Download L23 – Apr 03/08

SCI238 W08
Lecture 23: Extraterrestrial Life
atmospheres have been
detected around two
extrasolar planets; but
no water yet…
L23 – Apr 03/08
ET Life
Today’s Lecture
 Final Exam: Sat. 12 April, 7:30-10pm in PAC 1,2
 see old exam on web for format
 more emphasis on second half of Term (60/40?)
 a little cosmology
 astrobiology
 a new scientific discipline
 the chemistry of life
 the search for life
 in our Solar System
 where might life exist in other planetary systems?
 the “Rare Earth” hypothesis
 the Drake Equation: intelligent life in the Universe
 communicating with intelligent life in the rest of
the Universe
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we observe that central black hole mass
correlates with galaxy luminosity (mass)
so initial gravitational potential well must also play a part
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Models of Galaxy Formation
 hierarchical
 lots of small galaxies form, then merge to make bigger ones
 problem: simulations leave far too many small ones around
 monolithic
 large clouds of inter-galactic gas collapse to form large galaxies
 problem: difficult to make such large clouds of gas in such a
short time
 and combinations
 how long ago were merger events?
 how massive were merging galaxies?
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Are We Alone?
 Humans have speculated throughout history about life
on other worlds.
• it was assumed by many scientists & thinkers of the
17th & 18th Centuries that life was present on other
worlds
• and widely accepted by the public at the turn of the
20th Century
• scientists became more sceptical once we began to
explore the planets
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Are We Alone?
 Recent advances in astronomy and biology have renewed
interest.
• discovery of extrasolar planets indicate that
planetary systems are common
• indications that liquid water can exist on other worlds
• organic molecules are found throughout the Solar
System and Galaxy
• geological evidence suggests life on Earth arose as
soon as it was possible
• discovery that living organisms can survive in the
most extreme conditions
 This interest has spawned a new science called
astrobiology.
• the study of life in the Universe
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Astrobiology
This new discipline includes:
 cosmoschemistry
 chemical evolution
 the origin and evolution of life
 planetary biology and chemistry
 the formation of stars and planets
 space science
 expansion of terrestrial life into space
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Questions in extraterrestrial life
Is there intelligent life in our Galaxy?
… in the Universe? …in the solar system?
Can we communicate with other forms of life?
When will we communicate with these other forms?
How?
Is there any other form of life in our Solar System?
… in out Galaxy?
Was there life in any form in the past in our Solar
System?
How did life originate on our planet?
 spontaneously?
 carried to Earth by (?) a meteorite?
Could we recognize other forms of life?
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it is “easy” to produce the building blocks of life
•this simple
experiment
produces amino
acids – the basic
building blocks of
life as we know it…
•but not so easy to
turn them into life
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Clues to growth of amino acids?
clustering of millions
of amino acids into
droplets, which
“grow, divide”
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fossils ~2Gyr old (left)
compared with modern
blue-green algae
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oily droplets
made by exposing
freezing mixture
to strong UV light
produce cell-like
membranes
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•life exists under very
harsh conditions
•boiling water at right
and rich sulphur clouds
in ocean depths
•but water is a common
factor
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The Murchison Meteorite: arrow points to organic matter
=> chemical evolution has occurred elsewhere
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From organic compounds to
life: not an easy step
 exploration of the Solar System has revealed no sign
of large life forms/civilizations
 we must search for microbial life
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Mars: the best candidate for
extraterrestrial life?
 some reasons why Mars is the best
candidate to host such life:
 Mars was apparently warm & wet for
some periods in its distant past
 conditions, similar to early Earth,
made it possible for life to evolve
 had the chemical ingredients for life
 has significant amounts of water ice
 pockets of underground liquid water
might exist if there is still volcanic
heat
Mars at 2001 opposition
Hubble Space Telescope image
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Viking Lander (1976)
 We have searched for life on
Mars.
 Viking scooped up soil and ran
tests
 looked for products of
respiration or metabolism of
living organisms
 results were positive, but
could have been caused by
chemical reactions
 no organic molecules were
found
 results inconsistent with life
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Viking Lander (1976)
 this is not the final word.
 Viking sampled only surface soil
 took readings at only two locations
 life could be elsewhere or underground
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Martian Meteorite ALH84001
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Martian Meteorites
 rocks ejected by impact from
Mars have been found in
Antarctica.
 analysis of one revealed…
 age of4.5 billion years old
 landed on Earth 13,000 yrs ago
 contained complex organic
molecules & chains of crystals
similar to those created by
Earth bacteria
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Martian Meteorites
 also found fossils of nanobacteria.
 recently discovered on Earth
 they have DNA, but are they life?
 big news, but since then…
 structures seen could also be formed by chemical &
geological porcesses
 Earth bacteria have been found living in the meteorite
 CONTAMINATED!
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Possibly Life on Jovian Moons?
 beneath its icy surface, Europa
may have an ocean of liquid water.
 tidal heating keeps it warm
 possibly with volcanic vents on the
ocean floor
 conditions may be similar to how
Earth life arose
 life need not only be microbial
 Ganymede & Callisto may also have
subsurface oceans, but tidal
heating is weaker.
