Our Solar System

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					    Our Solar System

Origins of the Solar System
Astronomy 12
Learning Outcomes (Students will…)
-Explain the theories for the origin of the solar system

-Distinguish between questions that can be answered by science and those
that cannot, and between problems that can be solved by technology and
those that cannot with regards to solar system formation.

-Estimate quantities of distances in parsec. Estimate the age of the solar

-Describe and apply classification systems and nomenclature used in the
sciences. Classify planets as terrestrial vs. Jovian, inner vs. outer, etc.
Classify satellites. Classify meteoroid, asteroid, dwarf planet, planet.
Classify comets as long period vs. short period. etc

-Formulate operational definitions of major variables. Given data such as
diameter and density describe the properties that divide the planets and
moons into groups.

-Tools and methods used to observe and measure the inner and the outer
planets and the minor members of the solar system
Our Solar System
 Our solar system is made
   up of:
  Sun
  Eight planets
  Their moons
  Asteroids & Meteroids
  Comets
     Inner Planets
The inner four rocky planets
 at the center of the solar
 system are:

 Planet nearest the sun
 Second smallest planet
 Covered with craters
 Has no moons or rings
 About size of Earth’s moon
 Sister planet to Earth
 Has no moons or rings
 Hot, thick atmosphere
 Brightest object in sky besides sun and
  moon (looks like bright star)
 Covered with craters, volcanoes, and

 Third planet from sun
 Only planet known to have life and
  liquid water
 Atmosphere composed of Nitrogen
  (78%), Oxygen (21%), and other gases

 Fourth planet from sun
 Appears as bright reddish color in the
  night sky
 Surface features volcanoes and huge
  dust storms
 Has 2 moons: Phobos and Deimos
         Asteroid Belt

 Separates the inner, terrestrial planets
  from the outer, Jovian planets
 Contains ~100,000 asteroids.
 Largest known asteroid: 4 Vesta
 Largest object : Ceres (dwarf planet)
       Outer Planets
The outer planets composed
 of gas are :

 Largest planet in solar system
 Brightest planet in sky
 At last count, 65 moons: 5 visible from
 Strong magnetic field
 Giant red spot
 Rings have 3 parts: Halo Ring, Main
  Ring, Gossamer Ring
   6th planet from sun
   Beautiful set of rings
   62 moons
   Largest moon, Titan,
   Easily visible in the night
   Voyager explored Saturn
    and its rings.
   7th planet from sun
   Has a faint ring system
   27 known moons
   Covered with clouds
   Uranus sits on its side with the north
    and south poles sticking out the
 8th planet from sun
 Discovered through math
 12 known moons
 Triton largest moon
 Great Dark Spot thought to be a
  hole, similar to the hole in the
  ozone layer on Earth
        A Dwarf Planet

   Pluto is a small solid icy
    planet is smaller than the
    Earth's Moon.
   Never visited by
   Orbits very slowly

   Charon, its moon, is
    very close to Pluto
    and about the same
Two Types of Planets: Terrestrial and Jovian

 Small bodies
 Believed to be left over
  from the beginning of
  the solar system
  billions of years ago
 100,000 asteroids lie in
  belt between Mars and
 Largest asteroids have
  been given names

 Small icy bodies
 Travel past the Sun
 Give off gas and dust as
  they pass by
Anatomy of a Comet
Anatomy of a Comet
Anatomy of a Comet
   How was the Solar System
A viable theory for the formation of the solar
  system must be:

      • based on physical principles (angular
        momentum, the law of gravity, the law of

      • able to explain all (at least most) the
        observable facts with reasonable

      • able to explain other planetary systems
      How was the Solar System
A viable theory for the formation of the solar system
  must account for 4 characteristics:

           1.   Patterns of motion
           2.   Two types of planets
           3.   Asteroids & comets
           4.   Exceptions to patterns
                  Patterns of Motion
•   All the planets orbit the Sun in the same direction
•   The rotation axis of most of the planets and the Sun are
    roughly aligned with the rotation axis of their orbits.
•   Orientation of Venus, Uranus, and Pluto’s spin axes are    Why do they spin in
    not similar to that of the Sun and other planets.          roughly the same

