Planets
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For life on a planet, so far we
have three important questions:
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How far is it from its Sun?
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How massive is it?
–
What type of planet is it: is
it rocky?
Distance from Star
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Must be at distance from star
that liquid water can exist with
an environment
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Not too close (Venus)
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Not too far (further than Mars)
Period of orbit
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P
1 year
2
1
M
M sun
a
1 AU
3
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Lenth of time takes to complete
one orbit
Period of orbit and distance are
related
For a given star mass, period
squared is proportional to
distance cubed
Large distance – takes more
time to orbit
Closer – orbits faster
If know (or can estimate) star's
mass, period <-> distance
Size of Planet
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Rocky planets in our system:
–
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Gas Giants
–
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0.4 – 1.0 Earth diameters
4.0 – 11.0 Earth diameters
Size alone gives an idea what
sort of planet it is
Size + mass of planet cinches it
(why?)
We can see proto-planetary disks...
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Observed around very young
stars
Obscures new star in visible
light
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Glows in infrared (why?)
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Can we see planets?
–
Much harder
–
Condensed objects
–
Lost in glare of star
–
Until 10 years ago, answer:
NO.
Can we see planets?
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Answer today: yes!
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>110 extra solar planets discovered
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More almost every month
Can we see planets?
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Easier to make
measurements of
nearby stars
Many of the stars with
known planets are
easily visible to the
eye, even in Chicago
Gamma Cephei is near
the north pole
(Polaris)
Can we see planets?
●
●
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Easier to make
measurements of
nearby stars
Many of the stars with
known planets are
easily visible to the
eye, even in Chicago
47 Ursae Majoris is
below the big dipper
Finding Extra-solar Planets
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Techniques
–
Direct(ish) measuring of planet
–
Indirect measurement – effect on star
Results of search so far
–
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`Hot Jupiters'
Implications
–
Can life be found in these systems?
–
Are most systems like this?
–
Migration vs. Direct Formation
Finding Extra-solar Planets
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Direct(ish) Methods
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Light from planet
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–
Dark from planet
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–
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Visible, Infrared
Planet transits
Bending light from other
object
Indirect methods – gravitational
effect on star
–
Pulsar Timing
–
Astrometry
–
Doppler Shift
Direct Methods
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Light from planet
–
Reflected visible light
–
Reflected+generated infrared
Dark from planet
–
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Transits (shadows from planets)
Light bent by planet
–
Gravitational Lensing
Light from the planet
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Small brown dwarf (not
planet) companion to a
star directly imaged
Stars observed by emitting
their own light
Planets don't emit light, but do
reflect sunlight
Problem: reflect a billionth or
less of the light from the
companion star
Light from the planet
●
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Has yet to be observed
What sort of planets/systems does
this work best for?
Light from the planet
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Small brown dwarf (not
planet) companion to a
star directly imaged
Would work best for:
–
Large planets (more
reflecting surface)
–
Reflective planets
(ammonia clouds?)
–
Near enough star to
reflect lots of light
–
Far enough not to be
overwhelmed by light
from star
Light from the planet
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Small brown dwarf (not
planet) companion to a
star directly imaged
Large planets near star: `Hot
Jupiters'
Gas giants (presumably) very
near star
Light from the planet
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How observed?
