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Astronomy 110: SURVEY OF ASTRONOMY
14. Life in the Universe
1. The Universe
2. The Solar System
3. Other Stars
Is there life elsewhere in the universe? This question has fascinated people for centuries. Finding life on another planet would change our perspective on many scientific and philosophical issues. We may soon discover simple life-forms on Mars or Europa. But most people are hoping for something more than bacteria — we want somebody to talk to.
1. THE UNIVERSE
a. Can Life be Defined?
b. Is the Universe Fine-Tuned?
c. Is Life Inevitable?
Characteristics of Life on Earth
Properties of life as we find it on earth include:
• Order: structure composed of cells
• Reproduction: ability to produce new individuals
• Growth: increase size while maintaining structure
• Metabolism: harvest energy to fuel activities
• Response: react to changes in environment
• Evolution: pass on favorable traits to offspring
Cell biology
Cells
Reproduction
Reproduction
Core Science Knowledge
Growth
Phototropism
Response
Evolution
Evolution
Essential Characteristics of Life
Life is process, not substance!
• Order: structure composed of cells
• Reproduction: ability to produce new individuals
• Growth: increase size while maintaining structure
• Metabolism: harvest energy to fuel activities
• Response: react to changes in environment
• Evolution: pass on favorable traits to offspring
• Response: essential for ‘interesting’ life
• Order: regular structure packed with information
• Growth or assembly in final form
Essential Characteristics of Life
Life is process, not substance!
• Reproduction: ability to produce new individuals
• Metabolism: harvest energy to fuel activities
• Evolution: pass on favorable traits to offspring
Alive, or not Alive?
Viruses— reproduce and evolve— hijack host’s metabolism
Digital Organisms— reproduce and evolve— compete for resources
Bacteriophage
Avida-ED
Self-Replicating Machines— make identical copies— extract raw materials
Self-replicating machine
Alive, or not Alive?
Viruses— reproduce and evolve— hijack host’s metabolism
Bacteriophage
1. Are viruses alive?
A. yesB. noC. don’t know
Alive, or not Alive?
Digital Organisms— reproduce and evolve— compete for resources
Avida-ED
2. Are digital organisms alive?
A. yesB. noC. don’t know
Alive, or not Alive?
Self-Replicating Machines— make identical copies— extract raw materials
Self-replicating machine
3. Are self-replicating machines alive?
A. yesB. noC. don’t know
Is the Universe Fine-Tuned?
Most of the universe does not seem hospitable to life, but it does enable life to emerge:
• Supernovae make the chemical elements life needs.
• Galactic recycling allows these elements to build up.
• Stars provide dependable energy sources.
• Planets provide stable environments.
Proton:electron mass ratio— exact value mp/me = 1836.152672...— separates atomic and nuclear scales
Arbitrary Features of the Universe
Relative strength of fundamental forces
— no obvious reason for ratios— gravity much weaker than others
p
e
Matter and Dark Energy Content— inflation explains ‘flat’ geometry— dark energy surprisingly small
Small changes in the basic parameters can have a big effect on the universe’s ability to support life:
Examples of Fine-Tuning
1. A ~2% increase in the strong force would make the ‘diproton’ stable, permitting the reaction:
p + p → 2He
This would make hydrogen a rare trace element!
2. A small increase in dark energy would start runaway expansion before galaxies had time to form.
Such changes could ‘spoil’ the universe for life!
. . . imagine a puddle waking up one morning and thinking, ‘This is an interesting world I find myself in —an interesting hole I find myself in — fits me rather neatly, doesn’t it? In fact it fits me staggeringly well, must have been made to have me in it!’ This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, it’s still frantically hanging on to the notion that everything’s going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise.
Douglas AdamsBiography of M.C. Escher
Is Fine-Tuning Necessary?
1. Universes very different from ours may support life.
2. Stars can exist in a wide variety of universes.
“Anyone who insists that our form of life is the only one conceivable is making a claim based on no evidence and no theory.” — Victor Stenger
Long-lived stars could provide energy, while explosions of degenerate stars could produce elements for life.
3. The weak nuclear force may not even be necessary!Big-bang nuclear synthesis, star formation, long-lived stars, and supernovae are all still possible without weak interactions.
4. However, dark energy still seems to need fine-tuning.
10120 = 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,
000,000
Dark Energy
Expected amount of dark energy (assuming it exists at all) is 10120 times observed value!
Why the tiny, but nonzero amount we observe?
Is Life Inevitable?
• No — we’re just (very) lucky.
• Yes — there’s some deep physical reason.
• No — but nature tries all possible universes.
• Yes — because a creator designed our universe.
• Yes — because the universe tends towards life.
• Yes — we’re living inside a virtual reality simulation.
In other words, is there some reason to think that the universe must be capable of supporting life?
THE TOPOLOGY OF THE UNIVERSE
Eternal or Chaotic Inflation
Quantum events may trigger inflation in microscopic regions which then expand as separate universes.
These universes could ‘bud’ further universes, which continue the process infinitely.
Fundamental constants and parameters may take different values in different universes.
Our universe could just happen to be one of the (few) which can support life.
