Download How the Solar System formed

Elements of the Solar System,
Exploring Extrosolar Planets
and
Evolution of Planetary Systems
FIZ466, İTÜ
The Astronomical Unit (AU)
• The appropriate length unit for studying
the Solar System is AU
• AU is the average distance between the
Sun and the Earth
• 1 AU = 150 Million km=8 light minutes
1-The Solar System
Not only the Sun and the Planets
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The Sun
Planets (terrestrials and Jovians)
Moons of the planets
Meteorites
Astroid belts
Comets
Oort Cloud
Kuiper Belt
Interplanetary dust
Mass Distribution
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Sun: 99.85%
Planets: 0.135%
Comets: 0.01% ?
Satellites: 0.00005%
Minor Planets 0.0000002% ?
Meteoroids: 0.0000001% ?
Interplanetary Medium: 0.0000001% ?
Simply: Sun 99.9 % & 0.1% Jupiter
The Nine Planets
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Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
• Pluto(?)
MNEMONIC: My Very Educated Mother Just Sent Us Nine Pizzas
Imagening the distances
• Imagine the Solar System being a soccer ground
(about 100 m long).
• The Sun would be a glaring orange in the centre.
• Pluto would encircle the sun at the edge of the
soccer ground, having the size of a dust particle.
• The Earth would be 1,30m away from the “orange“,
having the size of a sesame seed.
Bode’s Relation
• a simple rule that gives the distances of
the planets from the Sun
N 4
Orbit Radius of a Planet  RN 
AU
10
where N=0, 3, 6, 12, 24…for Mercury, Venus,
Earth, Mars, etc.
Planet
N
Bode’s Law Radii
Mercury
0
(0+4)/10 = 0.4 AU
0.39 AU
Venus
3
(3+4)/10 = 0.7 AU
0.72 AU
Earth
6
(6+4)/10 = 1.0 AU
1.00 AU
Mars
12
(12+4)/10 = 1.6 AU
1.52 AU
____
Ceres
Jupiter
24
48
(24+4)/10 = 2.8 AU
(48+4)/10 = 5.2 AU
_______
2.88 AU
5.2 AU
Saturn
96
(96+4)/10 = 10.0 AU
9.5 AU
Uranus
192
(192+4)/10 = 19.6 AU19.2 AU
Neptune
?
Pluto
384
?
True Orbital Radii
30.1 AU
(384+4)/10 = 38.8 AU39.5 AU
What does Bode’s Law tell us?
• Bode's Law predicted that there should be
a planet between the orbits of Mars and
Jupiter.
• The "missing planet" turned out to be the
asteroid belt.
Obliquity of the Planets
The orbit of the planets lie on a
plane (except for the Pluto’s)
Terrestrial Planets
• The inner four planets at the center of
the solar system:
• Mercury, Venus, Earth, Mars
• They all are small, rocky, rotate slow,
they have small number of moons.
• Metal cores.
Jovian Planets
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Outer planets of the Solar System
Jupiter, Saturn, Uranus & Neptun
They are made of gas/liquid/ice
No solid surface
Small solid core (rock)
They have rings
Large number of moons
Terrestrial and Jovian Planets
Interiors of Jovian Planets:
cross-cuts
Gas giant planets: Jupiter & Saturn
• Dominant composition:
– Hydrogen + Helium, like the sun
– Surface clouds: ammonia ice, water ice....
– Deep in interior: liquid metallic hydrogen
– Even deeper: rocky core of ~ 10...15 M
• These are model results which depend on
equation of state of hydrogen
• For Saturn this is certain (unless models are
wrong)
• For Jupiter the uncertainty includes Mcore=0
Ice giant planets: Uranus & Neptune
• Dominant composition:
– Water + Ammonia + Methane ices
– Only atmosphere contains H, He (in total only minor)
• Uranus:
– 25% Iron + Silicates
– 60% Methane + Water + Ammonia
– 15% Hydrogen + Helium
• Neptune:
– 20% Iron + Silicates
– 70% Methane + Water + Ammonia
– 10% Hydrogen + Helium
Thermal emission of Jupiter and Saturn
• Jupiter and Saturn emit more radiation than they receive
from the sun.
• They are not massive enough for nuclear burning (need
at least 13 Mjup)
• Kelvin-Helmholz cooling time scale much shorter than
current age (at least for Saturn)
• Possible solution:
– Helium slowly sediments to center, releases
gravitational energy
Why U+N ice, J+S hydrogen?
