Download Lecture (Powerpoint)

Today
●
●
Today:
–
Review of Parts 4 & 5 of the text (Weeks 8-12)
–
Cover the last of the material
Next week
–
Assignment covering Weeks 8-13 due
–
Projects also due next week
–
Class summary
–
Any student presentations
Review: Weeks 8-12
●
History of our Planetary system
●
Planets Other than Our Own in our Solar System
●
Habitability of places other than Earth
●
Finding Planets outside of the Solar System
●
Visiting or Communicating?
Our Solar System
●
Orbits and Gravity
●
Planetary System Formation
Orbits
●
●
●
●
●
Planets are falling towards Sun
due to gravitational acceleration
Moving toward the side fast
enough that they miss
Moving too fast – escape entirely,
leave Sun
Move too slowly – fall into Sun
Same with satellites circling Earth,
or Sun orbiting in our galaxy, or...
Gravity
●
●
●
●
●
Gravity acts between all massive
objects
Gravitational force is equal on
both objects
If orbiting, both objects move, not
just one, since both are being
acted on by gravity
Both orbit the center of mass of
the system
Equal mass objects; center of mass
is at the center of the two objects
Gravity
●
●
●
●
●
If one body is more massive,
then gravitational force is
increased
Center of mass tilts towards
more massive body
Forces still equal
Equal force on lighter body
moves it more than the same
force on the heavier body
Lighter object moves larger
distance than heavier object
Gravity
●
●
Force of gravity also increases
as objects get nearer
Inverse Square Law (same as
light)
Orbits
●
Kepler's Laws: (EMPERICAL)
–
Planets travel in ellipses,
with sun at one focus of
ellipse
–
Area swept out by radius is
equal over any equal amount
of time
–
Square of the planet's period
(the `year' for that planet)
proportional to the distance
to the sun cubed.
–
P2 ~ a3
Planets
●
●
●
●
●
Almost all planets are in same
plane
All planets (except Uranus)
rotate more or less in the same
plane, as does Sun
Very suggestive of the idea that
planets, Sun formed from a
disk, as we discussed before
Suggested by Laplace in 1600s.
Disk near star is depleted in
Hydrogen, Helium by
evaporation
Planet Formation
●
●
●
●
As disk cools, gas/dust disk can begin
condensing
Grains form, which themselves
agglomerate to larger particles
Regions where disk is originally dense
condense faster, gravitationally attract
more material
Process of continued agglomeration can
form planets
Instability
●
●
●
Some processes are naturally stable
–
Burning in main sequence stars
–
Core heats up – outer layers puff
up – core cools down
–
Automatically stabilizes itself
–
Ball in a right-side-up bowl
Once there's a region of high density in a
gas cloud or disk, increase in
gravitational attraction to that region...
Unstable
–
Ball on an up-side-down bowl
Planet Formation
●
●
●
●
Proto-planetary-core starts
sweeping out material and
planetesimals at its radius
Accrete material streams in from
just outside or inside its radius
There is a limit to this process; if
there are planets forming on either
side, eventually the gaps collide –
no more new material
This process of slowly sweeping
up and accreting material can take
millions of years
Mystery: `Hot Jupiters'
●
●
A Jupiter couldn't form at 1AU;
evaporation would prevent such a
gas giant from forming
Many of the extra-solar planets
observed are gas giants at
distances ~ 1AU
●
What happened?
