Wayne State University College of Liberal Arts & Sciences Department of Physics and Astronomy AST 2010 Descriptive Astronomy Chapter 26 Astrobiology: Search/Study of Life on other worlds AST 2010: Chapter 26 Professor Claude A Pruneau Physics and Astronomy Department Wayne State University 1
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Wayne State UniversityCollege of Liberal Arts & SciencesDepartment of Physics and Astronomy
AST 2010Descriptive AstronomyChapter 26Astrobiology: Search/Study of Life on other worlds
AST 2010: Chapter 26
Professor Claude A PruneauPhysics and Astronomy DepartmentWayne State University
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AST 2010Descriptive Astronomy
Chapter 26: Astrobiology: Life on other worlds
AST 2010: Chapter 26
Lecture 1: Astrobiology
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AST 2010: Chapter 26
Introduction
• The Universe evolution from the Big Bang through the formation of stars has created conditions (here on Earth) enabling life.
•Could life also exist elsewhere in the Universe?
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AST 2010: Chapter 26
Life On Earth
• Life has existed on Earth “nearly” since its formation.
• Fossil algae and bacteria are found in rocks 3.5 billion yeas old.
• Likely life could not emerge before that time
• Earth was molten
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AST 2010: Chapter 26
Evolution of life of Earth
Age measured by radioactive dating.
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AST 2010: Chapter 26
Some facts of life...
• Extremely simple life forms were first formed and existed for 3/4 of Earth’s history.
• 600 million years ago, more complex forms of life formed/evolved.• Invertebrates
• 500 million years ago• Shells and Crustaceans
• 250 millions year ago•Mammals & Dinosaurs
• Dinosaurs wiped out 65 million years ago (Cretaceous extinction)
•Hominids - 5.5 million years ago.
•Homo Sapiens - 500,000 years ago.
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AST 2010: Chapter 26
Life Diversity
• Large variety of complex forms of life from butterflies, to whales, to mushrooms, to trees...
• Yet, all forms of life on Earth have the same underlying structure, reproduction, and metabolism.
• Same kinds of atoms: Hydrogen, Oxygen, Carbon, Nitrogen
• Same chemical substances
•Materials that are abundant on Earth
• Products of nucleosynthesis in stars.
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AST 2010: Chapter 26
Chemical Basis of Life
• Living structures and organisms based on large molecules, and chains of molecules.
• Many of these chains of molecules based on 20 amino acids.
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AST 2010: Chapter 26
All living organisms use....
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•Amino acids.
• More complex molecules called proteins.
• Scales of reptiles made of keratin
• Cartilage of mammals made of protein collagen
• Some proteins supply energy to the cell: Adenosine Triphosphate (ATP).
• Chlorophyll (plants) built around magnesium.
• Hemoglobin built around iron.
• Deoxyribonucleic acid (DNA) determines the genetic code of living organisms.
AST 2010: Chapter 26
Important observations
• All life forms are very similar.
• Same structure based on proteins
• Same reproduction control based on DNA and RNA.
•Implies all life had a common origin.
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AST 2010: Chapter 26
Origin of Life
• Life originated (very likely) from chemical reactions amongst complex molecules present on the young Earth.
• Idea goes back to Charles Darwin.
• Strong support by the Miller/Urey experiment - 1953.
•Mixed water, hydrogen, methane, ammonia.
• Use electric sparks
• After a week, tested the substances to find organic molecules & 5 five amino acids.
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AST 2010: Chapter 26
Origin of Life
• U.S. Sydney Fox carried more complex and realistic experiments
• Took complex organic molecules
• Repeatedly heated, cooled then
• Created short strands of proteins called proteinoid.
• These spontaneously formed droplets reminiscent to cells.
• Organic matter can spontaneously form acquire a structure.
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AST 2010: Chapter 26
Origin of Life
• C. Ponnamperuma (SriLanka), and Carl Sagan (US) showed ATP can be generated spontaneously from compounds produced in the Miller/Urey experiment.
• Demonstration of an additional important step in the making of living organisms.
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AST 2010: Chapter 26
Why did it stop?
• Why has the process of spontaneous generation stopped?
• Presence of oxygen in the atmosphere...
• Today’s organisms consume the material needed to form life.
• For these reasons, scientists speculate life likely emerge from the bottom of oceans near sea floor vents, rich in minerals, energy
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AST 2010: Chapter 26
Emergence of Complexity
• RNA vs DNA,
• Role of mutations,
• First cells were “simple”: Prokaryotes
• cells without a nucleus.
• Prokaryotes likely evolved in eukaryote cells (with a nucleus).
• Cells also have mitochondria - small bodies within the cell where food is converted into energy used by the cell.
• Multicelled organisms - motricity.1 billion-year-old fossil of
eukaryote algae
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AST 2010: Chapter 26
Gaia Hypothesis
• 1974 - James Lovelock (British chemist) & Lynn Margulis (US microbiologist) suggested life creates a single larger entity with a planet.