• Theory:
– All four formed at similar location, first forming
a rock+ice core by accumulating icy bodies
– Somehow U + N were moved outward and did
not accrete much gas anymore
– J + S remained and accreted large quantities
of hydrogen gas
Summary - What do the inner
planets look like?
They are all…
• rocky and
small!
• No or few
moons
• No rings
Summary - The Jovian Planets
They are all…
• gaseous and
BIG!
• Rings
• Many moons
Quantitative Planetary Facts
What are Moons?
• Moons are like little planets that encircle the real
planets.
• Usually, they are much smaller than planets.
• Planets can have no moons (like Mercury and
Venus), one moon (like Earth) or up to a very
large number of moons (e.g. >63 for Jupiter).
• Mars (2), Saturn (>34), Uranus (>27), Neptun
(>13), Pluto (1)
Asteroids
• Small bodies
• planetoid, minor planet
• Their mass is not sufficient to make them
spherical
• Many of the asteroids are part of the
asteroid belt between Mars and Jupiter.
• Believed to be left over from the early
evolution of the solar nebula.
• Largest object Ceres is about 1000 km
accross
Asteroid Belt
• The doughnut-shaped
concentration of asteroids
orbiting the Sun between
Mars and Jupiter
• More that 200000
asteroids
• Total mass, a few 1024 g,
is 1/30 of the Moon.
• if the estimated total mass
of all asteroids was
gathered into a single
object, this object would
be less than 1,500
kilometers across
The Origin of the Asteroid Belt
• The asteroid belt may be material that
never coalesced into a planet, perhaps
because its mass was too small; the total
mass of all the asteroids is only a small
fraction of that of our Moon.
• A less satisfactory explanation of the origin
of the asteroid belt is that it may have
once been a planet that was fragmented
by a collision with a huge comet.
This slide is not essential for the exam and can be skipped
Kirkwood Gaps
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If you plot the radius of the orbits of
the asteroids you do not get a smooth
`bell-curve' shape. There are
concentric gaps in the asteroid belt
known as Kirkwood gaps.
These gaps are orbital radii where the
gravitational forces from Jupiter do
not let asteroids orbit (they would be
pulled into Jupiter).
For example, an orbit in which an
asteroid orbited the Sun exactly three
times for each Jovian orbit would
experience great gravitational forces
each orbit, and would soon be pulled
out of that orbit.
There is a gap at 3.28 AU (which
corresponds to 1/2 of Jupiter's
period), another at 2.50 AU (which
corresponds to 1/3 of Jupiter's
period), etc. The Kirkwood gaps are
named for Daniel Kirkwood who
discovered them in 1866.
This is an example of resonance. This
resonance phenomenon has Jupiter
passing by any asteroid in the Kirkwood
gaps every two or three asteroid years,
depending on which gap. The repeated
tugging induces an asteroid into larger,
longer orbits closer to Jupiter.
Eventually, however, an asteroid's
resonance with Jupiter disappears as its
orbit increases.
Comets
a white dust tail and a blue gas (ion) tail.
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A comet consists of a tiny nucleus with diameter less than 10 km. The
nucleus is made up of frozen gases and dust.
Eccentric orbit around the Sun.
Most comets spend most of their time at vast distances from the Sun.
When they approach the Sun, some gases will be vaporized and an
extended coma will then be produced (of size 100000 km).
The tail can be up to 1AU long.
Orbits of a comet may be open or close. A comet with an open orbit will only
visit the Sun once. However, a comet with a closed orbit (actually it is
elliptical) will visit the Sun again and again. Perhaps, the most famous one
is the Comet Halley, it has a closed orbit with a period of 76 years.
• When a comet moves
close to the Sun, the
solar wind (charged
particles ejected from
the Sun) and the
Sun's radiation
pressure push the
dust and gases of the
comets away, this will
result in a beautiful
long tail.
• From this, we know
why the comet tail is
always pointing away
from the Sun.
Comet Tails
The dust trail is made of particles that are the size of sand grains and pebbles.
They are large enough that they are not affected much by the Sun's light and
solar wind.
The gas tail, on the other hand, is made of grains the size of cigarette-smoke
particles. These grains are blown out of the dust coma near the comet nucleus
by the Sun's light.
