Today

1
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

2
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?

3
Our Solar System

• Orbits and Gravity
• Planetary System Formation

4
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...

5
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

6
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

7
Gravity

• Force of gravity also increases as objects get
  nearer
• Inverse Square Law (same as light)

8
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

9
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

10
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

11
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

12
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

13
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

14
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

15
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

16
Our Solar System

• Other Bodies
• Mercury
• The Moon
• Venus
• Mars
• Gas Giants
• Gas Giant Moons

17
The Moon

• No atmosphere
• No geological activity
• No water
• -gt no erosion
• Can provide information about formation of solar
  system that is absent from Earth

18
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

19
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

20
Possible Moon Formation Scenario
21
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

22
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

23
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

24
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!)

25
Mars

• Near asteroid belt
• Likely more collisions than Earth
• Large impacts can blow off significant rocky
  material
• Meteorites
• As well as gases (atmosphere)

26
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

27
Mars

• Too little gravity to be able to hold onto a
  significant atmosphere
• Atmospheric pressure less than 1 of Earth's

28
Evaporation

• What causes evaporation of liquid, and what
  prevents it?

29
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

30
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

31
Evaporation

• Effect of atmospheric pressure happens on our own
  planet
• Reason for high-altitude cooking instructions'
  on some boxes
• Higher altitude -gt lower air pressure -gt
  evaporation is easier -gt lower boiling point

32
Evaporation

• Martian atmospheric pressure lt 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

33
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'

34
The Giants

• The Giants are sometimes all called Jovian'
  planets after Jupiter
• After more exploration showed their diversity,
  this term lost favour

35
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?

36
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

37
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

38
The Birth of Giants

• Large mass -gt 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

Jupiter in Infra-red
39
The Birth of Giants

• 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?

Jupiter in Infra-red
40
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)

41
The Moons of Giants

• Formation like planets around sun
• Rotating body, disk forms
• Moons generally along plane of rotation of planet

42
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

43
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')

44
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

45
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

46
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

47
How Unique is Earth?

• What is special about Earth?
• How important/rare are those things?
• How many such planets are there likely to be?

48
Earth

• Atmosphere
• Large surface gravity
• Reasonable temperature
• Rocky surface
• Large moon
• Lots of heavy elements

49
How Important/Rare are these?

• Heavy elements
• Likely ubiquitous in planets around Pop I stars

50
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)

51
How Important/Rare are these?

• Atmosphere
• Requires not too close to sun
• Requires massive enough planet

52
How Important/Rare are these?

• Reasonable Temperature
• Goldilocks zone
• Needs to be right distance to star

53
How Important/Rare are these?

• So we require
• Rocky Planet
• Of the right mass
• At the right distance from the star

54
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.

55
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 arent stable

56
Finding Other Planets

• Light from planet
• Reflected visible light
• Reflectedgenerated infrared
• Dark from planet
• Transits (shadows from planets)
• Light bent by planet
• Gravitational Lensing
• Star's Motion from planet
• Proper Motions
• Doppler Shift

57
Light from the planet

• 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

Small brown dwarf (not planet) companion to a
star directly imaged
58
Light from the planet

• Has yet to be observed
• What sort of planets/systems does this work best
  for?

59
Light from the planet

• 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

Small brown dwarf (not planet) companion to a
star directly imaged
60
Light from the planet

• Large planets near star Hot Jupiters'
• Gas giants (presumably) very near star

Small brown dwarf (not planet) companion to a
star directly imaged
61
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

62
Light from the planet

• 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

Small brown dwarf (not planet) companion to a
star directly imaged
63
Planetary Transits/Occultations

• 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

Brightness
Time
64
Planetary Transits/Occultations

• 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

?
Brightness
Time
65
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

66
Planetary Transits/Occultations

• Large survey
• Dedicated telescope
• Look at large fraction of sky every night (or
  nearly)

67
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

68
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

69
Gravitational lensing

• At least one planet has been seen' this way
• Results
• Mass of planet, star
• Distance to star
• Distance planet lt-gt star
• Difficult, because only get one chance at
  measuring system

70
Gravitational lensing

• Works best for what systems?
• Dim Stars
• Massive planets
• (relatively) insensitive to distance between star
  and planet
• Jupiters at any radii / temperature

71
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

72
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

73
Astrometry Proper Motions

• Has been successfully used to detect white-dwarf
  companions
• Shown below Sirius
• No successful measurement of planets however

74
Astrometry Proper Motions

• Would work best for?

75
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

76
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)

77
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

78
Doppler Shifting

• Works best for

79
Doppler Shifting

• Works best for
• Large planets
• Close in
• Faster period (easier to detect)

80
Interstellar Travel, Interstellar Communication

• Interstellar Travel
• Rockets
• Fuel
• Speeds
• Time Dilation
• Interstellar Communication
• What frequencies do we use?
• Meaningful signals
• SETI_at_home

81
Rockets

• 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

Net Force -gt acceleration
Gravitational Force
Force exerted by exhaust
82

• Easy to accelerate upwards
• Hard to keep from falling back down!
• Can either
• Accelerate very quickly to escape vel (25,000
  mph) and coast up
• 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

Net
Grav Force
exhaust
83
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

84
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

85
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

86
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.

87
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

88
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

89
Travel Difficult

• Communication much simpler than Transportation.

90
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

91
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

92
SETI_at_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

93
SETI_at_home

• Done as part of screen saver on thousands of
  volunteer's computers

94
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...

95
Has the Search Happened Already?

• UFO sightings
• What Evidence is Necessary?
• If no UFOs yet, why not?

96
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

97
What Evidence is Required?

• Large amount of documentary evidence that the
  Universe has apparently searched for life here
• Why not accept this as truth?

98
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?

99
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?

100
What Evidence is Required?

• Photographs are easily misinterpreted
• Photographs also easily faked
• These Robert Schaefer

101
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

102
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

103
Same lab change blindness'

• http//viscog.beckman.uiuc.edu/grafs/demos/10.html

104
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
• ...

105
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?

106
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
• ...

107
Fermi's Paradox

• No signals from aliens yet.
• No visitors yet either, perhaps.
• Why not?

108
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

109
Next week

• Assignment covering Weeks 8-13 due
• Projects also due next week
• Class summary
• Any student presentations