●
Two possibilities:
–
Migration
–
Different formation
mechanism
Planet Formation
●
●
●
Migration is possible
As planets form and
accrete material, they
experience a drag force
Drag takes energy from
planets motion and they
fall inwards
Planet Formation
●
●
●
Fast formation is also
possible
In sufficiently massive disk,
instabilities can occur much
faster, and on larger scales
Can happen quickly enough
that perhaps giants can form
near star
Our Solar System
●
Other Bodies
–
Mercury
–
The Moon
–
Venus
–
Mars
–
Gas Giants
–
Gas Giant Moons
The Moon
●
No atmosphere
●
No geological activity
●
No water
●
-> no erosion
●
Can provide information
about formation of solar
system that is absent from
Earth
Mercury
●
Similar to moon
●
Similar size
●
Small, empty, simple
●
Very close to Sun
●
No atmosphere to mediate
temperature swings:
–
+750o F in sun
–
-230o F in shade
Moon's Cratering
●
●
●
Nothing to alter surface
Complete history of
cratering in Moon's history
From predicted cratering
rate, one expects that crust of
moon formed very quickly in
solar system history
Possible Moon Formation Scenario
Possible Moon Formation Scenario
●
Explains similar Oxygen abundances
–
●
Very different from meteorites
Explains fewer volatiles
–
If Earth's iron core had already settled,
impact would have dislodged crust material
–
Heat of impact would have vaporized
volatiles
Venus
●
●
●
●
Closest to Earth
¾ as far away from Sun as
Earth is
Very similar to Earth's size,
density
Covered by thick, opaque
clouds
Venus
●
Runaway greenhouse effect
●
Hot: very near sun
●
Water begins to evaporate
●
●
Water vapor is a greenhouse
gas!
Surface gets hotter, more water
evaporation
●
Surface is hundreds of degrees
●
No liquid water
Mars
●
●
Red planet between Earth and
Asteroid Belt
Half again as far away from Sun
as the Earth is
–
Expect it to be ~100o F
colder than Earth on average
–
Average too cool for water
–
Peak temps ~ 70o F (but -130
at night!)
Mars
●
Near asteroid belt
–
●
Large impacts can blow off
significant rocky material
–
●
Likely more collisions than
Earth
Meteorites
As well as gases (atmosphere)
Mars
●
~1/2 radius of Earth
●
~1/10 mass
●
~40% surface gravity
–
Force of a 1 lb weight less
than ½ lb on Mars
–
Less gravity holding the
atmosphere in place
Mars
●
●
Too little gravity to be able to hold
onto a significant atmosphere
Atmospheric pressure less than
1% of Earth's
Evaporation
●
What causes evaporation of liquid,
and what prevents it?
Evaporation
●
●
●
●
What causes evaporation of liquid,
and what prevents it?
Fastest moving water (say)
molecules can escape into
atmosphere
Water molecules in atmosphere
can collide into water and become
part of the liquid
Balance is reached when
evaporating water = condensing
water
Evaporation
●
Can change balance:
–
Little water in atmosphere,
evaporation happens faster
●
–
(Why feel so sticky on a
humid day)
If air pressure is very low,
evaporated water molecules
can move very far away
from pool of water
●
Fewer around to condense
●
Faster evaporation
Evaporation
Boiling Point at Alt it ude
●
220
Boiling Pt of Wat er (F)
210
200
190
●
180
170
160
150
●
140
130
120
110
100
0
2000
5000
Alt it ude (ft )
7500
10000
Effect of atmospheric pressure
happens on our own planet
Reason for `high-altitude cooking
instructions' on some boxes
Higher altitude -> lower air
pressure -> evaporation is easier > lower boiling point
Evaporation
●
Martian atmospheric pressure <
1% of Earth's
–
●
●
(Earth's atmosphere at 15
miles / 80,000 ft)
Water boiling point is so low that
any liquid water evaporates
immediately
No free water possible on surface
Evaporation
●
But water ice DOES exist on
Mars:
–
Polar ice caps
●
●
–
Mostly (on top) dry ice
(frozen CO2)
Underneath, visible when
CO2 has sublimated, water ice
Quite likely some trapped
under surface: `permafrost'
The Giants
●
●
The Giants are sometimes all
called `Jovian' planets after
Jupiter
After more exploration
showed their diversity, this
term lost favour
The Giants
●
●
●
●
The giant planets can be
weighed very accurately by
measuring the speed of their
moons.
Much heavier than Earth, but
not so heavy considering their
size
Densities 600 – 1600 kg/m3,
compared with Earth's 5700
kg/m3
Mostly made of gas/liquids?
The Birth of Giants
●
●
●
●
In outer solar system, cooler
Less evaporative stripping of
volatile gases
If sufficiently massive cores
form, can keep even volatile
gases
These gases will be
representative of the very
early solar system
The Birth of Giants
●
●
●
●
Since early solar system is
largely composed of
Hydrogen, so will gas giants
Rocky or Icy or Slushy core
High-hydrogen atmosphere
has some similarities to
atmosphere in Miller-Urey
experiment
Can form lots of organics
The Birth of Giants
●
●
●
●
●
Jupiter in Infra-red
Large mass -> high pressure,
temperature at centre
Temperature at centre of
Jupiter ~ 4 times surface of
Sun!