Comet Orbits
Meteoroids, Meteors and Meteorites
• When asteroids collide with one another they
can produce small fragments known as
meteoroids.
• If a meteoroid enters the atmosphere of the
Earth, it glows due to heat generated by friction.
These are called meteors.
• If the rock survives the trip through the
atmosphere and strikes the surface of the Earth,
the remnant is called a meteorite.
• Only 2 documented cases in which a person is
hit by a meteorite.
This slide is not essential for the exam and can be skipped
Two documented Cases
• Annie Hodges of Sylacauga, Alabamawas napping on
her couch on November 30, 1954 when an eight-pound
meteorite crashed through the roof. It bounced off a
large console radio and hit her in the arm and then in the
leg, leaving her bruised but okay.
• On the afternooon of June 21, 1994, Jose Martin and his
wife, Vicenta Cors, were driving in Spain from Madrid to
Marbella. As they zoomed past the town of Getafe, a
three-pound meteorite smashed through their windshield
on the driver’s side, ricocheted off the dashboard, and
bent the steering wheel, breaking the little finger on
Martin’s right hand. It then flew between the couple’s
heads and landed on the back seat. Other than the
broken little finger, they were okay.
Meteor Shower
• Comets exposed to the heat of the inner
solar system slowly disintegrate
• This is another source of meteoritic
material
• When the Earth passes through the debris
left in a comet’s orbit, the result is a metor
shower of micrometeorites.
Perseid Meteor Shower
• Usually the best meteor
shower of the year.
• It starts in August 10
and peaks the following
2 days
• Specs of rock that have
broken off the comet
Swift-Tuttle.
August 10, 1998
November 13, 1833
Kuiper Belt & Oort Cloud
• Kuiper Belt is a "junkyard" of countless icy bodies left
over from the solar system's formation.
• Kuiper Belt is shaped like a disk.
• The Kuiper Belt extends from inside Pluto's orbit to the
edge of the solar system.
• Kuiper Belt was discovered in 1992
• There are at least 70,000 "trans-Neptunians" with
diameters larger than 100 km in the radial zone
extending outwards from the orbit of Neptune (at 30 AU)
to 50 AU.
• The Oort Cloud, which is much further (50000 AU), is a
vast spherical shell of billions of comets.
Kuiper Belt & Oort Cloud
Kuiper Belt & Oort Cloud
When is a planet not a planet?
Recently, the International Astronomical Union (IAU) had a fierce to try to iron
out the definition of a planet.
They decided that a planet:
 Is in orbit around the Sun.
 Has sufficient mass for their self-gravity to overcome rigid body
forces so that it assumes a hydrostatic equilibrium (nearly round)
shape.
 Has cleared the neighbourhood around its orbit.
Objects that pass the first two tests, but fail the third, and which are not themselves
satellites of other planets, are now called dwarf planets.
Quaoar and Sedna:
new planets?
Quaoar is a Kuiper belt object discovered by Trujillo and
Brown in 2002 with the Palomar Telescope.
It orbits outside Pluto and was the largest Solar System object
discovered since Pluto in 1930. Its diameter is about 1300km
(half the size of Pluto), and it is on a very circular orbit
currently one billion miles outside Pluto.
Sedna is a similar object that is even further away, and takes
over 10,000 years to orbit the Sun. It was discovered in 2004
by the same astronomers.
2003UB313, aka Xena
In 2003, a Kuiper-belt
object was found which is
bigger than Pluto. It even
has its own moon! Its
orbital period is 560 years
on a highly-inclined orbit.
Although colloquially
known as Xena, it is called
2003UB313 until an official
name is decided.
Xena and its moon Gabrielle,
imaged by the Keck telescope.
2-Formation of the Solar System
How was the Solar System Formed?
A viable theory for the formation of the solar system must be
1. based on physical principles (conservation of energy, momentum, the law of
gravity, the law of motions, etc.),
2. able to explain all (at least most) the observable facts with reasonable
accuracy, and
3. able to explain other planetary systems.
How do we go about finding the answers?
1.
2.
3.
4.
Observe: looking for clues
Guess: come up with some explanations
Test it: see if our guess explains everything (or most of it)
Try again: if it doesn’t quite work, go back to step 2.
Planetary Nebula or Close Encounter?
Historically, two hypothesis were put forward to explain the formation of the solar
system….
