Download origin of solar system

Guiding Questions
1. 
2. 
3. 
4. 
5. 
6. 
Why are some elements (like gold) quite rare, while
others (like carbon) are more common?
How do we know the age of the solar system?
How do astronomers think the solar system
formed?
Did all of the planets form in the same way?
What must be included in a viable theory of the
origin of the solar system?
Are there planets orbiting other stars? How do
astronomers search for other planets?
12.1 Diversity of the solar system
The diversity of the solar system is a result of its origin and
evolution. A model of solar system origin shall explain it.
Q1: compare terrestrial and Jovian planets and list
prominent similarities and differences between the two
groups.
o  Similarities:
- motion
o  Differences:
- distance
- size
- density and composition
Orbits of planets
Sizes of the planets in scale.
12.2 The abundances of the chemical elements
Abundant H and He from the Big Bang
A star ejecting its
mass into cloud
Gravitational contraction of giant
clouds generate proto-stars
Heavy elements form from
thermonuclear reaction in stellar cores
birth of new stars
Stellar remnants returned to interstellar
medium and recycled into new stars
The solar system is born out of star dust.
stars in old galaxies are metal poor, and stars in young
galaxies are metal rich.
Abundances (# of atoms) of light elements in the Milky Way.
Hydrogen and helium are dominant, and all other heavier
elements are relatively rare.
By mass, H and He make 98% of the Milky Way,
and other elements only 2%.
Therefore, terrestrial planets made of (rare) heavy elements
are small, and Jovian planets made of abundant hydrogen
and helium can be big.
By mass, heavy elements (excluding H and He) make 2% of
the solar system (as well as the Milky Way). Jupiter’s rocky
core makes 2.6% of its total mass.
Q1: Had the Earth retained hydrogen and helium in the
same proportion to the heavier elements that exist
elsewhere in the universe,
(a) what would its mass be relative to its actual mass?
(b) how does this compare to the masses of Jovian
planets?
However, Earth could not retain all the light elements to
grow as big as Jovian planets.
12.3 Formation of the solar system (nebular hypothesis)
o  The solar system formed from
a cloud of interstellar
material, the solar nebula,
4.56 billion years ago.
o  The chemical composition
of the solar nebula, by mass,
was 98% hydrogen and
helium and 2% heavier
elements.
o  The nebula flattened into a
disk (protoplanetary disk) in
which all the material orbited
the center in the same
direction, just as do the
planets today.
The Birth of the Solar System
Protoplanetary Disks (proplyds)
A Disk around a Young Star
o  The protosun formed by
gravitational contraction of
the center of the nebula.
o  After about 108 years,
temperatures at the
protosun’s center became
high enough to ignite
thermonuclear reactions
that convert hydrogen into
helium, thus forming a true
star, the Sun.
o  The planets formed by the
accretion of planetesimals
and the accumulation of
gases in the solar nebula.
Formation of the Sun and planets
Temperature distribution in the solar nebula
Temperature
(determined by
distance to the
center) is the
key to explain
the differences
between
terrestrial and
Jovian planets.
Terrestrial planets: close, hot, small, rocky, heavy elements
Jovian planets: far, cold, big, gaseous, hydrogen and helium
Chondrules:
remnants of
planetesimals
that did not
form planets.
(a crosssection of the
interior of a
meteorite)
In the inner regions, only
materials, iron,silicon,
magnesium, sulfur etc.,
with high condensation
temperatures remain
solid.
Computer simulation of formation of terrestrial planets.
Accumulation of
these materials,
called accretion,
form terrestrial
planets. They are
small because
heavy elements
are rare.
There are two models about formation of the Jovian planets.
  Core accretion model: dense iron core formed the same
way as how terrestrial planets are formed. At low
temperature, light gases of hydrogen and helium are captured
and maintained by gravity.
⎛ 3kT ⎞1/ 2
2GM
v esc =
v = ⎜
⎟
(6 v > vesc: gas escapes
)
R
⎝ m ⎠
  Disk instability model: Jovian planets formed directly from
the solar nebula gas, and rocky heavy core sank into the
center by gravity.
€
€
Asteroids, Kuiper belt objects, and comets are left-over
debris from the original solar nebula. Their distributions are a
result of gravitational force by Jovian planets.
Q2: how does gravity lead to formation of the solar system?
Gravitational contraction shrinks the nebula, which then rotates
faster and flattens.
Gravitational contraction produces the protosun, converting
gravitational energy into heat and producing the temperature profile
in the protoplanetary disk.
Gravitational accretion forms the terrestrial planets or cores of the
Jovian planets.
Gravity of Jovian cores retains large amounts of light elements.
Gravity shapes the solar system and governs the motions in the
solar system.
Such may happen at numerous places in the Universe!
12.4. Discovery of extrasolar planets
By measuring the Doppler effect in
stars that wobble because of planets
orbiting around them, astronomers
have found more than 100 extrasolar
planets since 1995.
A brown dwarf
4875
843
489
Most of the extrasolar
planets discovered to
date are quite massive
and have orbits that are
very different from
planets in our solar
system.
(www.exoplanets.org)
More than 5000 found,
and over 3000 of them
confirmed.
(www.nasa.gov/kepler)
Kepler's Planet Candidates: A Family Portrait
http://www.nasa.gov/kepler
Earth-like Planet Candidates
http://www.nasa.gov/kepler
Nearest exo-planet?
Proxima Centauri, the star nearest the sun, has a planetary
system consisting of at least one planet. The new study
analyzes and supplements earlier observations. These new
measurements show that this planet, named Proxima
Centauri b or simply Proxima b, has a mass close to that of
Earth (1.3 times Earth’s mass) and orbits its star at a
distance of 0.05 astronomical units (one tenth of the sunMercury distance). Contrary to what one might think, such a
small distance does not imply a high temperature on the
surface of Proxima b because the host star, Proxima
Centauri, is a red dwarf with a mass and radius that are only
one-tenth that of the Sun, and a brightness a thousand times
smaller than the sun’s. Hence Proxima b is in the habitable
zone of its star and may harbor liquid water at its surface.
http://www.nasa.gov/kepler
Key Words
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
accretion
atomic number
chondrule
condensation
temperature
conservation of angular
momentum
core accretion model
disk instability model
extrasolar planet
half-life
interstellar medium
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
jets
gravitational contraction
nebular hypothesis
planetesimal
protoplanet
protoplanetary disk
(proplyd)
protosun
radioactive age-dating
radioactive decay
solar nebula
summary