AST 2010 Descriptive Astronomy Chapter 26 Astrobiology ...motor1.physics.wayne.edu/~giovanni/ast2010/Chapter26.pdf · AST 2010: Chapter 26 Origin of Life • Life originated (very

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


Lecture 1: Astrobiology

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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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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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Evolution of life of Earth

Age measured by radioactive dating.

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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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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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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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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.


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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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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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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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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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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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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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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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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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


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.


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


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


• Endurance crater as seen by Opportunity.

• Strata suggests this area was once covered by water.

• Other evidence of abundant water long ago.


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…


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.


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.


“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).


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


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.


• 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


fP : Other planetary factors…

• Rotation

• affects temperature difference between day/night sides

• Axial tilt: stability to limit climate change

• large moon needed?

• Orbit

• ellipticity


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.


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


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).


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.


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.


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


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.


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


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.


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.


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


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.


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.

Documents

AST 2010 Descriptive Astronomy Chapter 26 Astrobiology ...motor1.physics.wayne.edu/~giovanni/ast2010/Chapter26.pdf · AST 2010: Chapter 26 Origin of Life • Life originated (very