• Gravitational Collapse of Planetary Nebula (Latin for “cloud”)
Solar system formed form gravitational collapse of an interstellar cloud or gas
• 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…
Common
Characteristics
and Exceptions
of the Solar
System
We need to
be able to
explain all
these!
Common Characteristics and Exceptions
The Nebular Theory* of Solar System
Formation
Interstellar Cloud (Nebula)
*It
is also called the
‘Protoplanet Theory’.
Gravitational Collapse
Protosun
Heating  Fusion
Sun
Leftover Materials
Asteroids
Protoplanetary Disk
Condensation (gas to solid)
Metal, Rocks
Gases, Ice
Accretion
Nebular
Capture
Terrestrial
Planets
Jovian
Planets
Leftover Materials
Comets
A Pictorial
History
Gravitational
Collapse
Interplanetary Cloud
Accretion
Condensation
Nebular Capture
Pre-main Sequence Evolution
10
105 yr
Protostar+
primordial
disk
Lstar
104 yr
Planet building
107 yr
109 yr
1
Planetary system
+ debris disk
100 AU
Cloud collapse
8,000
5,000
2,000
Tstar (K)
The Interstellar Clouds
• The primordial gas after the Big
Bang has very low heavy metal
content. The interstellar clouds
that the solar system was built
from gas that has gone through
several star-gas-star cycles.
Collapse of the Solar Nebula
Gravitational
Collapse
Denser region in a interstellar cloud, maybe compressed
by shock waves from an exploding supernova, triggers
the gravitational collapse.
1.
2.
3.
Heating  Prototsun  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.
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.  demonstration
Flattening  Protoplanetary disk. Check out the animation in the e-book!
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!
Condensation of the Solar Nebula
Composition of the Solar Nebula
As the protoplanetary disk cools, materials in the disk condensate into planetesimals
• The solar nebular contains 98% Hydrogen and Helium (produced in the Big
Bang), and 2% everything else (heavy elements, fusion products inside the stars).
• Local thermal environment (Temperature) determines what kind of material
condensates.
−
−
−
Water and most hydrogen compounds have low sublimation temperature, and cannot
exist near the Sun. They exist far away from the Sun.
Metals and rocks have high sublimation temperature, and can form near the Sun.
Frost line lies between the orbit of Mars and Jupiter.
The Four Phases of Matter
There are in fact more than three phases of matter.
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Plasma – when the temperature is very high, high energy collision
between atoms will knock the electrons lose, and they are not bounded to
the atoms anymore…
Core and corona of the Sun
and stars
Surface of the Sun and
stars
Surface of Earth
White dwarfs, CMB
Transition Between Phases
Liquidation
Solid Solidification
Liquid
Evaporation
Condensation Gas
Condensation
Sublimation: atoms or
molecules escape into the
gas phase from a solid.
Initially, small dust and ice particles in the early solar nebula collided, sticking
electrostatically. As this accretion process continues, gravity plays a greater role in
forming these planetesimals. These can be as large as asteroids. Within a few
million years, some of these planetesimals have grown to hundreds of kilometers
and are nearly spherical as a result of their self gravitation. They start to affect the
orbits of nearby planetesimals, increasing the number of collisions.
Accretion: Formation of the Terrestrial Planets
Accretion The process by which small ‘seeds’ grew into planets.
• Near the Sun, where temperature is high, only metals and rocks can condense.
The small pieces of metals and rocks (the planetesimals) collide and stick
together to form larger piece of planetesimals.
• Small pieces of planetesimals can have any kind of shape.
• Larger pieces of planetesimals are spherical due to gravity.
• Only small planets can be formed due to limited supply of material (~0.6% of the
total materials in the solar nebula).
• Gravity of the small terrestrial planets is too weak to capture large amount of gas.
• The gas near the Sun were blown away by solar wind.
Click it!
Solar Winds
Solar wind is the constant outflow of
gas from the Sun…
Evidences of Solar Wind
1. Tails of Comet always point away
from the Sun, indicative of the
existence of solar wind.
2. SOHO (SOlar and Heliospheric
Observatory) C2 and C3 movies.
Effects of Solar Wind on Planet
Formation
At certain stage of the planet forming
process, Solar winds blow away the
gases in the planetary nebula, ending
the formation of the planets.
Nebula Capture: Formation of the Jovian Planets
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In the regions beyond the frost line, there are abundant supply of solid materials
(ice), which quickly grow in size by accretion.
