characteristics of our solar system

					           Characteristics of the Solar System
           (formation theory must explain all)

1. All of the planets orbit in the same direction and in the same
   plane. (Within a few degrees) This plane corresponds with
   the equator of the sun.
   – Exceptions: Mercury (7 degrees), Pluto (17 degrees)
2. Most of the planets rotate in the same direction and have
   their equators roughly aligned with the plane of the solar
   – Exception: Venus rotates in the opposite direction (retrograde).
   – Exception: Uranus and Pluto highly tilted (~90 degrees) with respect
     to the plane of solar system.
           Characteristics of the Solar System

3. Orbits of moons around planets are in the planet’s equatorial
   – Exception: Earth’s moon rotates in plane of the solar system
   – One of Neptune’s moons orbits retrograde.
4. Two (three?) types of planets…
   – Small rocky planets (Mercury, Venus, Earth, Mars)
   – Large gaseous planets (Jupiter, Saturn, Uranus, Neptune)
   – Tiny icy planets? (Pluto?)
Terrestrial Planets
                Terrestrial Planets

Small and Rocky. Crust mainly composed of Silicon
  and Oxygen (Silicates).
Atmosphere ranges from none to thick. Atmosphere
                     Terrestrial Planets

Near to the sun. (< 5 astronomical units)
   – Reminder of what an astronomical unit is….
Small and Rocky. (From 0.1 to 1 Me)
   – High density (volume/mass) (3-5x the density of water).
      • How do we measure the density?
   – Crust mainly composed of Silicon and Oxygen (Silicates)
     with small fraction of Sulfates.
      • Exception: Earth has higher percentage of carbonates in its rocks.
   – Cores of Nickel and Iron
      • Some solid, some liquid
                          Terrestrial Planets

Cratering is common on the
   surface of terrestrial planets.

From the ages of the craters, we
   can tell that impacts were
   much more common early in
   the history of the solar system
                    Terrestrial Planets

Atmosphere ranges from none (Mercury) to thick
  – Typical atmospheric components are Carbon Dioxide
    (CO2) and Nitrogen (N2)
     • Exception: Earth has Oxygen (O2) and Water (H2O) in its
Typically have no moons.
  – Exception: Earth has a very large moon.
  – Exception: Mars has two tiny moons.
Terrestrial Planet Interiors
Jovian (Gas Giant) Planets
                  Jovian (Gas Giant) Planets

Primarily composed of hydrogen (H2) and Helium (He)
Massive (15 to 300 Me)
Thick Atmosphere
   – H2, He, Methane (CH4), Ammonia (NH3)
Low density (0.7 to 1.8x the density of water)
   – Saturn would float!
Small rocky core surrounded by huge ocean of liquid hydrogen.
   – Is the core a terrestrial planet?
All are found more distant than about 5 astronomical units from
  the sun.
                   Jovian Planets

Rings! All Jovian planets have them.
                  Jovian Planets

Moons! Jovian planets tend to have very many
                              Jovian Planets



                   Liquid     Metallic
                   Hydrogen   Hydrogen   Core
                Jovian Planets

• Some (Jupiter and Saturn at least) radiate
  more energy than they receive from the sun.
  – They generate energy from gravitational
               Tiny Icy Planets

• Pluto? Charon? Are they really planets
• The moons of the Jovian planets?

• These are like the terrestrial planets, but
  instead of SiO2 they have H2O
• Tiny rocky core underneath the ice.
      Characteristics of the Solar System

5. Three types of space debris:
    – Asteroids:
        • Chunks of rock between 10
          meters and a thousand km
          in size.
        • ~20,000 of them
        • Concentrated in the plane
          of the solar system between
          Mars and Jupiter
    Characteristics of the Solar System

• Comets:
   – Chunks of ice and rock between
     10 meters and a thousand km in
   – Billions? of them
   – Most reside outside the orbit of
     Pluto in the Oort cloud. Not
     concentrated into the plane of
     the solar system
   – Occasionally one will fall into
     the inner solar system (on a
     very elliptical orbit).
    Characteristics of the Solar System

• Meteoroids:
   – Tiny bits of rock and metal
   – Most <1 gram
   – Heated by atmospheric
     friction until they glow.
   – Most follow along the orbits
     of comets
       • Debris left behind when a
         comet goes by.
     Characteristic of the Solar System

6. Age:
• The objects in the solar system are all about 4.6
  billion years old.
• How do we know this?
   – Radioactive dating
      • A radioactive element decays into a daughter
          – We don’t know what time a specific atom will decay but
            we know how long it will take for half the atoms to decay.
          – (Demo)
                Radioactive Dating

• U238>Pb206
  – Halflife:
     • 4.5 billion years
  – Oldest earth rocks
     • 3.96 billion years
  – Meteors and Moon
     • 4.6 billion years
     • This is the time they
       solidified… The
       solar system is older
       than this.
         Theory of the formation of the Solar

