Transcript 9. Formation of the Solar System

Announcements
Second Mid Term Exam Weds Mar 14
On energy, Newtons Laws, light, telescopes, the Solar System ,
solar system formation, extrasolar planets
New homework assignment 6, available today, due Weds Mar 14
2pm
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Origin of the Solar System:The
Nebular Theory
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And temperature depended on distance from the Sun
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Building the Planets
So only rocks & metals condensed within 3.5 AU
of the Sun… the so-called frost line.
Hydrogen compounds (ices) condensed beyond the
frost line.
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Origin of the Asteroids
• The Solar wind cleared the leftover gas, but not the leftover
planetesimals.
• Those leftover rocky planetesimals which did not accrete onto a
planet are the present-day asteroids.
• Most inhabit the asteroid belt between Mars & Jupiter.
– Jupiter’s gravity prevented a planet from forming there.
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Origin of the Comets
• The leftover icy
planetesimals are the
present-day comets.
• Those which were
located between the
Jovian planets, if not
captured, were
gravitationally flung in
all directions into the
Oort cloud.
• Those beyond
Neptune’s orbit
The nebular theory predicted the existence remained in the ecliptic
plane in what we call
of the Kuiper belt 40 years before it was
the Kuiper belt.
discovered!
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Exceptions to the Rules
So how does the nebular theory deal with exceptions,
i.e. data which do not fit the model’s predictions?
• There were many more leftover planetesimals than we
see today.
• Most of them collided with the newly-formed planets
& moons during the first few 108 years of the Solar
System.
• We call this the heavy bombardment period.
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Exceptions to the Rules
Close encounters with and impacts by planetesimals could explain:
• Why some moons orbit opposite their planet’s rotation
– captured moons (e.g. Triton)
• Why rotation axes of some planets are tilted
– impacts “knock them over” (extreme example: Uranus)
• Why some planets rotate more quickly than others
– impacts “spin them up”
• Why Earth is the only terrestrial planet with a large Moon
– giant impact
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Formation of the Moon
(Giant Impact Theory)
• The Earth was struck by a
Mars-sized planetesimal
• A part of Earth’s mantle
was ejected
• This coalesced in the
Moon.
– it orbits in same direction as
Earth rotates
– lower density than Earth
– Spun up Earth
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9.5 How Old is the Solar System?
Our goals for learning:
• How do we measure the age of a rock?
• How old is the Solar System and how do we
know?
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Radiometric Dating
• Isotopes which are unstable are
said to be radioactive.
• They spontaneously change in
to another isotope in a process
called radioactive decay.
– protons convert to neutrons
– neutrons convert to protons
• The time it takes half the
amount of a radioactive isotope
to decay is called its half life.
• By knowing rock chemistry, we chose a stable isotope which does
not form with the rock…its presence is due solely to decay.
• Measuring the relative amounts of the two isotopes and knowing
the half life of the radioactive isotope tells us the age of the rock.
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The Age of our Solar System
• Radiometric dating can only measure the age of a rock since it
solidified.
• Geologic processes on Earth cause rock to melt and resolidify.
 Earth rocks can’t be used to measure the Solar System’s age.
• We must find rocks which have not melted or vaporized since they
condensed from the Solar nebula.
– meteorites imply an age of 4.6 billion years for Solar System
• Radioactive isotopes are formed in stars & supernovae
– suggests that Solar System formation was triggered by supernova
– short half lives suggest the supernova was nearby
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Extrasolar Planets
• Since our Sun has a family of planets, shouldn’t
other stars have them as well?
– Planets which orbit other stars are called extrasolar
planets
• We finally obtained direct evidence of the
existence of an extrasolar planet in the year 1995
– A planet was discovered in orbit around the star 51
Pegasi
– ~160 extrasolar planets are now known to exist
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Why is it so difficult to detect
planets around other stars?
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Brightness Difference
• A star like the Sun would be a billion times
brighter than the light reflected off its planets.
• As a matter of contrast, the planet gets lost in the
glare of the star.
• Also, planets are relatively close to their stars, need
high angular resolution to separate the bodies
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Detecting Extrasolar Planets
• Can we actually make
images of extrasolar
planets?
– this is very difficult to do.
• The distances to the
nearest stars are much
greater than the distances
from a star to its planets.
• The angle between a star
and its planets, as seen
from Earth, is very small
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Planet Detection
• Direct: Pictures of the planets themselves tricky due to contrast and separation issues
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Direct Detection
• Special techniques can eliminate light from brighter
objects
• These techniques are enabling direct planet detection
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Indirect: Measurements of stellar
properties revealing the effects of
orbiting planets
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Gravitational Tugs
• Sun and Jupiter orbit
around their
common center of
mass
• Sun therefore
wobbles around that
center of mass with
same period as
Jupiter
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Astrometric Technique
• We detect the planets indirectly by observing the
star.
