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Elements of the Solar System, Exploring Extrosolar Planets and Evolution of Planetary Systems


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Title: Elements of the Solar System, Exploring Extrosolar Planets and Evolution of Planetary Systems

Elements of the Solar System,Exploring
Extrosolar PlanetsandEvolution of Planetary
  • FIZ463, ITÜ

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 km8 light minutes

1-The Solar System
Not only the Sun and the Planets
  • The Sun
  • Planets (terrestrials and Jovians)
  • Moons of the planets
  • Meteorites
  • Astroid belts
  • Comets
  • Oort Cloud
  • Kuiper Belt
  • Interplanetary dust

Mass Distribution
  • 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
  • Mercury
  • Venus
  • Earth
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune
  • Pluto(?)

MNEMONIC My Very Educated Mother Just Sent Us
Nine Pizzas
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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
  • The Earth would be 1,30m away from the orange,
    having the size of a sesame seed.

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Bodes Relation
  • a simple rule that gives the distances of the
    planets from the Sun

where N0, 3, 6, 12, 24for Mercury, Venus,
Earth, Mars, etc.
Planet N Bodes Law Radii
True Orbital Radii
Mercury 0 (04)/10 0.4 AU 0.39 AU
Venus 3 (34)/10 0.7 AU 0.72 AU
Earth 6 (64)/10 1.0 AU 1.00 AU
Mars 12 (124)/10 1.6 AU 1.52 AU
____ 24 (244)/10 2.8 AU _______
Ceres 24 2.88 AU
Jupiter 48 (484)/10 5.2 AU 5.2 AU
Saturn 96 (964)/10 10.0 AU 9.5 AU
Uranus 192 (1924)/10 19.6 AU 19.2 AU
Neptune ? ? 30.1 AU
Pluto 384 (3844)/10 38.8 AU 39.5 AU
What does Bodes 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.

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Obliquity of the Planets
The orbit of the planets lie on a plane (except
for the Plutos)
Terrestrial Planets
  • The inner four planets at the center of the solar
  • Mercury, Venus, Earth, Mars
  • They all are small, rocky, rotate slow, they
    have small number of moons.
  • Metal cores.

Jovian Planets
  • 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
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Interiors of Jovian Planets cross-cuts
Interiors of Jovian Planets cross-cuts
Saumon Guillot (2004)
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
  • For Jupiter the uncertainty includes Mcore0

Ice giant planets Uranus Neptune
  • Dominant composition
  • Water Ammonia Methane ices
  • Only atmosphere contains H, He (in total only
  • 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 UN ice, JS hydrogen?
  • Theory
  • All four formed at similar location, first
    forming a rockice core by accumulating icy
  • 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. gt63 for Jupiter).
  • Mars (2), Saturn (gt34), Uranus (gt27), Neptun
    (gt13), Pluto (1)

  • Small bodies
  • planetoid, minor planet
  • Their mass is not sufficient to make them
  • 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

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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
  • 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.

Kirkwood Gaps
This slide is not essential for the exam and can
be skipped
  • 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
  • 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.
a white dust tail and a blue gas (ion) tail.
  • 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

Comet Tails
  • 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.

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.

Two documented Cases
This slide is not essential for the exam and can
be skipped
  • 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 drivers side,
    ricocheted off the dashboard, and bent the
    steering wheel, breaking the little finger on
    Martins right hand. It then flew between the
    couples 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 comets orbit, the result is a metor shower of

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Perseid Meteor Shower
  • Usually the best meteor shower of the year.
  • It starts in August 10 and peaks the following 2
  • Specs of rock that have broken off the comet

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
  • 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
  • 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
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
2-Formation of the Solar System
  • How was the Solar System Formed?
  • A viable theory for the formation of the solar
    system must be
  • based on physical principles (conservation of
    energy, momentum, the law of gravity, the law of
    motions, etc.),
  • able to explain all (at least most) the
    observable facts with reasonable accuracy, and
  • able to explain other planetary systems.
  • How do we go about finding the answers?
  • 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 doesnt 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
  • Probability for such encounter is small in our

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.
A Pictorial History
Gravitational Collapse
Interplanetary Cloud
Nebular Capture
Pre-main Sequence Evolution
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.
  • 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
  • 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

The Four Phases of Matter
  • There are in fact more than three phases of
  • 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
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
  • 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

Click it!
Solar Winds
  • Solar wind is the constant outflow of gas from
    the Sun
  • Evidences of Solar Wind
  • Tails of Comet always point away from the Sun,
    indicative of the existence of solar wind.
  • 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
  • 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
  • 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
  • Rocky leftover planetesimals of the inner solar
  • Most of the asteroids are concentrated in the
    asteroid belt between the orbit of Mars and
  • Jupiters 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
  • Comets
  • Icy leftover planetesimals of the outer solar
  • 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 Shoemakers collision with Jupiter
  • Surface of the Moon and Mercury,
  • More in Chapter 7
  • Effects of Impact
  • Tilt of the rotation axis of planets (Venus,
  • 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
  • Triton orbits in a direction opposite to
    Neptunes 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
  • 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 K-40
  • For every 1.25 billion years, half of the
    Potassium-40 decay and turn into Argon-40
  • 1.25 billion years is called the half-life of

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 2
Orbit in the Solar System, Part 4
History of the Solar System, Part 3
Do we Have a Viable Theory?
  • YES!
  • We can explain most of the properties of the
    solar system, including the exceptions.
  • We used only good physics.
  • Testing Our Theory against other solar system
  • Can we find protoplanetary disks (before planets
    were formed)?
  • Can we find other solar system?
  • If we do find other solar system, does our theory
    explain the other solar system?

Evidences Of Protoplanetary Disks
Do we have any evidence of the existence of
planetary nebulae outside of the solar system?
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
  • (3) The planetesimals collided to form larger
  • 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 disk-shaped nebula.
  • The Solar System started as a large, low-density
    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 Orion Nebula.

  • The contraction of the solar nebula made it spin
    faster and heat up. (Compressed gas gets hotter.)

Temperature of solar nebula
gt 2000 Kelvin near Sun lt
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,
  • 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,
  • Hydrogen and helium and gaseous everywhere.

(3) The planetesimals collided to
form larger planets.
  • Planetesimals attracted each other
  • 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
  • Not every planetesimal was incorporated into a
  • Comets leftover icy planetesimals.
  • Asteroids leftover rocky and metallic

  • 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
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
  • 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)

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Astrometric (Wobble) Detections
If a stars position on the sky (proper motion)
wobbles with time, it could be due to an unseen
companion. Only Jupiter-mass 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
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
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
  • It is estimated that there are at least 20
    billion planetary systems in our Galaxy.

What has been found?
More Known Planets
Whats wrong with this picture?
These are all Jupiter-sized planets orbiting very
close to the star!
Selection Effect
  • Actually, our methods of detecting extra-solar
    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
  • 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 gasesSo, 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 planetsby indirect
  • 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 dont find any solar system like ours?
  • May be we just havent found them yet!
  • Possible Explanation ? Detection Limit
  • Larger planets at close distance to the host
    stars produce larger Doppler effect and intensity
    dropSmaller planets far away from the star
    produce much smaller effect, and are more
    difficult to detect.

  • 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

  • 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/lectur
  • http//liftoff.msfc.nasa.gov/academy/space/solarsy
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