Chapter 6 The Solar System

6.1 An Inventory of the Solar System

  • Early astronomers knew Moon, stars, Mercury, Venus, Mars, Jupiter, Saturn, comets, and meteors

  • Now known

    • One star

    • 165 moons

    • Eight planets

    • Eight big asteroids

    • More than 100 Kuiper belt objects more than 300 km in diameter

    • Smaller asteroids, comets, and meteoroids

  • Extrasolar planets: planets that are not in our solar system

  • More than 200 extrasolar planets have been found

  • Understanding planetary formation is our own solar system helps understand it’s formation as well as formation of other systems

6.2 Measuring the Planets

  • Orbital period can be observed

  • Distance from Sun known by Kepler’s law

    • p²/ a³ =1

    • p=period

    • a= semi major axis

  • Radius known from angular size

    • angular size also called apparent size

    • Apparent size is proportional to the actual size

    • universal proportional to the distance

  • Masses from Newton’s laws

    • gravitational law and 2nd law

  • Rotation period from observations

  • Density can be calculated knowing radius and mass

    • density = mass/volume

6.3 The Overall Layout of the Solar System

  • All orbits but Mercury’s are close to same plane

  • Because the planet’s orbits are close to being in a plane, it is possible for them to appear in a straight line as viewed from Earth.

6.4 Terrestrial and Jovian Planets

  • Terrestrial Planets:

    • Mercury, Venus, Earth, Mars

  • Jovian Planets

    • Jupiter, Saturn, Uranus, Neptune

  • Difference among the terrestrial planets

    • All have atmospheres, but they are very different; surface conditions vary as well

    • Only Earth has oxygen in its atmosphere and liquid water on its surface

    • Venus’s rotation is slow and retrograde

    • Only Earth and Mars have moons

    • Only Earth and Mercury have magnetic fields

6.5 Interplanetary Matter

  • Asteroids and meteoroids: rocky composition

  • The Only Difference: Size

  • Asteroids are bigger, larger than 100m in diameter.

  • Comets: icy, with some rocky parts

  • Pluto, once classified as one of the major planets, is the closest large Kuiper Belt object to the Sun

  • Pluto: in 2006, reclassified as dwarf planet

  • Planet:

    • orbits the sun

    • massive enough that its own gravity has caused its shape to be approximately spherical

    • has ‘cleared the neighborhood’; has collided with/kicked out the debris

  • The discovery of Pluto was a pure accident. If it was discovered today, it wouldn’t be classified as planet in the first place.

  • It doesn’t fit in with either Terrestrial or Jovian planets

6.6 Spacecraft Exploration of the Solar System

  • Mariner 10: Flew by Mercury, 1974-1975

  • Next visit to Mercury: Messenger, 2011

  • Soviet Venera probes landed on Venus from 1970 to 1978

  • The Most recent Venus expedition from the United States was the Magellan orbiter, 1990-1994

  • Viking landers arrived at Mars in 1976

  • Sojourner was deployed on Mars in 1997

  • Pioneer and Voyager flew through outer solar system

  • Gravitational “slingshots” can change direction of spacecraft, and also accelerate it

  • Cassini mission arrived at Saturn in 2004, will stay 4 years

More Precisely 6-2: Angular Momentum

  • Conservation of angular momentum says that products of radius and rotation rate must be constant

6.7 How Did the Solar System Form?

  • Nebular contraction:

    • Cloud of gas and dust to gravity conservation of angular momentum means it spins faster and faster as it contracts

  • The observation of disks surrounding newly formed stars supports this theory