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Possible Life on Jovian Moons
 Titan has a thick atmosphere and
oceans of methane & ethane.
• water is frozen
• perhaps life can exist in liquids
other than water
 pockets of liquid water might
exist deep underground.
 and in 5Gy – it will be warmer…
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Life in other planetary systems…
 what are the requirements on a planet for it to
produce life as we know it?
 liquid water
 the planetary system must be old enough for life to
have evolved
 the star must survive long enough for life to evolve
 development of planetary crust – planet has to cool
 magnetic field?
 composition of planet?
 stability of life-sustaining conditions – e.g. no very
large variations in surface temperature over ? 500
million years
 large single Moon provides climate stability
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Which Stars make Good Suns?
 which stars are most likely to have planets harboring life?
 must be long lived and old enough that life could arise in a
few x 108 y
• this rules out massive O & B main sequence stars
 they must allow for stable planetary orbits
• this rules out binary and multiple star systems
 they must have relatively large habitable zones
• habitable zone is the region where large terrestrial planets could
have surface temperature that allow water to exist as a liquid
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Width and location of habitable zone depend
on the type of star
•in the solar system the habitable zone is from
inside Earth’s orbit to just beyond Mars’
•this will change as the sun evolves
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need both habitable zone and time…
•if life takes ≥ 109 yr to evolve => best bet is stars ≤ 3MSun
•but below 1MSun size of habitable zone shrinks rapidly
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Planets and binary systems?
a planet in a binary
star system must
have a stable,
uniform orbit to
maintain the right
conditions for life
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Have we detected habitable planets around other
stars?
 NO…current technology is insufficient, because an
earth-like planet
 is too small to be resolved or for us to notice its
gravitational pull on its parent star
 would be lost in the glare of its parent star
 launch of the Kepler mission (October 2008?) which will…
 measure the light curves of stars to look for transits of
Earth-sized planets
 measure planets’ orbits to determine if they are in the
star’s habitable zone
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Detecting habitable planets around other stars
 In the next decade, NASA plans to launch Terrestrial
Planet Finder.
 an interferometer in space
 will take spectra and make crude images of Earth-sized
extrasolar planets
 spectrum of a planet can tell us if it is habitable.
 look for absorption lines of ozone and water
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Earth-like Planets: Rare or Common?
 expectation (models, some
observations) is that Earth-like
planets are common.
 billions of stars in our Galaxy
have at least medium-size
habitable zones
 theory of planet formation
indicates terrestrial planets
form easily in habitable zones
 some scientists have proposed a
“rare Earth” hypothesis.
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Earth-like Planets: Rare or Common?
 Rare Earth Hypothesis: Life on Earth resulted from a
series of lucky coincidences…
• terrestrial planets may form only around stars with
high abundances of heavy elements
• the presence of Jupiter deflects comets and asteroids
from impacting Earth
• yet Jupiter did not migrate in towards the Sun
• Earth has plate tectonics which allows the CO2 cycle to
stabilize climate
• our Moon, result of a chance impact, keeps tilt of
Earth’s axis stable
 are these “coincidences” unique or fairly ordinary?
• we will not know the answer until were have more data
on other planets in the Galaxy
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location in the galaxy matters too
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# civilizations in the galaxy:
the Original Drake Equation
The Drake Equation estimates the number of civilizations
detectable at radio wavelengths (first expressed in
1961 by Cornell University astronomer Frank Drake.):
 Rs = the rate of forming stars
 fp = fraction of stars with planets
 np = number of planets in habitable zone
 fb = fraction where life developed
 fi = fraction where intelligent life developed
 fc = fraction that communicate
 Lc = lifetime of communicating civilization
N  Rs f p n p f b f i f C Lc
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a “picture” form of the Drake equation
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How Many Civilizations Exist in the Galaxy with
whom we could make contact?
A simpler version of the Drake Equation
NHP = number of habitable planets in the Galaxy
flife = fraction of habitable planets which actually contain
life
fciv = faction of life-bearing planets where a civilization
has at some time arisen
fnow = fraction of civilizations which exist now
Number of civilizations = NHP x flife x fciv x fnow
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if we are a good example: it takes a long time for
intelligent life to evolve on a planet
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on Earth life took a long time to evolve
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How Many Civilizations Exist in the Galaxy
with Whom We could make Contact?
 We can not calculate the actual number since the values of
the terms are unknown.
 The term we can best estimate is NHP
 including single stars whose mass < few M AND…
 assuming 1 habitable planet per star, NHP  100 billion
 unless the “rare Earth” ideas are true
 Life arose rapidly on Earth, but it is our only example.
 flife could be close to 1 or close to 0
 Life flourished on Earth for 4 billion yrs before
civilization arose.
 value of fciv depends on whether this was typical, fast, or slow
 We have been capable of interstellar communication for
50 years out of the 10 billion-year age of the Galaxy.
 fnow depends on whether civilizations survive longer than this or not
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Search for ExtraTerrestrial Intelligence
 IF we are typical of intelligent
species and…
 IF there are many intelligent
species out there…
• then some of them might also
be interested in making
contact!
