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Summary for Final
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Last week of classes 12/10 to 12/14
First hour (Monday):
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Second hour (Wednesday):
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Final, Part 3: SCANTRON Test (Form 882, #2 pencils),
70 questions from Review questions, cumulative,
includes question on SGA and Planispheres (70 pts)
During your Third Hour
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Planetarium Sky Quiz (30 pts)
Final, Part 1 (Solar System Object Quiz, 20 pts)
Final, Part 2: Group effort on questions relating to 3rd
hour (20 pts)
All extra credit due by 12/14 at NOON
Also: HW this week due on Friday, 11:59 PM as
usual
Lecture 13: Other Planetary Systems
Planetary Systems around Other Stars?
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Until recently, we had no proof that there
were other planets around other stars
(extrasolar planets).
Throughout history the meaning of planets
has changed
Defining a planet is tricky and was done for
our solar system just recently
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Not officially defined for other solar systems
Web site: http://exoplanets.org
© Sierra College Astronomy Department
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Lecture 13: Other Planetary Systems
Planetary Systems around Other Stars?
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The catastrophe theories of solar system
formation were prominent until about 50
years ago
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Many of these ideas required a (near) collision
with another star
As a consequence, these would predict that other
solar systems are very rare
Since disks of dust and gas have been seen
around other stars, could it be more
commonplace to have stars with planets
around them?
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Lecture 13: Other Planetary Systems
Planetary Systems around Other Stars?
The difficulty of detecting other planets
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Giant planets are likely easier to detect
because they are more massive and
brighter that terrestrial planets
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Directly observing a Jupiter-like planet
around a Sun-like star would be difficult
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If the Sun were a the size of a grapefruit, Jupiter
would be a marble 80 meters away.
The star would shine one-billion times
brighter than the reflected light from the
planet
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Lecture 13: Other Planetary Systems
Planetary Systems around Other Stars?
Successful detection
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There are 2 basic ways to detect an extrasolar planet:
 Directly: with pictures or spectra
 Indirectly: with precise measurements of stellar
properties
Most of the efforts of the last 10-15 years deal with
looking for the gravitational influence of the planet on its
parent star.
COM
While it appears that a planet orbits the sun, it is better to
say that both orbit a common center of mass
This means the star is slowly moving about this point if at
least one planet is present. Multiple planets may make
this motion very complicated.
There are two techniques: astrometric and Doppler Sun
COM 5
© Sierra College Astronomy Department
Demo
Lecture 13: Other Planetary Systems
Planetary Systems around Other Stars?
Astrometric Technique
 This involves looking at the very precise
motion of a star over time
 As an unseen planet tugs on a star, the
star will move side to side with a period
equal to that of the planet
 This
method is difficult: e.g. if the Sun and
Jupiter were 10 light-years away the Sun
would appear to move only 0.003 arcsec
and it would take 12 years to happen
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Lecture 13: Other Planetary Systems
Planetary Systems around Other Stars?
Doppler Technique
 This involves looking at the stars
motions using the Doppler shift
Doppler
 As
the unseen planet tugs on the stars, the
star will move alternatively closer and
further from us
 The causes the spectral lines to alternately
blue and red shift.
 This leads to an orbital speed of the star
51 Peg
around its center of mass
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Lecture 13: Other Planetary Systems
Planetary Systems around Other Stars?
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This was first used successfully on 51 Peg to
detect a planet
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51 Peg
Doppler
The star wobbled at 57 m/s with a 4-day period
Therefore the planet was going around the star in 4
51 Peg A
days!
Drawing
This planet had to lie close to star and had a surface
temp of 1000 K
Current techniques allow us to measure velocities
to 1 m/s – walking speed
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This allows to find smaller planets, further away than
the one around 51 Peg
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other stars
Lecture 13: Other Planetary Systems
Planets around Other Stars
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How easy would it be to find a system like
ours?
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For the Jupiter-Sun system the speed of the
Sun around the center of mass is only 13 m/s
This leads to a Doppler shift of only one part in
20 million
The details of this motion allows us to come
up with the orbital period of the planet and
the average distance the planet is from the other stars
star
From this info, what method do we use to
find the mass of the star (plus the planet)?
