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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
system.
-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:
Mercury
Venus
Earth
Mars
Mercury
Planet nearest the sun
 Second smallest planet
 Covered with craters
 Has no moons or rings
 About size of Earth’s moon

Venus
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
mountains

Earth
Third planet from sun
 Only planet known to have life and
liquid water
 Atmosphere composed of Nitrogen
(78%), Oxygen (21%), and other gases
(1%).

Mars
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

Outer Planets
The outer planets composed
of gas are :
Jupiter
Saturn
Uranus
Neptune
Jupiter
Largest planet in solar system
 Brightest planet in sky
 60+ moons, 5 visible from Earth
 Strong magnetic field
 Giant red spot
 Rings have 3 parts: Halo Ring, Main
Ring, Gossamer Ring

Saturn






6th planet from sun
Beautiful set of rings
31 moons
Largest moon, Titan,
Easily visible in the night
sky
Voyager explored Saturn
and its rings.
Uranus





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
sides.
Neptune
8th planet from sun
 Discovered through math
 7 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.
Pluto



Never visited by
spacecraft
Orbits very slowly
Charon, its moon, is
very close to Pluto
and about the same
size
Asteroids
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
Jupiter
 Largest asteroids have
been given names

Comets
Small icy bodies
 Travel past the Sun
 Give off gas and dust as
they pass by

SOLAR SYSTEM
FORMATION
Bode’s Law

Also known as the Titius-Bode Law

Titius: German astronomer who introduced the
idea (1766) that the planets in the Solar System
were spaced apart in a mathematical sequence
Bode: German astronomer who popularized
Titius’s idea (1772)


Bode’s Law: hypothesis that the bodies in
some orbital systems, including the Sun's, orbit
at semi-major axes in an exponential function of
planetary sequence.
Bode’s Law

Step 1: Create a sequence of 10 numbers that
follow the pattern
0, 3, 6, 12, …
Pattern
0
3
6
12
Bode’s Law

Step 2: Add 4 to each number in the sequence.
Pattern
0
3
6
Add 4
4
7
10
12 24 48 96 192 384 768
Bode’s Law

Step 3: Divide by 10.
Pattern
0
3
6
12 24
48
96
192
384
768
Add 4
4
7
10
16 28
52
100
196
388
772
Divide
by 10
0.4 0.7 1.0
Bode’s Law

Step 4: Compare!
Pattern
0
3
6
12
24
48
96
192
384
768
Add 4
4
7
10
16
28
52
100
196
388
772
Divide
by 10
0.4 0.7
19.6 38.8
77.2
19.2
39.44
1.0 1.6 2.8 5.2 10
Distanc 0.39 0.72 1.0 1.52 2.77 5.20 9.54
e in AU
Planet
Mer Ven
cury us
Eart Mar
h
s
Ast Jupi Satur Uran
eroi ter
n
us
d
Belt
Cer
es
30.06
Neptu Pluto
ne
Why is Bode’s Law Important?



Bode’s Law correctly determined where the
planets were at the time of its introduction
(Mercury to Saturn, with a missing planet in
between Mars and Jupiter)
Bode’s Law helped to predict where “missing”
planets (Ceres, Uranus) would be found!
Bode’s Law can be used for moons around
planets as well
Moons of Uranus
What are the shortcomings of
Bode’s Law?
Bode’s Law is a hypothesis – there is no
evidence that this is nothing more than
coincidence (scientifically speaking)
 Bode’s Law did not determine Neptune’s
orbit accurately
 Bode’s Law cannot be tested properly on
other planetary systems because only 1
system has enough planets (55 Cancri)

Questions from Bode’s Law
1) What does Bode’s Law describe?
 2) Why is Bode’s Law criticized?
 3) What has Bode’s Law determined?
 4) What else could Bode’s Law be used to
determine?

How old is the Solar System?

Approximately 4.5 to 4.6 billion years old
Planetary Nebula or Close
Encounter?
Historically, two hypothesis were put forward to explain the formation of the solar
system….

#1 – Nebular Theory or 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


Hot gas expands so planets should not form
Probability for such encounter is small in our neighborhood…
Astronomers favour Hypothesis #1
THE NEBULAR THEORY
6- Steps to Form a Solar System
Step 1: Solar Nebula
Huge cloud of cold gas and dust forms.
 This cloud spins slowly.
 The solar nebula is made up of 98% gas
(H and He) but also contains 1.4%
hydrogen compounds (water, methane,
ammonia), 0.4% silicates and 0.2%
iron/nickel/aluminum

Step 2: Protosun
The solar nebula (cloud) condenses
into a dense central region and a lessdense outer region.
 The protosun begins to spin faster and
flatten into a disk.

Step 3: Rings and Planetesimals

Instabilities in the rotating disk caused
regions within it to condense into rings.
Planetesimals formed in these rings.
Planetesimals are a few cm to a few km
in size
 Planetesimals are attracted to each
other by gravity
 They collide to build planets


This process explains the orderly
motion of most of the solar system
objects!
Step 4: Gas Giants
In the outer part of the disk, the
planetesimals are made of rock and ice.
 When big enough, they will attract large
amounts of gas around them.

Step 5: Rocky Planets
Near the protosun only rocky material
and metals can withstand the heat.
 Therefore any planets formed here will
be rocky.

