LECTURE 13: Solar System Overview

• What is the Sun?
• What are the most common chemical elements in the solar system?
• What are planets?
• What is the difference between terrestrial and Jovian planets?
• What are asteroids?
• How did the solar system form?

THE STORY SO FAR

• Sun is at the center of the solar system.
• Planets orbit the Sun, following Kepler's laws (ellipses, equal area, P²=a³).
• Planet motions (including small departures from Kepler's laws) explained by Newton's laws of gravity and motion.
• Comets also follow Kepler's/Newton's laws, move on elongated elliptical orbits.
• Some planets have moons, which orbit under influence of planet's gravity.
• M_Sun ~ 1000 M_Jup ~ 300,000 M_Earth
• All planets orbit Sun in the same direction, and in nearly the same plane.

THE SUN

The Sun is:

• 100 times the Earth's diameter, 300,000 times the Earth's mass.
• A large ball of gas, mostly hydrogen and helium.
• Held together by gravity, supported against collapse by pressure of hot gas.
• Radiating energy (mostly visible light) because it is much hotter than its surroundings.
• Replenishing that energy by fusing hydrogen into helium in its core.
• Surface temperature: 6000 K. Central temperature: 15 million K.
• A fairly typical star.

TOP TEN ELEMENTS

Ten most common elements in the Sun, by fraction of mass:

10. Iron (0.014 percent)
9. Sulfur (0.04 percent)
8. Neon (0.058 percent)
7. Magnesium (0.076 percent)
6. Nitrogen (0.096 percent)
5. Silicon (0.099 percent)
4. Carbon (0.40 percent)
3. Oxygen (0.97 percent)
2. Helium (27.1 percent)
1. Hydrogen (71.0 percent).

This top ten list also describes the solar system as a whole, since the Sun has most of the mass.

PLANETS

Planets are balls of matter, held together by gravity, supported against collapse by pressure.
Two basic types:

• Terrestrial planets:
  • Mercury, Venus, Earth, Mars
  • Solid. Mostly heavy elements (carbon, oxygen, silicon, iron, etc.)
  • Masses ~ 0.05 M_Earth - 1 M_Earth
• Jovian planets (a.k.a. gas giants):
  • Jupiter, Saturn, Uranus, Neptune
  • Solid cores of heavy elements.
  • Massive gaseous envelopes, mostly hydrogen and helium.
  • Masses ~ 15 M_Earth - 300 M_Earth
• Pluto is solid, small, but rather different from inner terrestrial planets.

MOONS AND RINGS

• Each of the Jovian planets has many moons, ranging enormously in mass.
• Earth has a large moon, Mars has two small moons.
• Saturn has a dramatic ring system.
• Uranus and Neptune have fainter rings.

ASTEROIDS AND COMETS

• There are many solid bodies smaller than planets orbiting the Sun.
• They range in size from meters to hundreds of km.
• The largest reservoirs are between the orbits of Mars and Jupiter (the asteroid belt) and beyond the orbit of Neptune (the Kuiper belt).
• Icy bodies on elongated elliptical orbits appear as comets when they come close to the Sun.

ORIGIN OF THE SOLAR SYSTEM -- A SKETCH

Proto-stars and proto-planetary disks:

• Stars form by gravitational contraction of clouds of gas.
• Central region heats up as it contracts, makes a star.
• Slowly rotating gas cloud spins up as it contracts, due to conservation of angular momentum.
• Spin makes collapse into a sphere hard. Instead, proto-star is surrounded by a large, thin, spinning disk, tens of AU across.
• We see such disks around forming stars.

• Inner disk: radiation from star drives away gas, melts ices.
• Small solid grains collide and stick. Snowballing process forms km-sized planetesimals after a few thousand years.
• Gravity of planetesimals accelerates growth. Collisions and sticking lead to Moon-sized proto-planets.
• Proto-planets collide, form planets. Sweep up remaining grains and planetesimals.

• Outer disk: radiation from star too weak to melt ices, drive away gas.
• Similar process builds up solid cores, several Earth masses.
• These cores attract and retain hydrogen/helium envelopes.

• Inner terrestrial planets, outer gas planets, rotating around Sun in same direction, approximately the same plane.
• Asteroids and comets are leftover solid material that didn't get swept up into planets.