Very careful imaging of nearby
stars
Probably with telescopes above
atmosphere (Hubble)
As long as planet isn't in front
of/behind star, will be reflecting
light towards Earth
Just a question of being able to
observe it
Light from the planet
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Small brown dwarf (not
planet) companion to a
star directly imaged
This is actually an infrared image
Jupiter-type planets may emit their
own infrared light
Terrestrial planets reflect a lot of
infrared
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Star emits most of its light in visible
●
Better chance in IR
Light from the planet
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Infrared is between visible light
and radio
`Near' infrared most easily detected
with telescopes
Very far infrared can be observed
with radio telescopes
Light from the planet
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Interferometry
Allows (with some computation)
using several radio telescopes as if
it were one large telescope
Easier to do with radio than with
visible light
Amount of signal proportional to
total area
Resolution increases with size of
array
Infrared interferometry has some
promise for observing planets
directly
Dark from the planet
●
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Brightness
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Time
Light from planet can be blocked
by orbiting planet
Careful measurement of total
light from star can show this
Can't see directly; the star is just a
point
Planetary Transits/Occultations
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Brightness
●
Time
Light from planet can be blocked
by orbiting planet
Careful measurement of total
light from star can show this
Can't see directly; the star is just a
point
Planetary Transits/Occultations
●
●
Brightness
●
Time
Light from planet can be blocked
by orbiting planet
Careful measurement of total
light from star can show this
Can't see directly; the star is just a
point
Planetary Transits/Occultations
●
●
Brightness
●
Time
Light from planet can be blocked
by orbiting planet
Careful measurement of total
light from star can show this
Can't see directly; the star is just a
point
Planetary Transits/Occultations
●
●
Brightness
●
Time
Light from planet can be blocked
by orbiting planet
Careful measurement of total
light from star can show this
Can't see directly; the star is just a
point
Planetary Transits/Occultations
●
●
Brightness
●
Time
Light from planet can be blocked
by orbiting planet
Careful measurement of total
light from star can show this
Can't see directly; the star is just a
point
Planetary Transits/Occultations
●
●
Brightness
●
Time
Light from planet can be blocked
by orbiting planet
Careful measurement of total
light from star can show this
Can't see directly; the star is just a
point
Planetary Transits/Occultations
●
What sort of planets/systems does
this work best for?
Planetary Transits/Occultations
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Brightness
?
What information can we get?
–
If can watch until repeats, can
find period of planets orbit
–
Length of dip: amount of time
planet in front of star
–
Time
●
Speed of Planet
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Size of Star
Amount of dip: Size of planet
/ size of star
Planetary Transits/Occultations
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Brightness
?
Time
If period is measured (multiple
transits) and mass estimate for star
exists, have:
–
Planet's distance
–
Planet's size
–
Planet's orbital period
–
Star's size
Planetary Transits/Occultations
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How are these observed?
Planetary Transits/Occultations
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How are these observed?
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Fairly rare events:
–
Has to be exactly along line of
sight
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Only planetary systems aligned
along line of sight
Planet directly in front of star
only very briefly (Jupiter: ~1
day / 11 yrs)
Fairly careful measurements must
be made
–
Jupiter: 1% decrease in Sun's
brightness
Planetary Transits/Occultations
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Large survey
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Dedicated telescope
–
Look at large fraction of sky
every night (or nearly)
Planetary Transits/Occultations
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Works best for:
–
Large planets (blocks more of
star)
–
Planets near star (shorter
period – easier to observe)
–
Hot Jupiters
Has been used to find planets
Gravitational lensing
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A very powerful technique to
measure dim objects
Used in searches for brown dwarfs
or other large clumps of `dark
matter'
Requires
–
distant, bright, source star,
–
very accurate measurements
of the brightness of the
source star over time
Gravitational lensing
Gravitational lensing
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Similar requirements to transit
searches
Lots of careful images of large
amount of sky
Comparison to see any changes
Lensing searches get transit data
`for free'
Both transit search, lensing data
here from same operation (OGLE)
Gravitational lensing
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At least one planet has been `seen'
this way
Results:
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Mass of planet, star
–
Distance to star
–
Distance planet <-> star
Difficult, because only get one
chance at measuring system
Gravitational lensing
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Works best for what systems?
Gravitational lensing
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Works best for what systems?
–
Dim Stars
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Massive planets
–
(relatively) insensitive to
distance between star and
planet
–
Jupiters at any radii /
temperature
Indirect Methods
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Gravitational Effect on Star
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Pulsar Timing
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Astrometry
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Doppler Shift
Center of Mass
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`For every action there is an equal
and opposite reaction’
Gravitational force Earth exerts on
Sun the same as the force the Sun
exerts on the Earth
So why does the Earth orbit the Sun,
and not vice-versa?