The large planetesimals attract materials around them gravitationally, forming the
jovian planets in a process similar to the gravitational collapse of the solar nebula
(heating, spinning, flattening) to form a small accretion disk.
Abundant supply of gases allows for the creation of large planets.
However, the jovian planets were not massive enough to trigger nuclear fusion at
their core.
The Results of Selective
Condensation…
• Not much light gases were available for the formation of
planets near the Sun, but small amount of metals and rocks are
available:
– The planets close to the Sun are small and rocky…
• There are abundant supply of light gases farther out…
– The planets far away from the Sun are big and composed of
gases of hydrogen components…
These processes can explain the two types of major planets,
their size differences, locations, and composition.
Origin of Comets and Asteroids
Asteroids
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Rocky leftover planetesimals of the inner solar system.
•
Most of the asteroids are concentrated in the asteroid
belt between the orbit of Mars and Jupiter.
•
Jupiter’s strong gravity might have disturbed the
formation of a terrestrial planet here.
•
Jupiter also affects the orbit of these asteroids and sent
them flying out of the solar system, or sent them into a
collision cause with other planets.
Comets
•
Icy leftover planetesimals of the outer solar system.
•
Comets in between Jupiter and Neptune were ‘bullied’
away from this region, either collide with the big
planets, or been sent out to the Kuiper belt or the Oort
cloud.
•
Comets beyond the orbit of Neptune have time to grow
larger, and stay in stable orbit. Pluto may be (the
biggest) one of them.
Explaining the Exceptions: Impact and Capture
Heavy Bombardment There were many impact events
during the early stage of the solar system formation process,
when there were still many planetesimals floating around.
Evidences of Impact
• Comet Shoemaker’s collision with Jupiter
• Surface of the Moon and Mercury
Effects of Impact
• Tilt of the rotation axis of planets
(Venus, Uranus)
• Creation of satellites (May be our
moon)
• Exchange of materials (Where did
the water on Earth come from if
most of the gases were blown
away by solar wind after Earth
was formed?)
• Catastrophes (Where did all the
dinosaurs go?)
Where did the moons come from?
Giant Impact
• Our moon may have been formed in a giant impact between the Earth and
a large planetesimal…
Captured Moons
• Phobos & Deimos of Mars may be captured asteroids.
• Triton orbits in a direction opposite to Neptune’s rotation
Capture of Comet Shoemaker by
Jupiter
The Age of the Solar System
Through radioactive dating, the age of the solar system
is determined as 4.6 billion years…
Potassium-40 (an isotope of Potassium [K19]) decays to
Argon-40 by electron capture, turning a proton in its
nuclei into neutron (thus changing its chemical
properties)…
– Potassium-40 exists naturally
– Argon is an inert gas that never combine with anything, and did
not condense in the solar nebula…
– By determining the relative amount of Potassium-40 to Argon-40
trapped in rock, we can determine the age of rock, assuming that there
were no Argon-40 initially…
Formation of the Solar System
• Formed 4.568 Gigayears ago (=age of oldest known solids
in solar system)
• Mars formed about 13 Megayears later
• Earth formed 30 to 40 Megayear later
– Leading theory for formation of the moon is that about
100 Myr after the birth of the solar system Earth was hit
by a Mars-size object. The heavy cores of both objects
formed the new Earth and the light silicate crusts formed
the moon.
• Jovian planets (Jupiter, Saturn, Uranus, Neptune) must
have formed in less than 10 Myrs (life time of gaseous
protoplanetary disks)
Radioactive Dating Using K40
• For every 1.25 billion
years, half of the
Potassium-40 decay
and turn into Argon40…
• 1.25 billion years is
called the half-life of
Potassium-40.
The Formation Of Solar System: Simulations
Simulations from www.astronomyplace.com. Check them out!
History of
the Solar
System,
Part 1
History of
the Solar
System,
Part 3
History of
the Solar
System,
Part 2
Orbit in the
Solar
System,
Part 4
Do we Have a Viable Theory?
YES!
1. We can explain most of the properties of the solar system,
including the exceptions.
2. We used only good physics.
Testing Our Theory against other solar system
1. Can we find protoplanetary disks (before planets were formed)?
2. Can we find other solar system?
3. If we do find other solar system, does our theory explain the other
solar system?
Do we have any evidence of the existence of
planetary nebulae outside of the solar
system?