• A symbiosis of life and planet called Gaia Hypothesis. (greek goddess of Earth)
• Life does not simply responds to its environment, it alters the planet’s atmosphere, its temperature, ...
• Photosynthesis carried by plants created the oxygen atmosphere we have today, producing O2, and removing CO2.
• Other environmental factors such as humidity, salinity, sea level,
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AST 2010: Chapter 26
Anthropic Principle
• 1961 Robert Dicke, Princeton Physicist, discovered some cosmological coincidences.
• Age of the Universe not too different from the lifetime of stars such as the Sun
• Argued it is a necessity.
• Emergence of life required existence of elements (e.g. oxygen, carbon, etc), planets, and stars.
• Elements were formed in stars...
• Life can only appear when the age of the Universe is within certain limits.
• 1974, English physicist, Brandon Carter proposed the anthropic principle.
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AST 2010: Chapter 26
Anthropic Principle
• What we can expect to observe must be restricted by the conditions for our presence as observers.
• “How truly marvelous it is that conditions on Earth are just right for life.”
• Of course, if they were not, there would be no life here to do the marveling.
• This suggests there may be many other planets “out there” that can or actually host life.
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AST 2010Descriptive Astronomy
Chapter 26: Astrobiology: Life on other worlds
AST 2010: Chapter 26
Lecture 1I: The Search forLife Elsewhere
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AST 2010: Chapter 26 20
Introduction
• People have wondered whether life exists elsewhere for thousands of years.
• Epicurus: “In all worlds there are living creatures and plants.”
• Plato/Aristotle: life exists only on Earth.
• This view prevailed in medieval Europe under the authority of the Church.
AST 2010: Chapter 26 21
Origin of Life: Hypotheses
• 1. Life arises independently everywhere it arises.
• 2. Panspermia: simple life forms drift across space…
• driven by radiation pressure?
• on surface of meteors or comets?
• still had to originate somewhere
fossilized stromatolites
AST 2010: Chapter 26 22
Life on Mars?
• Speculation since Earth understood as a planet.
• In 1820 K. Gauss suggested planting trees and wheat in shape of a huge right triangle…
• calculated it would be visible to Martians
• In 1877 G. Schiaparelli announced observations of canali (“channels”) on Mars.
•mistranslated into English as “canals”
• sparked Mars mania (e.g. Percival Lowell)
•mid 1960s: Mariner probe showed only real channel is the enormous Valles Marineris
AST 2010: Chapter 26 23
• Endurance crater as seen by Opportunity.
• Strata suggests this area was once covered by water.
• Other evidence of abundant water long ago.
AST 2010: Chapter 26 24
Martian fossils?
• Meteorite ALH 84001
• Discovered 1984 in Antarctica
• Martian origin
• By analysis of composition
• Possible microfossils
• Announced in 1996 (D. McKay)
• Terrestrial (by contamination)?
•Nonbiological (chemistry)?
• Evidence inconclusive…
AST 2010: Chapter 26 25
Searching for Life on Exoplanets
• Lovelock (late 1960s): oxygen in Earth’s atmosphere would disappear if life on Earth vanished.
• Highly reactive, combines with rock.
• Suggests: search for spectroscopic evidence of O2 in atmosphere of transiting planets.
AST 2010: Chapter 26 26
Exoplanetary O2
• At least one such planet has been discovered.
• HD 209548 b
•Oxygen and carbon detected in its atmosphere.
• Also H20 & methane.
• But it’s a “hot Jupiter”.
• atmosphere boiling away
• unlikely to signify life
• Still difficult to detect an Earth-mass exoplanet.
AST 2010: Chapter 26 27
“Many-worlders” vs. “Loners”
• M-Wers: Life is plentiful in the Galaxy!
• Planetary systems are common.
• Earthlike planets are (probably) common.
• Life arose on Earth almost as soon as the surface was cool enough.
• Loners: we are alone (or nearly so).
•Our transmissions will fill the Galaxy in a short time on the cosmic scale.
• Yet we detect nothing (Fermi paradox).
AST 2010: Chapter 26 28
The Drake Equation
• Devised by Frank Drake in 1960
• to estimate the number of civilizations in the Galaxy.
• NI = N* × fS × fP × fL × fI
•N* : number of stars in the Galaxy (~100 billion)
• fS : fraction of stars suitable for development of life
• fP : fraction of suitable stars with habitable planets
• fL : fraction of habitable planets where life arises
• fI : fraction of inhabited planets with intelligent life
•NI : number of planets with intelligent life
AST 2010: Chapter 26 29
fS : Stellar Criteria
• Only consider stars suitable for continuous habitability over billions of years.
• Implies: spectral types F, G, or K (10%)
• Luminous blue stars don’t live long enough.
• Planets around red stars would need to be very close (probably tidally locked).
• Pop I (metal rich to form planetary systems, 2%)
• Estimate: fS ~ 0.002.
AST 2010: Chapter 26 30
• Require planet to be in star’s habitable zone.