• A story the fits the facts….
   – Needs to explain, or at least be consistent with all the
     characteristics that we listed.
   – Needs to also be consistent with what we know about the rest of
     the galaxy. Other stars and solar systems should form in the same
                 The Solar Nebula Theory

• The sun and solar system formed from the collapse of a
  cloud of gas and dust.
   – The cloud was slowly rotating, so centrifugal force made it into a
     disk (accretion disk) transferring matter to the center.
   – Conservation of angular momentum made it rotate more quickly
              The Solar Nebula Theory

– Instabilities in the disk may have formed smaller sub-disks where
  giant planets formed
              The Solar Nebula Theory

– Dust, rock and ice condense and stick together to make small
  bodies called planetesimals.
– Heat from the forming sun only allowed certain elements to
  condense nearby. Ices could only condense far away.
                  The Solar Nebula Theory

• Two ways of building planets
   – Larger planetesimals attract smaller ones. They collide and merge
     to make a bigger planetesimals. These attract more and eventually
     form the planets
   – Near the sun, the nebular hydrogen gas is too hot (moving to fast)
     to form an atmosphere around the planets. Distant planets begin to
     form hydrogen atmospheres once they get big enough.
      • The Jovian planets captured their atmospheres.
• As time goes on the nebula cools, making the “frost line”
  move inward.
                         The Solar Nebula Theory
• The young planets start out fairly warm (in a liquid or nearly liquid state). Heavy
  elements start to sink… This concentrates the radioactive elements in the center
  (and explains why the earth’s core is hot).

•   This process is still occurring in the giant planets. It releases gravitational energy.
                 The Solar Nebula Theory

• On the terrestrial planets, gasses are released from the hot
  interior to form atmospheres.
   – Volcanic processes release H2S, SO2, CO2, H2O, NH3, N2
   – Solar UV radiation breaks apart NH3, hydrogen escapes, leaving
   – Solar UV radiation also breaks apart H2O, hydrogen escapes
     leaving O, which reacts with rock to form solid oxides.
   – H2O combines with H2S, and SO2 to make sulfuric and sulfurous
     acid. This eats away rock to form solid sulfates.
• What’s left? CO2 and N2 in the atmosphere. Oxygen and Sulfur in
  the rocks. (Why is Earth’s atmosphere different?)
                    Where did the nebula go?

• Solar wind, heat, and light pressure drove the gas away.
• What about the left over planetesimals?
   – Most of the rocky ones in the inner solar system eventually collided with
     planets. (That’s why the rate of impacts was high 4 Bya, but is low now.)
       • There’s about 20,000 left over mostly between Mars and Jupiter (Asteroids!)
       • Jupiter’s gravity prevented a planet from forming there.
   – Encounters with the giant Jovian planets kicked most of the remaining icy ones
     into the outer solar system or interstellar space
       • These are comets!
       • The encounters would kick them in any direction. (This explains why comets aren’t
         concentrated in the plane of the solar system.)
              How does this theory fit the
          characteristics of the Solar System?

1. & 2. Collapse to a disk explains the concentration in the plane of the
   solar system, and why almost everything moves in the same direction.
3. The giant planets had disks of their own so their moons orbit in their
   equatorial plane
4a. Because the inner solar system was hot, only rock and metal could
   condense which resulted in terrestrial planets
4b. The outer solar system was cold enough for ices to condense and for
   hydrogen gas to be captured by a massive enough body. This resulted
   in Jovian planets.
4c. If an object in the outer solar system wasn’t massive enough to
   capture hydrogen gas, it remained as a small icy body. (Pluto, the
   outer planet moons, comets)
              How does this theory fit the
          characteristics of the Solar System?

4d. The terrestrial planets released their atmospheres from their interiors.
   The Jovian planets captured theirs. The icy planets weren’t massive
   enough to capture one, or hot enough to release one.
4e. The inner structure of the planets is explained by differentiation.
   Heavier elements sink to the core. Lighter ones float to the surface.
5. Asteroids and comets are left over planetesimals. Meteors are bits of
   dust that have fallen off of comets
6. Everything is the same age because it all formed at about the same

               What about the exceptions?
          For every exception there is a rule...

• Tilted orbits of Mercury and Pluto.
   – Mercury probably suffered a large impact late in its formation
   – Pluto might be a left-over planetesimal.
• Retrograde rotation of Venus:
   – Probably due to a large impact late in formation.
   – Probability favors, but does not require, rotation in the same direction as the
• High axial tilt of Uranus and Pluto:
   – Also likely to be due to a large impact
   – Also, in the outer solar system, computer models suggest the nebula was less
     concentrated in the plane, which could result in large tilt of sub-disks.
          For every exception there is a rule...

• Retrograde moon of Neptune.
   – Probably a captured planetesimal.
• Oxygen in the atmosphere of earth.
   – Earth’s atmosphere is highly modified by life.
• Earth’s moon orbits in the plane of the solar system.
   – This is likely because the moon was formed from an impact with another body
     traveling in the plane of the solar system.

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