Evidences Of Protoplanetary Disks
We now have many
observational evidences of
the existence of the
protoplanetary Disks.
Hubble Space Telescope image of the dust disk
surrounding Beta Pictoris
Each disk-shaped “blob” is a disk of material orbiting a
star…
Origin of the Solar System: Key
Concepts
How the Solar System formed:
(1) A cloud of gas & dust contracted to form a
disk-shaped solar nebula.
(2) The solar nebula condensed to form small
planetesimals.
(3) The planetesimals collided to form larger
planets.
When the Solar System formed:
(4) Radioactive age-dating indicates the Solar
System is 4.56 billion years old.
Clues to how the Solar System formed:
How things move (dynamics)
All planets revolve in the same direction.
Most planets rotate in the same direction.
Planetary orbits are in nearly the same
plane.
(1) A cloud of gas and dust
contracted
to form a diskshaped nebula.
The Solar System
started as a
large, lowdensity cloud of
dusty gas.
Such gas clouds
can be seen in
our Milky Way
and other
galaxies today.
The flat, rapidly
rotating cloud of
gas and dust was
the solar
nebula.
The central dense
clump was the
protosun.
Similar flat, rotating
clouds are seen
around
protostars in the
The contraction of the solar nebula made it spin faster
and heat up. (Compressed gas gets hotter.)
Temperature of
solar nebula:
> 2000 Kelvin
near Sun; < 50
Kelvin far from
Sun.
(2) The solar nebula condensed
to form small planetesimals.
Approximate condensation
temperatures:
1400 Kelvin: metal
(iron, nickel)
1300 Kelvin:
rock (silicates)
200
Kelvin: ice (water, ammonia, methane)
Inner solar system: over 200 Kelvin, only
metal and rock condense.
Outer solar system: under 200 Kelvin,
ice condenses as well.
As the solar nebula cooled, material
condensed to form planetesimals
a few km across.
Inner Solar System:
Metal and rock = solid planetesimals
Water, ammonia, methane = gas.
Outer Solar System:
Metal and rock = solid planetesimals
Water, ammonia, methane = solid, too.
Hydrogen and helium and gaseous
everywhere.
(3) The planetesimals collided
to form larger planets.
Planetesimals attracted each other
gravitationally.
Planetesimals collided with each other to form
Moon-sized protoplanets.
Protoplanets collided with each other (and
with planetesimals) to form planets.
Inner Solar System:
Smaller planets, made of
rock and metal.
Outer Solar System:
Larger planets, made of
rock, metal and ice.
In addition, outer planets are massive
enough to attract and retain H and He.
Collisions between protoplanets were not
gentle!
Venus was knocked “upside-down”, Uranus
and Pluto “sideways”.
Not every planetesimal was incorporated
into a planet.
Comets = leftover icy planetesimals.
Asteroids = leftover rocky and metallic
planetesimals.
How does this “nebular theory” explain the
current state of the Solar System?
Solar System is disk-shaped:
It formed from a flat solar nebula.
Planets revolve in the same direction:
They formed from rotating nebula.
Terrestrial planets are rock and metal:
They formed in hot inner region.
Jovian planets include ice, H, He:
They formed in cool outer region.
More Protoplanetary Disks
MAUNA KEA, Hawaii (August 12, 2004) The
sharpest image ever taken of a dust disk around
another star has revealed structures in the disk
which are signs of unseen planets.
Dr. Michael Liu, an astronomer at the
University of Hawaii's Institute for Astronomy,
has acquired high resolution images of the
nearby star AU Microscopii (AU Mic) using
the Keck Telescope, the world's largest infrared
telescope. At a distance of only 33 light years,
AU Mic is the nearest star possessing a visible
disk of dust. Such disks are believed to be the
birthplaces of planets.
http://www.ifa.hawaii.edu/info/press-releases/Liu0804.html
3-Extra-solar Planets
Do you believe solar systems like our own are
common or rare among sun-like stars in the disk
of the Milky Way galaxy? Why?
We expect to find planetary systems around other systems because of the
Copernican Principle.
Are there more planets in the
Universe?
• Yes, there are other planets, so-called
extra-solar planets (around stars other
than the Sun).
• But it is very difficult to spot them, since
they are far far away.
• Recall that a planet is much smaller than a
star.
• How can planets of other stars be spotted
then?