                                                                  Why are they
What does the solar system look like
          from far away?
•   Sun, a star, at the center

•   Inner (rocky) Planets
    (Mercury, Venus, Earth,
    Mars) ~ 1 AU

•   Asteroid Belt ~ 3 AU

•   Outer (gaseous) Planets
    (Jupiter, Saturn, Neptune,
    Uranus) ~ 5-40 AU

•   Kuiper Belt ~ 30 to 50 AU
    -includes Pluto

•   Oort Cloud ~ 50,000 AU
                                 Bode’s Law
•A rough rule that predicts the spacing of the planets in the Solar System

•To find the mean distances of the planets, beginning with the following simple sequence of
                              0 3 6 12 24 48 96 192 384
•With the exception of the first two, the others are simple twice the value of the preceding

•Add 4 to each number:                                                              Works
                                4 7 10 16 28 52 100 196 388                           for
•Then divide by 10:                                                                 moons
                        0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.6 38.8                       too!
                      Planet           Actual Distance (AU)    Bode’s Law

                      Mercury                  0.39               0.4

                      Venus                    0.72               0.7

                       Earth                   1.00               1.0

                       Mars                    1.52               1.6

                      Jupiter                  5.20               5.2

                      Saturn                   9.54               10.0

                      Uranus                   19.2               19.6

                      Neptune                  30.1               38.8
            Where are the asteroids?
Most asteroids are
located in two regions:

•Asteroid belt

•Orbit of Jupiter… the
Hildas (the orange
"triangle" just inside the
orbit of Jupiter) and the
Jovian Trojans (green).
The group that leads
Jupiter are called the
"Greeks" and the trailing
group are called the
            Where are the comets?
Kuiper Belt
A large body of small
objects orbiting (the short
period comets <200 years)
the Sun in a radial zone
extending outward from the
orbit of Neptune (30 AU) to
about 100 AU. Pluto maybe
the biggest of the Kuiper
Belt object.

Oort Cloud
Long Period Comets
(period > 200 years) seems
to come mostly from a
spherical region at about
50,000 AU from the Sun.
                             Exceptions to Patterns

•Uranus has different axial tilt
•Some moons larger than others
•Some moon have unusual orbits
             Planetary Nebula or Close
Historically, two hypothesis were put forward to explain the formation of the solar

   #1 - Gravitational Collapse of Planetary Nebula
    Solar system formed form gravitational collapse of an interstellar cloud of gas

   #2 - Close Encounter (of the Sun with another star)
    Planets are formed from debris pulled out of the Sun during a close encounter
    with another star. But, it cannot account for
            The angular momentum distribution in the solar system,
            Probability for such encounter is small in our neighborhood…

                  Astronomers favour Hypothesis #1
    The Nebular Theory* of Solar System
                                                          *Itis also called the
                            Interstellar Cloud (Nebula)
                                                          ‘Protoplanet Theory’.

                              Gravitational Collapse

     Protosun                  Protoplanetary Disk
Heating  Fission/Fusion   Condensation (gas to solid)    (depends on temperature)

         Sun               Metal, Rocks     Gases, Ice

                            Accretion        Nebular

 Leftover Materials         Terrestrial       Jovian            Leftover Materials
    Asteroids                Planets          Planets               Comets
          Collapse of the Solar Nebula

                               Denser region in a interstellar cloud, maybe compressed
                               by shock waves from an exploding supernova, triggers
                               the gravitational collapse.
1.   Heating  Protosun  Sun
     In-falling materials loses gravitational potential energy, which were converted into kinetic
     energy. The dense materials collides with each other, causing the gas to heat up. Once the
     temperature and density gets high enough for nuclear fusion to start, a star is born.
2.   Spinning  Smoothing of the random motions
     Conservation of angular momentum causes the in-falling material to spin faster and faster
     as they get closer to the center of the collapsing cloud.
3.   Flattening  Protoplanetary disk.
     The solar nebular flattened into a flat disk. Collision between clumps of material turns the
     random, chaotic motion into a orderly rotating disk.
                  This process explains the orderly motion of
                       most of the solar system objects!
     The Solar
  Basis of modern theory
   of planet formation.

    Planets form at the
    same time from the
  same cloud as the star.

