This gives us important information about the planet’s composition
  • The inner planets have average densities similar to earth, they are made up of rocks and metals
  • Detailed analysis shows that the fraction of rock and metal varies.
  • The outer planets have much lower average densities than Earth \rightarrow they are mostly hydrogen gas and fluids with a small fraction of rock and metal.
  • The rock and metal is in the core, covered by very deep oceans and atmospheres.

Age of the Solar System

  • 4.6- 4.8 billion years

Other Planetary Station

  • Astronomers assumed for along time that other planetary systems exist, but detecting them is VERY hard.
  • Planets are tiny and very faint.
  • Stars are huge and very bright.
  • Stars are light-years from Earth.
  • About 20 years ago we developed methods to detect exoplanets.

Detecting Exoplanets

  • Doppler method uses the Doppler shift we learned in Ch 4.
  • In CH 3 we learned Newton’s 3rd law of motion: action = reaction.
  • Because of this, as a star’s gravity causes a planet to move in orbit, the planet’s gravity also causes the star to orbit.
  • Both the planet and the star orbit around the balance point between them.
  • The planet is very far from the balance point and does a very big orbit.
  • The star is very close to the balance point and does a very small orbit.
  • But we observe the star’s spectrum and can measure its motion using the Doppler shift of its spectral lines as it reacts to the planet.
  • If there are several planets, each has an effect, causing a complicated pattern of spectral shifts.
  • The Doppler method works best for:
  • Massive planets similar to Jupiter because they cause the star to react faster. ( because more mass means a stronger force of gravity)
  • Planets very close to the star because the orbit is completed more quickly so we make the discovery faster.
  • Therefore, we are certain to miss smaller planets farther from the star.
  • The transit method is another way to discover exoplanets.
  • “Transit” means the planet passes in front of the star as seen from Earth, but only a tiny fraction of planets have the perfect alignment.
  • During the transit the planet blocks some of the star’s light, making the star very slightly fainter. (that is what we’re looking for when trying to discover planets)
  • The star’s brightness dips every orbit.
  • The drop in brightness \rightarrow the planet’s size.
  • If there are several planets these cause dips of different depths and with different periods.
  • The transit method has been very successful using the satellite named Kepler because it was able to take images of about 150,000 stars continuously 24 hours every day for 4 years.
  • A similar satellite named TESS was launched in March to detect exoplanets.

Exoplanet Results

  • We have learned that none of the systems are exactly like our solar system.
  • The planets often orbit very close to their star.
  • Even planets like Jupiter that are mostly gas - gas giants can be very close to their star (Very surprising since the gas doesn’t evaporate.
  • Exoplanets have a larger range of average density than we find in our solar system.
  • Exoplanets can have very elliptical orbits
  • Very large variety of planetary systems.

Formation of Planetary Systems

  • Any theory must explain:
  • Why planetary systems appear to be flat.
  • In our solar system.
  • Why planets orbit in the same direction.
  • Why some planets are close to the Sun.
  • Why gas planets are far from the Sun.
  • Why all planets seem to have the same age.

Solar Nebula Theory

  • Developed independently by German philosopher Immanuel Kant (1724-1804) and French mathematician Pierre-Simon Laplace (1749-1827)
  • They imagined that a planetary system forms from a large (a few light-years interstellar cloud - “nebula” in Latin.
  • The mass of the cloud is made of hydrogen (71%), helium (27%), and 2% other elements.
  • The nebula began to collapse - we still do not know why.
  • Once the collapse started, the pull of gravity makes if continue.
  • Because everything spins, the original nebula would have had some very tiny rotation.
  • As the nebula collapsed, it rotated more rapidly (due to a very fundamental physical law).
  • The rotating nebula flattened into a disk.
  • The gas that collapsed to the centre of the disk was hotter while the gas farther out was colder.
  • As the gas in the disk became more concentrated it condensed into solid particles of metal, rock close the centre ++ ice farther out.
  • The solid particles collided and stuck together - accretion, forming larger particles.
  • The gravity of the larger particles attracted each other to form planetesimals, which grew into planets.
  • In the warm inner disk only rock and metal could condense \rightarrow “terrestrial” planets formed there.
  • In the cold outer disk rock, metal, and icy matter could condense \rightarrow “Jovian” planets formed there.
  • Because hydrogen and helium are 98% of all matter, the Jovian planets are much larger than the terrestrial planets.
  • However, the exoplanets show us that this explanation is incomplete.
  • Planet building continued for a long time as the planetesimals continued to collide with the planets.
  • Cratered surfaces of the rocky planets.
  • Probably formed Earth’s Moon.
  • Creation of our Moon’s maria.
  • Some planetesimals still exist - asteroids, so collisions are still possible.

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