• Planet gravitationally tugs the star, causing it to
wobble.
• We can detect planets by measuring the change in a
star’s position on sky
• However, these tiny motions are very difficult to
measure (~0.001 arcsecond)
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Easier is the Doppler Technique
• Measuring a star’s
Doppler shift can tell
us its motion toward
and away from us
• Current techniques
can measure motions
as small as 1 m/s
(walking speed!)
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First Extrasolar Planet detected
1995 using Doppler Technique
• Planet around 51 Pegasi has a mass similar to
Jupiter’s, despite its small orbital distance
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First Extrasolar Planet
• Doppler shifts of star
51 Pegasi indirectly
reveal a planet with
4-day orbital period
• Short period means
small orbital distance
• First extrasolar planet
to be discovered
(1995)
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Other Extrasolar Planets
Large
planet
mass
• Doppler data curve
tells us about a
planet’s mass and the
shape of its orbit
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Planet Mass and Orbit Tilt
• We cannot measure an exact mass for a planet without
knowing the tilt of its orbit, because Doppler shift tells
us only the velocity toward or away from us
• Doppler data gives us lower limits on masses
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Measuring the Properties of Extrasolar Planets
• A plot of the radial velocity shifts forms a wave.
– Its wavelength tells you the period and size of the
planet’s orbit.
– Its amplitude tells you the mass of the planet.
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Transit Method
• The Doppler technique yields only planet masses and orbits.
• Planet must eclipse or transit the star in order to measure its radius.
• Size of the planet is estimated from the amount of starlight it blocks.
• We must view along the
plane of the planet’s orbit for
a transit to occur.
– transits are relatively rare
• They allow us to calculate the
density of the planet.
– extrasolar planets we have
detected have Jovian-like
mass/density
– But these would be easiest
to detect
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Transits and Eclipses
• A transit is when a planet crosses in front of a star
• The resulting eclipse reduces the star’s apparent
brightness and tells us planet’s radius
• No orbital tilt: accurate measurement of planet mass
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Spectrum during Transit
• Change in spectrum during transit tells us about
composition of planet’s atmosphere
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Transit Result from Spitzer
HD 209458b
and TrES-1detected using
“transit”
method in
infra-red light
-ie observers
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The Nature of Extrasolar Planets
• What have we learned about extrasolar
planets?
• How do extrasolar planets compare with
those in our solar system?
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Measurable Properties
• Orbital Period, Distance, and Shape
• Planet Mass, Size, and Density
• Composition
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Orbits of Extrasolar Planets
• Most of the detected
planets have orbits
smaller than Jupiter’s
• Planets at greater
distances are harder
to detect with
Doppler technique
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Orbits of Extrasolar Planets
• Orbits of some
extrasolar planets are
much more elongated
(greater eccentricity)
than those in our
solar system
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Multiple-Planet Systems
• Some stars
have more
than one
detected
planet
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Orbits of Extrasolar Planets
• Most of the detected
planets have greater
mass than Jupiter
• Planets with smaller
masses are harder to
detect with Doppler
technique
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How do extrasolar planets compare
with those in our solar system?
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Surprising Characteristics
• Some extrasolar planets have highly
elliptical orbits
• Some massive planets orbit very close to
their stars: “Hot Jupiters”
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Hot Jupiters
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Planets: Common or Rare?
• One in ten stars examined so far have
turned out to have planets
• The others may still have smaller (Earthsized) planets that current techniques cannot
detect
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How will we search for Earth-like
planets?
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Transit Missions
• NASA’s Kepler mission is
scheduled to begin looking
for transiting planets in
2008
• It is designed to measure
the 0.008% decline in
brightness when an Earthmass planet eclipses a Sunlike star
–Transit missions will be capable of finding Earthlike planets that cross in front of their stars (Kepler to
launch in 2008)
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Astrometric Missions
will be capable of measuring the “wobble”
of a star caused by an orbiting Earth-like
planet
• GAIA: A European mission planned for 2010 that will use
interferometry to measure precise motions of a billion stars
• SIM: A NASA mission planned for 2011 that will use
interferometry to measure star motions even more
precisely (to 10-6 arcseconds)
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Direct Detection
• Determining whether
Earth-mass planets
are really Earth-like
requires direct
detection
Mission concept for NASA’s
Terrestrial Planet Finder (TPF)
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• Missions capable of
blocking enough
starlight to measure
the spectrum of an
Earth-like planet are
being planned