Center of Mass
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Same force, but Sun is much heavier
than earth
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Same force moves Sun very little
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But Earth (say) a Lot
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Relative amount of motion =
relative masses of objects
Center of Mass
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Same force, but Sun is much heavier
than earth
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Same force moves Sun very little
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But Earth (say) a Lot
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Relative amount of motion =
relative masses of objects
Center of Mass
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Sun is 300,000 times more massive
than Earth
So Sun moves 1/300,000 as much as
Earth
Both orbit a Center of Mass which is
300,000x closer to center of Sun than
Earth
–
1/10% of Sun’s radius
Center of Mass
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Sun is 1,000 times more massive
than Jupiter
So Sun moves 1/1,000 as much as
Jupiter
Both orbit a Center of Mass which is
1,000x closer to center of Sun than
Jupiter
–
Sun’s radius
Pulsars
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`Cosmic Lighthouses'
Send out beam of high-energy
radiation
Rotates
If we're along line of sight, see very
regular bursts of light/energy
Easy visibility + regularity -> very
easy to detect changes
Pulsars
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Two planets have been so far
discovered around pulsars
Significance for life? Probably
small.
–
Pulsar likely the result of a
supernova
–
Neutron star doesn't emit
much energy
–
Column of high-energy
radiation every few seconds
probably not helpful
Pulsars
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What sort of systems would this
work well for?
Pulsars
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What sort of systems would this
work well for?
–
Need a pulsar
–
Massive planet (large
gravitational effect)
–
Near the pulsar (large
gravitational effect)
Astrometry: Proper Motions
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Stars motion towards/away from us
can be measured very accurately
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Doppler Shift
Motions `side-to-side' on the sky
take VERY long time to make
noticable changes
Astrometry: Proper Motions
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If star has a large enough proper
motion
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(probably means very near us)
Wobble in the star's motion could
indicate that the star is being
tugged on by a nearby planet
Astrometry: Proper Motions
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Has been succesfully used to detect white-dwarf
companions
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Shown below: Sirius
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No successful measurement of planets however
Astrometry: Proper Motions
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Would work best for?
Astrometry: Proper Motions
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Would work best for?
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Nearby strs
–
Large mass companion
–
Distant from planet: can pull further distance
–
Near planet: faster orbit, more visible wobble
Doppler Shifting
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Star has slight motion in
orbit
If that motion is largely
towards/away from us,
might be detected by
doppler shift
Motions towards/away can
be very accurately measured
(few meters/sec)
Doppler Shifting
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Has so far been extremely
succesful
If can watch for several
periods, can get very
accurate period
measurements
Sine wave: circular orbit
`Tilted' sine wave: elliptical
orbit
Get: period, total velocity
induced by planet
Doppler Shifting
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Works best for:
Doppler Shifting
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Works best for:
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Large planets
–
Close in:
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Faster period (easier to
detect)
Center of Mass
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Sun rotates around a circle 1/10% of
Sun’s radius in size every year
Maximum velocity: 3 inches/sec
Center of Mass
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Sun rotates about a circle its radius
in size every 11 years
–
10 yards/sec
Hot Jupiters
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How did these planets get so close to
their sun?
Normal planetary formation theory:
< 1 AU is too close
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Gasses would have evaporated
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No Juptiers from
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Migration?
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High eccentricities
–
Gas could stay, just not form…
Hot Jupiters
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Jupiters are large enough to disrupt
other planets
Asteroid belt
Jupiters less than 4 or 5 AU away
from sun would probably prevent
any Earth-like planets forming
within habitable zone.
Hot Jupiters
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Unlikely place for life
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BUT
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If some have ~ Earth sized moons:
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Rocky “planets” in habitable
zone?
Reading for Next Class (Apr 30)
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Chapter 19, 20: Interstellar spaceflight, communications
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Chapter 19: Interstellar spaceflight
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Energy, fuel requirements
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Time requirements
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Time dilation – Special Relativity
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Manned vs. Unmanned probes
Chapter 20: Interstellar communications
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Spectrum, and choice of frequency
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Choice of message
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Listening vs. Speaking
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SETI@home