• where temperatures allow liquid water on surface
• “fuzzy” − depends on atmosphere and reflectivity
•moves outward as star ages on main sequence
• Must be massive enough to hold onto atmosphere
• Can’t exclude gas giants or their moons.
fP : Planetary Criteria
AST 2010: Chapter 26 31
fP : Other planetary factors…
• Rotation
• affects temperature difference between day/night sides
• Axial tilt: stability to limit climate change
• large moon needed?
• Orbit
• ellipticity
AST 2010: Chapter 26 32
fP : Planetary system environment
• Binary systems?
•May be OK if planet’s orbital radius is < 1/5 companion star’s closest approach.
• “Hot Jupiters” probably bad.
•Migration would disrupt orbit of a planet in the HZ.
• Gas giants well outside HZ: probably good.
•Maybe necessary: perturb water-bearing comets into inner solar system.
AST 2010: Chapter 26 33
fP : Observational record
• One planetary science site* lists 57 candidate exoplanets in or near their star’s HZ
• All but 16 are in very eccentric orbits.
• One strong candidate terrestrial exoplanet in HZ
• Gliese 581c
• ~15 out of ~300 known exoplanets.
Assume this is a good sample − no bias.Optimistic: fP ~ 0.05.
*http://www.planetarybiology.com
AST 2010: Chapter 26 34
fL: If the conditions are right…
• What’s the chance that life will arise?
• Our sample size is 1!!
•Not enough info to estimate with any confidence.
• Optimistic guess: fL = 1.
• “based” on case Earth
• Life appeared quickly (few 100 million years).
AST 2010: Chapter 26 35
fI : The question of Intelligence
• Again, our sample size is 1.
• Many factors of unknown probability.
• Climate changes
• Asteroid impacts
• cause extinctions but also effect natural selection
• Conservative: fI = 0.001.
AST 2010: Chapter 26 36
Plugging it all in…
• NI = N* × fS × fP × fL × fI
• = 1011 × 0.002 × 0.05 × 1 × 0.001
• = 10,000
• Our estimate: 10,000 planets in the Galaxy with intelligent life at some time.
AST 2010: Chapter 26 37
Focusing on the now…
• How many civilizations at a given time?
• Depends on how long civilizations survive!
• Should really consider NI × L/Tg
• L : expected lifetime of civilization
• Tg: age of the Galaxy (1010 years)
• L ~ 106 y gives L/Tg = 0.0001.
•NI = 10,000: expect only 1 other civilization out there!
• But if L is indefinite, fraction L/Tg could be ~1.
• i.e. the Galaxy steadily accumulates civilizations
AST 2010: Chapter 26 38
How far away might they be?
• Let d be average distance between civilizations.• Imagine each civilization
is surrounded by a sphere of radius d/2.• The Galactic disk is
covered by NI such spheres.
AST 2010: Chapter 26 39
Let R be the radius of the Galactic disk:
or
Take:R = 40,000 light yearsNI = 10,000 We get d ~ 800 ly!
It’s a big Galaxy!
NI × π (d / 2)2 = πR2
d = 2R / NI
AST 2010: Chapter 26 40
PRO/CON of the Drake Equation
• CON:
• The sine qua non condition of science is that it generates testable hypotheses.
• Drake equation generates no testable hypotheses.
• Therefore, the Drake equation is not science.
• PRO:
• Provides framework for discussion
• Stimulates careful thought
• Focus becomes how to proceed experimentally.
AST 2010: Chapter 26 41
SETI: Search for Extra-Terrestrial Intelligence
• Search the sky for radio transmissions from other civilizations.
• Started with Project Ozma by F. Drake (1960)
• Monitored radio transmissions from nearby star systems.
• Found nothing of interest.
• The search continues today.
• So far nothing.
AST 2010: Chapter 26 42
Which wavelengths to monitor?
• Very long wavelengths: “noise” from interstellar gas
• Very short wavelengths: atmosphere blocks signals
• Some searchers focus on “the waterhole”
• Range between 21-cm H line and 18-cm OH line
AST 2010: Chapter 26 43
Where to look?
• Catalog of Habitable Stellar Systems (HabCat, 2002)
• J. Tarter & M. Turnbull working under auspices of SETI Institute
•Motivated by Allen Telescope Array : 350 radio dishes (42 operational 10/2007)
• Started from Hipparcos Catalog of 120,000 “nearby” stars (within ~500 pc)
• Applied selection criteria based on astrophysical principles
• Arrived at core group of 17,000 “HabStars”
• Likeliest identified candidates for continuous habitability over billions of years.
AST 2010: Chapter 26 44
Summary
• Hypotheses of origin of life include panspermia.
• Evidence of ancient water on Mars but no strong evidence of life.
• It might be possible to detect by-products of life in the atmospheres of exoplanets.
• Estimating abundance of life elsewhere rests on informed speculation: the Drake equation.
• Fermi paradox suggests life may be rare.
• SETI is Search for Extraterrestrial Intelligence by listening for radio signals from other civilizations.