Collapse from origin of planet
still slowly continuing
Releases heat energy
These planets have a source of
heat
The Birth of Giants
●
●
●
●
●
Jupiter in Infra-red
Gas giants emit more heat
than they absorb from Sun
At earlier times, would have
been much hotter
Moons, which are nearby,
heated by their nearby planet
Many of these moons are
large (planet-sized)
Moons might be interesting
for life?
The Moons of Giants
●
●
●
●
Planets large enough that many
moons were also formed
Many of them planet sized in their
own right
Get heat from planet
Some (Io/Jupiter) effected by planets
magnetic field
●
Atmosphere? (Titan, Saturn)
●
Water? (Europa, Jupiter)
The Moons of Giants
●
Formation: like planets around sun
●
Rotating body, disk forms
●
Moons generally along plane of
rotation of planet
Gas Giants
●
Convection is a fundamental
process
–
●
Happens everywhere
Fluid heated at bottom rises,
cools, falls back down
●
Gas giants have hot centres
●
Large-scale motions
●
Mix material
Gas Giants
●
●
●
●
Makes it difficult to imagine
life forming
No real surface to live on
Chemicals constantly being
mixed around
No originally contained
environment (`protocell')
Moons
●
●
●
Gas giants have planet-sized
moons
At least one (Titan) has a
significant atmosphere
Another (Europa) very likely
has liquid salty water under a
layer of ice
Europa
●
●
Very suggestive it has a liquid
underneath
–
No cratering
–
Many fractures, ridges on
surface
What would this mean for life?
–
If some source of energy
on inside (geothermal,
chemical), very real
possibility of some sort of
life
Titan
●
●
Very Cold
Massive, Cold enough to have
an atmosphere (1.5 x as dense
as ours!)
●
No oxygen
●
No liquid water
●
Hydrogen rich
●
Interesting organic chemistry
●
Lakes of hydrocarbons?
●
Huygens probe 2005
How Unique is Earth?
●
What is special about Earth?
●
How important/rare are those things?
●
How many such planets are there likely
to be?
Earth
●
Atmosphere
–
Large surface gravity
●
Reasonable temperature
●
Rocky surface
●
Large moon
●
Lots of heavy elements
How Important/Rare are these?
●
Heavy elements;
–
Likely ubiquitous in
planets around Pop I stars
How Important/Rare are these?
●
Rocky Surface
–
Can happen if there is
heavy elements (see
above)
–
Probably true of all
planets close enough to
have liquid water
–
(But planet migration)
How Important/Rare are these?
●
Atmosphere
–
Requires not too close to
sun
–
Requires massive enough
planet
How Important/Rare are these?
●
Reasonable Temperature
–
`Goldilocks zone’
–
Needs to be right distance
to star
How Important/Rare are these?
●
So we require
–
Rocky Planet
–
Of the right mass
–
At the right distance from
the star
Habitable Zone
●
●
●
Corresponds to further than
Venus to about Mars distance
for our Sun
Using inverse-square law, could
calculate for other stars
Main requirement: liquid water
in the presence of an
atmosphere.
Habitable Zone: Binary Stars
●
●
●
●
●
About half of all stars are in
binary systems
Stars orbit a common centre of
mass (more on that next week)
Can planets have reasonable
orbits in such systems?
Yes, but must orbit one star or
be far away from both;
`Figure 8’ orbits aren’t stable
Finding Other Planets
●
●
Light from planet
–
Reflected visible light
–
Reflected+generated infrared
Dark from planet
–
●
Light bent by planet
–
●
Transits (shadows from planets)
Gravitational Lensing
Star's Motion from planet
–
Proper Motions
–
Doppler Shift
Light from the planet
●
●
●
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
●
●
Has yet to be observed
What sort of planets/systems does
this work best for?
Light from the planet
●
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
●
●
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
●
●
●
●
●
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
●
●
●
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
●
Star emits most of its light in visible
●
Better chance in IR
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
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
●
How are these observed?