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Lecture 13: Other Planetary Systems
Planets around Other Stars
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COM
Doppler
Face-on
More scrutiny of the data allows us to get the
mass ratio of the system and therefore the mass
of the planet
COM
Caveat: the Doppler shift depends on the
Doppler
average
inclination of the orbit in the plane of the sky
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If the orbit is edge-on we get the “full” Doppler effect:
good and correct estimation of mass
If the orbit is tilted by some amount the Doppler shift
is reduced because not all the motion is towards and
away from us: underestimation of mass
If the orbit is face-on, no Doppler shift is measured
and the planet remains undetected
© Sierra College Astronomy Department
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Lecture 13: Other Planetary Systems
Planets around Other Stars
The search begins by looking at stars that
are the similar to the sun
 The newly discovered systems do not look
like our own
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This in part is due to a selection effect – we
are more likely to detect large planets close to
the star
 All planets currently detected are giant
planets: we don’t yet have the sensitivity to
detect Earth-size planets
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Lecture 13: Other Planetary Systems
Planets around Other Stars
Transiting planets
 In certain extrasolar systems where the planet is
edge-on to our line of site, the planet will pass
over or transit the star once every orbit.
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Occasionally, Venus and Mercury do this over the Sun
Venus
as seen from Earth
transit
The transit should dim the star slightly
The transit should repeat every orbital period
The size of the planet may be derived
Since the inclination is nearly edge-on an accurate
mass may be determined.
© Sierra College Astronomy Department
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Lecture 13: Other Planetary Systems
Planets around Other Stars
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transit
The first successful transit occurred with the star
HD209458
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Star was known to have a planet via Doppler
method.
The period between transits (3.5 days) exactly
matched what was derived with the Doppler
method.
The mass, radius (and therefore volume) were
derived allowing us to calculate a density. This
density was consistent with a Jovian planet.
Atmospheric information could be derived too.
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Lecture 13: Other Planetary Systems
Planets around Other Stars
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The planet was also seen to behind
HD209458 (i.e. an eclipse)
 This
resulted in a smaller drop in the total
light since the planet is much fainter than
the star
 The temperature of the planet could be
derived at about 1100 K
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Lecture 13: Other Planetary Systems
Planets around Other Stars
 This transiting method has strengths and
weakness
 Weakness:
Only a small fraction of
systems have planets which transit their
star.
 Weakness: short orbital period planets are
far more likely to be discovered this
technique.
 Strength: smaller planets can be detected
this way, including, potentially, Earth-size
planets
© Sierra College Astronomy Department
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Lecture 13: Other Planetary Systems
Planets around Other Stars
Direct detection
 Getting a direct picture of a planet could
more detail about their atmospheres and
surfaces.
 As mentioned before, the glare of the
parent star, makes it very difficult to get
picture of the planet
 A planet has been detected around a
Brown dwarf
failed star-type called a brown dwarf. planet
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Lecture 13: Other Planetary Systems
Planets around Other Stars
Other strategies
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Gravitational lensing
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Unseen planet may distort light of background
star
Looking the influence of planets around
the star’s disk of dust
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Ripples seen in Beta Pictoris indicate planets
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Lecture 13: Other Planetary Systems
The Nature of Extrasolar Planets
What do we learn?
By studying these extrasolar planets, we
learn what types of possible planets there
are.
 We also see whether the layout of our
solar system is rare or common.
 Can the solar nebular theory explain it all
or does it have to modified?
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Lecture 13: Other Planetary Systems
The Nature of Extrasolar Planets
What have we learned? Here are the
extrasolar planetary properties we have
derived in the last 15 years:
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Orbital period
Orbital distance
Orbital shape
Mass
Size (radius)
Density
Composition
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Lecture 13: Other Planetary Systems
The Nature of Extrasolar Planets
Orbits
masses
orbits
eccentricity
 Only
a handful of orbits are larger than
5 AU and quite a few are closer than
Mercury is from the Sun
 Most orbits are (very) elliptical
 Some multiple planet systems have
orbital resonances
© Sierra College Astronomy Department
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Lecture 13: Other Planetary Systems
The Nature of Extrasolar Planets
Masses
masses
orbits
eccentricity
 All
of the extrasolar planets are greater
than 5.5 Earth masses.
 Many are more massive than Jupiter.
 This is clearly a selection effect as
more massive object are easier to
detect via Doppler, astrometric, and
transit methods.