Planets of other stars
• There are three main ways that
astronomers search for these planets:
• Doppler method
• Transit method
• Gravitational (micro)lensing
Doppler Method
• The planet will pull the star into a small circle about the center of mutual
mass, called the system barycenter. On the sky, the star will move from
side to side.
If you observe a star very
accurately
with Doppler instruments,
you may be able to
measure a slight “wobble“
around
the center of mass. This
can indicate a planet.
Radial Motion of Stars due to Planets
• Astrometrically (via a
positional “wobble”)
• Spectroscopically (via
blueshifts and
redshifts of
absorption lines)
Astrometric (Wobble) Detections
If a star’s position on the sky (proper motion) wobbles with
time, it could be due to an unseen companion. Only Jupitermass planets have enough mass to be detected in this way.
First Success 1995
Transiting Planets
•If you can observe
many stars, you may
sometimes see one get
slightly fainter for a little
while. This happens if a
planet passes between
us and the star – like a
mini-eclipse.
Transiting Extra-solar Planets
Gravitational Lensing Detections
If a star/planet moves
exactly in front of a
background star, the
brightness of the
background star can
be greatly magnified
by the gravitational
lens effect.
Detection via microlensing
OGLE-2003-BLG-235
Foreground faint (invisible) star passes across background
faint (invisible) star. Gravity of foreground star amplifies
background star. Brightening of background star.
If planet is present around foreground star, AND one is
lucky that it also passes background star: one sees ‘blip’ in
the signal.
Detection via microlensing
OGLE-2003-BLG-235
Extrasolar planets to date
• First extrasolar planet was discovered around a
neutron star in 1991
• First extrasolar planet orbiting a normal star was
found in 1995 by Michel Mayor and Didier
Queloz of the Geneva Observatory in
Switzerland orbiting the star 51 Pegasi
• More than 200 planets have been discovered
see http://www.obspm.fr/encycl/catalog.html
• It is estimated that there are at least 20 billion
planetary systems in our Galaxy.
What has been found?
We have abundant indirect evidence of the existence of extrasolar planets!
More Known Planets
What’s wrong with this picture?
These are all Jupitersized planets orbiting
very close to the star!
Our Jupiter is way out here, 4.5 AU…
Selection Effect
• Actually, our methods of detecting extrasolar planets can find only massive
planets that are close to the stars.
• So it is not surprising that all we have
found are such planets.
• But we still need one explanation...
But, why are these large planets so
close to the stars?
According to our planetary nebular theory, large planets can only be
formed far away from the host star, behind the frost line, where there
are abundant quantities of gases…So, why do we see these large planets
so close to the stars?
Possible Explanation
Maybe these planets were formed far away from the stars as our
planetary nebular theory predicts. But for some reason (say friction
between the planets and the dense planetary gas) caused the planets
to lose their orbital angular momentum and migrate toward the stars.
(Planetary migration is an active research field)
Eccentricity of Planets
From: Review by G. Marcy Ringberg 2004
Is The Nebular Theory OK?
•
•
•
We have evidences for the existence of protoplanetary disks!
We have found many extrasolar planets…by indirect methods.
We have not found any solar system like ours!
− All the extrasolar planets we found so far are large, Jupiter-sized (or
larger) planets.
− All these planets are located very close to the host star, inconsistent
with the nebular theory.
Why we don’t find any solar system like ours?
 May be we just haven’t found them yet!
Possible Explanation  Detection Limit
Larger planets at close distance to the host stars produce larger Doppler effect and
intensity drop…Smaller planets far away from the star produce much smaller
effect, and are more difficult to detect.
Summary
• We have a viable theory to
explain the formation of our
solar system.
• We have evidences that
planetary nebulae exist in other
star systems.
• However, we have not found a
solar system similar to ours
outside of our own.
Extrasolar planets we found so
far do not agree with our
theory – The physics of our
theory is fundamentally
correct, but details of the
model may need adjustment…
Links
•
•
•
•
•
•
•
http://www.solarviews.com/eng/homepage.htm
http://www.solarsystem.org.uk/
http://learn.arc.nasa.gov/planets/
http://solarsystem.nasa.gov/planets/
http://pds.jpl.nasa.gov/planets/
http://exoplanet.eu/
http://exoplanets.org/
• http://observe.phy.sfasu.edu/courses/ast105/lectures105/
•
http://liftoff.msfc.nasa.gov/academy/space/solarsystem/solarsystemjava.html