   Planet formation sites
  observed today as dust
   disks of T Tauri stars.

  Sun and our solar system
formed ~ 5 billion years ago.
Beta Pectoris dust disk
Planetesimals forming planets
for Ongoing
    Many young
 stars in the Orion
    Nebula are
  surrounded by
    dust disks:
 Probably sites of
 planet formation
    right now!
Dust Disks

Dust disks around
some T Tauri stars
  can be imaged
  directly (HST).
   The Story of Planet Building
    Planets formed from the same protostellar material
      as the sun, still found in the sun’s atmosphere.

       Rocky planet material formed from clumping
     together of dust grains in the protostellar cloud.

 Mass of less than ~ 15          Mass of more than ~ 15
    Earth masses:                   Earth masses:
                                  Planets can grow by
Planets can not grow by
                            gravitationally attracting material
 gravitational collapse
                               from the protostellar cloud
    Earthlike planets
                                Jovian planets (gas giants)
                           Extrasolar Planets

                                                    An extrasolar planet,
                                                    or exoplanet, is a
                                                    planet beyond our solar
                                                    system, orbiting a star
                                                    other than our Sun

Information obtained primarily from wikipedia.org
      Types of Extrasolar Planets
Hot Jupiter
A type of extrasolar planet whose mass is close to or exceeds
that of Jupiter (1.9 × 1027 kg), but unlike in the Solar System,
where Jupiter orbits at 5 AU, hot Jupiters orbit within
approximately 0.05 AU of their parent stars (about one eighth
the distance that Mercury orbits the Sun)
Example: 51 Pegasi b
       Types of Extrasolar Planets
Pulsar Planet
A type of extrasolar planet that is found orbiting pulsars, or
rapidly rotating neutron stars

Example: PSR B1257+12 in the constellation Virgo
      Types of Extrasolar Planets
Gas Giant
A type of extrasolar planet with similar mass to Jupiter and
composed on gases

Example: 79 Ceti b
   Methods of Detecting Extrasolar
Transit Method
•If a planet crosses ( or
transits) in front of its parent
star's disk, then the observed
visual brightness of the star
drops a small amount.
•The amount the star dims
depends on the relative sizes
of the star and the planet.
   Methods of Detecting Extrasolar
•This method consists of precisely
measuring a star's position in the
sky and observing how that position
changes over time.
•If the star has a planet, then the
gravitational influence of the planet
will cause the star itself to move in a
tiny circular or elliptical orbit.
•If the star is large enough, a
‘wobble’ will be detected.
  Methods of Detecting Extrasolar
Doppler Shift (Radial Velocity)
•A star with a planet will
move in its own small orbit in
response to the planet's
gravity. The goal now is to
measure variations in the
speed with which the star
moves toward or away from
•In other words, the
variations are in the radial
velocity of the star with
respect to Earth. The radial
velocity can be deduced
from the displacement in the
parent star's spectral lines
(think ROYGBIV) due to the       •A red shift means the star is moving away from Earth
Doppler effect.                  •A blue shift means the star is moving towards Earth
   Methods of Detecting Extrasolar
Pulsar Timing
•A pulsar is a neutron star: the small,
ultra-dense remnant of a star that has
exploded as a supernova.
•Pulsars emit radio waves extremely
regularly as they rotate. Because the
rotation of a pulsar is so regular, slight
changes in the timing of its observed
radio pulses can be used to track the
pulsar's motion.
•Like an ordinary star, a pulsar will
move in its own small orbit if it has a
planet. Calculations based on pulse-
timing observations can then reveal
the geometry of that orbit
   Methods of Detecting Extrasolar
Gravitational Microlensing
•The gravitational field of a star acts like a lens, magnifying the light of a
distant background star. This effect occurs only when the two stars are
almost exactly aligned.
•If the foreground lensing star has a planet, then that planet's own
gravitational field can make a detectable contribution to the lensing effect.
   Methods of Detecting Extrasolar
Direct Imaging
•Planets are extremely faint light sources compared to stars and what little
light comes from them tends to be lost in the glare from their parent star.
•It is very difficult to detect them directly. In certain cases, however, current
telescopes may be capable of directly imaging planets.

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