●
Fairly rare events:
–
Has to be exactly along line of
sight
●
●
●
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
●
Large survey
–
Dedicated telescope
–
Look at large fraction of sky
every night (or nearly)
Planetary Transits/Occultations
●
●
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
●
●
●
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
●
●
●
At least one planet has been `seen'
this way
Results:
–
Mass of planet, star
–
Distance to star
–
Distance planet <-> star
Difficult, because only get one
chance at measuring system
Gravitational lensing
●
Works best for what systems?
–
Dim Stars
–
Massive planets
–
(relatively) insensitive to
distance between star and
planet
–
Jupiters at any radii /
temperature
Astrometry: Proper Motions
●
Stars motion towards/away from us
can be measured very accurately
–
●
Doppler Shift
Motions `side-to-side' on the sky
take VERY long time to make
noticeable changes
Astrometry: Proper Motions
●
If star has a large enough proper
motion
–
●
(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
●
Has been successfully used to detect white-dwarf
companions
●
Shown below: Sirius
●
No successful measurement of planets however
Astrometry: Proper Motions
●
Would work best for?
Astrometry: Proper Motions
●
Would work best for?
–
Nearby stars
–
Large mass companion
–
Distant from planet: can pull further distance
–
Near planet: faster orbit, more visible wobble
Doppler Shifting
●
●
●
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
●
●
●
●
●
Has so far been extremely
successful
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
●
Works best for:
Doppler Shifting
●
Works best for:
–
Large planets
–
Close in:
●
Faster period (easier to
detect)
Interstellar Travel, Interstellar
Communication
●
●
Interstellar Travel
–
Rockets
–
Fuel
–
Speeds
–
Time Dilation
Interstellar Communication
–
What frequencies do we use?
–
Meaningful signals
–
SETI@home
Rockets
●
Net Force
-> acceleration
Gravitational
Force
Force exerted
by exhaust
●
●
●
Have to exert force to overcome
that of gravity
Reactions from some sort of fuel
–
Chemical
–
Electrical...
Propel exhaust downwards
By Newton's 3rd law, propel
rocket upwards
●
●
●
Easy to accelerate upwards
Hard to keep from falling back
down!
Can either:
–
Net
Accelerate very quickly to
escape vel (25,000 mph)
and coast up
●
Grav
Force
exhaust
–
●
Gravity will keep
decelerating you but never
quite pull you back
Or accelerate slowly
through ascent
Luckily, further up you get,
weaker force from Earth's
gravity becomes
Rockets: Fuel
●
●
●
●
Takes a lot of fuel to move something
into Earth's orbit or further
Would take about as much fuel to
launch me into orbit as it takes to heat
a Chicago home through an entire
winter
Unlike a car trip, fuel starts weighing a
lot, even compared to rocket
Shuttle launch:
–
Empty Shuttle: 230,000 lb
–
Fuel :
2,700,000 lb
Fuel along the way?
●
●
●
●
●
Interstellar medium VERY
tenuous
Sprinkled with hydrogen
Could it be collected and then
burned (nuclear fusion?)
Hard to see how
–
Drag on ship
–
Power to magnetic fields
But would solve enormous fuel
problem
Special Relativity
●
Einstein:
–
Physics is the same in all
inertial frames of reference
–
Speed of light in a vacuum is
a fundamental physical
constant of the Universe
Special Relativity
●
●
●
●
But for higher velocities, can be
significant!
Astronaut goes to Alpha Centauri and
back at 95% of speed of light
Astronaut ages 3 years, people back
home 9
At closer and closer to speed of light,
effect gets bigger and bigger.
Special Relativity
●
●
●
●
Speed of light becomes moving target
Astronaut can put more and more
energy into traveling faster
But because can never pass light (light
must always travel at same velocity!)
can never pass speed of light
Takes infinite amount of energy to even
get to speed of light
Automated Probes?
●
High-tech Voyagers or Pioneers
●
Aim towards nearby stars
●
Enough fuel to accelerate
●
Enough smarts to navigate toward
system
●
Get solar power once near star
●
Send message
–
To nearby planets
–
To us
Travel Difficult
●
Communication much simpler than
Transportation.