© Sierra College Astronomy Department
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Lecture 13: Other Planetary Systems
The Nature of Extrasolar Planets
Sizes and Densities
masses
orbits
masses
Transiting planets allow us to get size and
then density.
 We have found that seven of these worlds
are larger but less dense than the Jovian
planets.
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These worlds lie close to the star and puffs up
its outer atmosphere making it larger and less
dense.
 There is one planet which has a Saturn-like
mass but a Neptune-like density and may
contain more rock and ice than the Jovian
planets.
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© Sierra College Astronomy Department
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Lecture 13: Other Planetary Systems
The Nature of Extrasolar Planets
Composition
 There
is little information on the
composition, except through transits
and eclipses.
 We
compare spectra during vs.
before/after transit
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see the Jovian-like planets have
hydrogen and water in their
atmosphere.
© Sierra College Astronomy Department
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Lecture 13: Other Planetary Systems
The Nature of Extrasolar Planets
Compare to our Solar System
layout
masses
 Most
of the extrasolar systems bear
little resemblance to our Solar System.
 The extra-jovian planet should look
similar to ones we have. However,
many are very close to their star and
have very eccentric orbits
 Labeled as “hot Jupiters” there origin is
still under debate
Jup vs. Hot Jup
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Lecture 13: Other Planetary Systems
The Foramation of Extrasolar Systems
Challenge to the Solar Nebula theory
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layout
Recall that the solar nebula theory suggested that
most of the protoplanetary disk was made of H,
He, a little hydrogen compounds, and still less
rocks, metals.
The rocks and metal were the only thing which
condensed close to the Sun while in the outer
solar system, everything condensed
This lead to the small rocky terrestrial planets in
the inner solar system and larger, more gaseous
Jovian planets in the outer solar system
These extrasolar systems challenges that basic
idea
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Lecture 13: Other Planetary Systems
The Foramation of Extrasolar Systems
layout
Why are the Jovian planets so close
to their star?
 It
does not seem likely that a Jovianlike planet could form so close to their
star
 Astronomer now feel that these “hot
Jupiters” formed far away and
somehow migrated inward
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Lecture 13: Other Planetary Systems
The Foramation of Extrasolar Systems
migration
Planetary Migration
In this scenario, a planet forms “quickly” in
the protoplanetary disk.
 As it orbits around a star it starts to bunch
up material on either side of it
 This material starts to interact with the
planet causing to migrate inward
 This did not happen so much with our Solar
System, because the solar wind cleared out
all this gas and dust
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Lecture 13: Other Planetary Systems
The Foramation of Extrasolar Systems
Encounters and Resonances
 The eccentric orbits of some of
these planets might be due
interactions they has with
neighboring planets.
 Resonances also may also forced
some of eccentric orbits.
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Lecture 13: Other Planetary Systems
The Foramation of Extrasolar Systems
Change the theory
 It is too early to tell whether the
basic solar nebular theory need to
be changed
 Our solar system may be unique
which might suggest that the Earth
is unique in this Galaxy
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Lecture 13: Other Planetary Systems
Finding More Extrasolar Planets
Kepler
 The
discovery which will most
significant is finding an Earth-like planet
 Current
technology: not quite there
 Transit detection missions: Kepler (Launch
2008) will be able to monitor 100,000 stars
looking transiting planets, down to the size
of Mercury!
 COROT (Lauched 2006) is not as
sensitive as Kelpler, but should be able to
detect planets down to a few Earth
masses
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Lecture 13: Other Planetary Systems
Finding More Extrasolar Planets
 Astrometric
migration
missions:
 GAIA (launch
2010) will be able to
measure star positions down to 10
microarcsecs
 SIM (Space Interferometer Mission;
launch 2011) will be able to measure star
positions down to 1 microarcsec
 That’s
good enough to find a Jupiter-like
planet 3000 light-year away
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Lecture 13: Other Planetary Systems
Finding More Extrasolar Planets
 Direct
TPF-I
detection missions:
 TPF
(Terrestrial Planet Finder;
launch 2014,2020)
These
two missions will be able to
detect planets around by effectively
blocking starlight from a photo leaving
only the planet(s)
Spectra and other analysis can lead to
determination of atmospheric
properties
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The End
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