 THEN: let’s listen for them!
That is the idea behind the
SETI program.
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Search for ExtraTerrestrial Intelligence
 Use radio telescopes to listen for
encoded radio signals.
• search strategies are used to
decide which stars to observe
• now they scan millions of
frequencies at once
 We sent a powerful signal once in
1974 to the globular cluster M13.
• now we just listen
 due to low chance of success and
large amount of time required,
SETI is now privately funded.
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(a) the 140 foot telescope at
Green Bank Observatory (West
Virginia)
Project Phoenix in the 1990’s
searched for extraterrestrial
signals
(b) a typical recording of an
extraterrestrial signal – the
Pioneer 10 spacecraft (from
beyond Pluto)
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message broadcast
toward direction of
globular cluster M13
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at what wavelength should we look and listen?
The “Water Hole”
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the number of civilizations/the
distance to the nearest (ly)
Probability of Life
Lifetime
1
10-2
10-4
10-6
10 years
102/104
1/-
10-2/-
10-4/-
102/104
1/-
10-2/-
103 years 104/2000
105 years
106/300
104/2000
102/104
1/-
107 years
108/70
106/300
104/2000
102/104
109 years
1010/15
108/70
106/300
104/2000
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Where are the Aliens?
 With our current technology it is plausible that…
• within a few centuries, we could colonize the nearby
stars
• in 10,000 years, our descendants could spread out to
100s of light years
• in a few million years, we could have outposts
throughout the Galaxy
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Where are the Aliens?
 Assuming, like us, most civilizations take 5 billion yrs to
arise.
• the Galaxy is 10 billion yrs old, 5 billion yrs older
than Earth
• IF there are other civilizations, the first could have
arisen as early as 5 billion yrs ago
• there should be many civilizations which are millions
or billions of years ahead of us
• they have had plenty of time to colonize the Galaxy
 So…where is everybody? Why haven’t they visited us?
• this is known as Fermi’s paradox
• named after physicist Enrico Fermi, who first asked
the question in 1950
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Possible Solutions to Fermi’s Paradox
 We are alone.
• civilizations are extremely rare and we are the first one to arise
• then we are unique, the first part of the Universe to attain selfawareness
 Civilizations are common, but no one has colonized the
Galaxy.
• perhaps interstellar travel is even harder or costlier than we
imagine
• perhaps most civilizations have no desire to travel or colonize
• perhaps most civilizations have destroyed themselves before they
could
• we will never explore the stars, because it is impossible or we will
destroy ourselves
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Possible Solutions to Fermi’s Paradox
 There is a Galactic civilization.
• it has deliberately concealed itself from us
• we are the Galaxy’s rookies, who may be on the verge of a great
adventure
 We may know which solution is correct within the near
future!!
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The Fine Structure Constant
α = 2πe2/hc = 137.03599976 ± 0.00000050
a “finely tuned universe”?
the anthropic principle
“weak”: the existence of life imposes a selection
effect on where and when we observe the
universe
”strong”: the presence of observers imposes
constraints on the physical constants of the
Universe
see:
http://physicsweb.org/articles/world/14/10/3
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And… some thoughts on
interstelllar travel
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Why is Interstellar Travel Difficult?
 The distance between the stars in HUGE!
 We will most likely be limited by the speed of light.
 Our current interstellar spacecraft, Pioneers 10 & 11
and Voyagers 1 & 2, will take 10,000 yrs to travel 1
light-year.
• our spacecraft need to go 10,000 times faster in
order to travel to the stars within human lifetimes
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Why is Interstellar Travel Difficult?
 This will require new types of engines and new energy
sources.
• accelerating the USS Enterprise to half the speed
of light would require 2,000 times the total annual
energy output of the entire world
 Constructing a starship would be expensive.
• would require political will and international
cooperation
 Theory of Relativity will complicate life for space
travelers.
• a 50 l.y. round trip to Vega might seem like 2 years
to the crew…
• while 50 years has passed on Earth!
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Starship Propulsion
 Chemical rockets are impractical for interstellar travel.
• going faster requires more fuel, which make the ship
more massive and harder to accelerate
 Nuclear powered ships produce more energy per kg of
fuel.
• two designs, employing fission & fusion, have been
studied
• the best they could achieve is 10% of the speed of
light
• travel to the nearby stars would take decades
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Starship Propulsion
 Matter-Antimatter engines would convert 100% of fuel
to energy.
• problem is where to get the fuel! antimatter does not
exist naturally
• producing large amounts of antimatter takes
tremendous amounts of energy
• storing antimatter is a big problem as well
 Ships that do not carry their own fuel:
• solar sails would harness the photon pressure from
sunlight
• interstellar ramjets would scoop up Hydrogen from
the ISM to fuel its nuclear fusion engine
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Starship Propulsion
nuclear (H-bomb) powered
solar sail
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interstellar ramjet
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