Messages
●
●
Its a lot easier sending signals than
things
Messages
–
Have no mass
–
Don't require fuel
–
Don't require food/provisions
for long journey
–
Cheap to produce
–
Travel at speed of light
What frequencies to use?
●
●
●
Two choices for long-distance
forces:
–
Gravity (difficult)
–
Electromagnetic
But there's an essentially infinite
range of frequencies to examine
Radio waves:
–
Easy/cheap to generate,
focus
SETI@home
●
●
●
●
●
Several different SETI listening
experiment
One is called `Project SERENDIP'
`Listen in' on other astronomical
uses of the Arecibo radio telescope
in Puerto Rico
Can't choose where the observers
are looking, but can listen (nearly)
24x7
Receiver installed which listens to
168 million narrow channels near
21cm Hydrogen line
SETI@home
●
Done as part of screen saver on thousands of volunteer's
computers
Results
●
●
●
●
Several candidate signals
discovered
2500 persistent gaussians (longish
spikes seen at least twice)
Need to be checked to make sure
not interference/noise
Also searching data for persistent
spikes, pulses, triplets...
Has the Search Happened Already?
●
UFO sightings
●
What Evidence is Necessary?
●
If no UFOs yet, why not?
UFO Sightings
●
●
No shortage of UFO observation
stories, photos
A moment spent with google
provides thousands of ernest,
probably mostly honest web
pages describing
–
UFO sightings
–
Abductions
What Evidence is Required?
●
●
Large amount of documentary
evidence that the Universe has
apparently searched for life here
Why not accept this as truth?
Extrordinary Claims require
Extrordinary Evidence
●
●
Let me make two claims
–
This morning, violence broke out in an
up-til-now quiet region of Iraq, in the
southern town of Rajaf. Four US
soldiers were killed.
–
With great effort, I can fly short
distances (10-20 ft) using the power of
my mind.
Which (if either) do you believe?
Extrordinary Claims require
Extrordinary Evidence
●
●
●
You have exactly the same evidence for both
claims: my say-so.
Clearly, the Iraq claim has more serious
immediate consequences (death, future
violence)
Why is the same evidence more likely to be
sufficient in one case (the more serious,
even) than in the other?
What Evidence is Required?
●
Photographs are easily
misinterpreted
●
Photographs also easily faked
●
These: Robert Schaefer
What Evidence is Required?
●
●
●
Eyewitness evidence notoriously
unreliable
Human brain very good at seeing
patterns, filling in blanks
Too good, in fact, to be good at
mundanely reciting uninterpreted
observations
Observation Test
●
●
●
Quantitative test
Count basketball passes by one team (dressed in
white) in a complicated, dynamic scene
http://viscog.beckman.uiuc.edu/grafs/demos/15.html
Same lab: `change blindness'
●
http://viscog.beckman.uiuc.edu/grafs/demos/10.html
Post-event Suggestibility
●
Elizabeth Loftus:
–
Film shown of car accident
–
Questionaire after film
–
Followup questionaire afterwards
–
Leading questions,
misinformation in questions could
cause people to misremember
event afterwards
●
●
●
Wrong color of car
`Remembering' stop signs, buildings
that weren't there
...
What Evidence is Required?
●
●
●
This doesn't mean that all the
evidence is proven
wrong/mistaken
Not enough evidence to be
convincing
What would be convincing
evidence?
What Evidence is Required?
●
●
●
This doesn't mean that all the
evidence is proven
wrong/mistaken
Not enough evidence to be
convincing
What would be convincing
evidence?
–
Chunk of spacecraft
material/technology
–
Cheek swab from alien
–
...
Fermi's Paradox
●
No signals from aliens yet.
●
No visitors yet either, perhaps.
●
Why not?
Fermi's Paradox
●
●
●
●
●
Even if 1,000,000 civilizations in our
galaxy today, that's one per ~300,000
stars
Would have to explore by chance to
find Earth
Radio signals identifying Earth are very
new: 1960s or so
Even if travel speed of light, on has
been time for 20ly round trip:
Only a handful of stars that close
Next week
●
Assignment covering Weeks 8-13 due
●
Projects also due next week
●
Class summary
●
Any student presentations