Step 6: Remaining Debris Blown
Away

Radiation from the Sun blows away
most of the remaining gas and loose
material. Some of the leftover
planetesimals form the Oort cloud.
Nebular Hypothesis
Formation of the Solar System –
Nebular Theory
Planetesimals forming planets
Where else has there been
evidence for this theory???
Beta Pectoris dust disk
Possible planetary system! Too young
to tell yet!
Evidence
for Ongoing
Planet
Formation
Many young
stars in the Orion
Nebula are
surrounded by
dust disks:
Probably sites of
planet formation
right now!
Dust Disks
around
Forming
Stars
Dust disks around
some T Tauri stars
can be imaged
directly (HST).
Planets form from the
same cloud as the star.
Planet formation sites
observed today as dust
disks of T Tauri stars.
T Tauri stars are YOUNG
stars with less mass that
are very bright due to
their large radii.
Encounter Hypothesis
Problems with Close Encounter
Theory
1) Does not explain angular momentum
 2) Hot gas expands so planets should not
form
 3) Probability for such encounter is small
in our neighborhood…(main problem!)

So…How WAS the Solar System
Formed?
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
motions)
• able to explain other planetary systems
• able to explain all (or at least most) of the
observable facts with reasonable
accuracy
How was the Solar System
Formed?
A viable theory for the formation of the solar system
must account for 4 characteristics or observable
facts:
1.
2.
3.
4.
Patterns of motion
Two types of planets
Asteroids & comets
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 (fairly vertical axis).
•
Orientation of Venus, Uranus, and Pluto’s spin axes are not
Why do they spin in
similar to that of the Sun and other planets. (axial tilts >> 30’)
roughly the same
orientation?
Why are they
different?
Patterns of Motion
The rotation axis of most of the planets and the Sun are roughly aligned with the
rotation axis of their orbits (fairly vertical axis).
Orientation of Venus, Uranus, and Pluto’s spin axes are not similar to that of the
Sun and other planets. Their axial tilts are much greater than 30’.
Questions
1) Explain how all the planets orbiting the
Sun in the same direction makes sense
using the Solar Nebula theory.
 2) What problems are still evident in this
theory in terms of patterns of motion?

Two Types of Planets: Terrestrial and Jovian
Why are there 2 types of planets according to
the Solar Nebular theory?
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 Earth masses:
Planets cannot grow by
gravitational collapse. These
planets are made of metal and rock
and can withstand the Sun’s heat.
Earthlike planets
Mass of more than~15 Earth masses:
Planets are large enough to grow
by gravitationally attracting
material from the dust/gas cloud
Jovian planets (gas giants)
What about Pluto???
Exceptions to Patterns
•Uranus has different axial tilt
•Some moons larger than others
•Some moons have unusual
orbits
So…
1) Uranus’ tilt must have occurred due to
an impact after it formed.
 2) Moons are formed either by collisions
of planetesimals or by disks of gas/dust
around the planets (similar to how the
planets formed).
 3) Unusual orbits…

The Jovian Problem
Two problems for the theory of planet formation:
1) Observations of extrasolar planets indicate that
Jovian planets are common.
2) Protoplanetary disks tend to be evaporated quickly
(typically within ~ 100,000 years) by the radiation of
nearby massive stars.
 Too short for Jovian planets to grow!
Solution:
Computer simulations show that Jovian planets can
grow by direct gas accretion without forming rocky
planetesimals.
Clearing the Nebula
Remains of the protostellar nebula were cleared away by:
• Radiation pressure of the sun
• Sweeping-up of space debris by planets
• Solar wind
• Ejection by close encounters with planets
Surfaces of the moon and Mercury show evidence for
heavy bombardment by asteroids.
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
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
Terrestrial Planets
Small, rocky planets that orbit close to the star
Types of Extrasolar Planets
Gas Giant
A type of extrasolar planet with similar mass to Jupiter and
composed of gases
Example 1: Corot-9b
This gas giant was discovered
March 17, 2010 over 1500
light years away!
Example 2: 79 Ceti b
Types of Extrasolar Planets
Super Earth
A super-Earth has a MASS between that of Earth and the
Solar System's gas giants. This term does not imply anything
about the surface conditions or habitability of the planet!
Example 1: Corot-7b
Example 2: GJ 1214b
Neptune
Earth
Corot-7b
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
Found orbiting a star (51 Pegasi) in
the constellation Pegasus about 50
light years away…
Types of Extrasolar Planets
Eccentric Jupiter
A gas giant that orbits the star in a highly eccentric path (like a
comet)
Examples:
16 Cygni Bb and HD 96167 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
Theoretical Extrasolar Planets…
1) Ocean Planet
2) Hot Neptune
Key Terms – Types of Extrasolar
Planets






1) Terrestrial Planet – small, rocky
2) Gas Giant – large, gaseous
3) Super Earth – size is up to 10 Earths (not as
large as the gas giants)
4) Hot Jupiter Planet: mass is close to Jupiter
(1.9 x 1027 kg) and orbit is within 0.5 AU of star
5) Eccentric Jupiter Planet: mass is close to
Jupiter but orbit is highly elliptical
6) Pulsar Planet: orbits pulsars (a pulsars is a
neutron star which is a remnant of a
gravitationally collapsed massive star)
Key Terms – Types of Extrasolar
Planets

Theoretical Planets
1) Ocean Planet
 2) Hot Neptune


http://channel.nationalgeographic.com/cha
nnel/known-universe-interactive
Methods of Detecting Extrasolar
Planets
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
Planets
Astrometry
•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
Planets
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
Earth.
•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
Doppler effect.
•A red shift means the star is moving away from Earth
•A blue shift means the star is moving towards Earth
Methods of Detecting Extrasolar
Planets
Pulsar Timing
•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 pulsetiming observations can then reveal
the geometry of that orbit
Methods of Detecting Extrasolar
Planets
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
Planets
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.