2. THE SOLAR SYSTEM
a. When Did Life Begin?
b. How Did Life Develop?
c. Life in the Solar System?
When Did Life Begin?
The oldest fossils are ~3.5 Gyr old.
Carbon isotopes suggest life was active ~3.85 Gyr ago.
Dating Rocks
Radioactive elements decay into stable ones; e.g.,
(Potassium-40) (Argon-40) (positron)
40K → 40Ar + e+
The rate of decay is fixedby the element’s half-life, the time for 50% to decay; for 40K, this time is 1.25 Gyr (1 Gyr = 1 billion years).
Rocks contain no 40Ar when they form; by measuring the ratio of 40Ar to 40K, the rock’s age can be found.
Sedimentary Rock Formation
1. Silt from rivers is deposited on ocean floors.
2. More layers, with different minerals, build up over time.
3. Tectonic uplift and erosion expose layers of rock.
Deepest layers are oldest!
Living Stromatolites
Shark Bay World Heritage Area
Cyanobacteria (blue-green algae)
sediment trapped by microbes
3.5 Gyr-old fossil stromatolites
Carbon Evidence
Two stable isotopes of carbon exist:
12C (6 p + 6 n): 98.9%13C (6 p + 7 n): 1.1%
Photosynthesis prefers 12C, so organic material (including fossils) has an even higher level of 12C.
The oldest rocks with higher 12C level are 3.85 Gyr old.
When Did Life Begin?
The oldest fossils are ~3.5 Gyr old.
Carbon isotopes suggest life was active ~3.85 Gyr ago.
Once conditions allowed, life arose very quickly!
How Did Life Develop?
Life began simply, and gradually — over very long spans of time — evolved to produce complex organisms.
This is evident from the fossil record, where layers with complex fossils are found above layers with simple ones.
All living things on earth have common features which imply they are all descended from a common ancestor.
How Does Evolution Work?
Evolution rests on three well-established facts:
1. An organism’s structure depends on the genetic material it inherits from its parent(s).
2. Organisms must compete for resources in order to reproduce.
3. Mutations and/or shuffling of genetic material produce variations among offspring.
Natural selection results because organisms with favorable traits can have more offspring.
The Genetic Code
All life use the same code — based on the DNA molecule — to store genetic information.
DNA is copied by chemical means, and copies are passed on to an organism’s descendants.
Genetic information in DNA is used to assemble proteins, the building blocks of cells.
The Tree of Life
Comparing DNA shows how all the different organisms alive on earth today are related in a ‘family tree’.
common ancestor
This tree hints at characteristics of a common ancestor.
Drawing Hands
genes
proteins
Early Life Forms
The common ancestor may have been like the bacteria we find near oceanic volcanic vents and hot springs, which have very simple metabolisms.
H2O NH3
CH4 H2
Pre-Biotic Chemistry
coo
l
heat
1. Take a Hydrogen-rich atmosphere (like the early earth’s).
2. Cycle gas through simulated lightening (electric spark).
Result: amino acids!
Chemistry to Biology?
The chemical building blocks can be made on the early earth; they can also arrive from space via comets, etc.
What was the next step, and how did it happen?
Metabolism First Replication First
1. Amino acids make proteins.
2. Proteins form pre-cells.
3. RNA made as by-product.
1. Nucleic acids make RNA.
2. RNA replicates and evolves.
3. RNA uses proteins to help.
Drawing Hands
genes
proteins
Which came first, the protein or the gene?
One Possible Scenario
Clay is a catalyst for formation of RNA and fatty membranes.
pre-cell membrane
RNA strands
“There's nothing crawling out of the test tubes yet.” — Jack Szostak, Harvard
Origin of Oxygen
Cyanobacteria (blue-green algae) began producing O2
between 3.5 and 2.5 Gyr ago.
O2 did not build up in the air at first; reactions with Fe in surface rocks used it up too fast.
Once free O2 became abundant, sunlight transformed some of it into O3, creating an ozone layer.
Free O2 made animals possible, while O3 eventually enabled life to colonize the land.
Timeline for Development of Life
cyanobacteria
Cyanobacteria begin generating O2.
Timeline for Development of Life
cyanobacteria
Cambrian period produces many animals — even fish!
Timeline for Development of Life
cyanobacteria
Mass extinctions close out many evolutionary periods.
Life in the Solar System?
Terrestrial life thrives under a wide range of conditions(oceanic vents, solid rock, acidic, alkaline, brine, etc).
1. Inorganic nutrients — to build cells.
2. Energy (light, heat, etc) — for biological activity.
3. Liquid water — medium for chemical reactions.
Where else are these available?
However, all known ecological systems require:
Pretty much anywhere liquid water exists!
Life on Mars?
Light Deposits Indicate Water Flowing on Mars
Mars had abundant surface water a few Gyr ago. It still has ice deposits, and possibly even flowing water.
Martian Meteor ALH84001
Formed on Mars during “wet” period ~4 Gyr ago.
Left Mars ~15 Myr ago.
Hit Earth 13,000 yr ago. Allan Hills 84001
ALH84001
Contains tiny rod-shaped objects resembling nano-bacteria found on Earth.
Biological origin not proved.
Methane on Mars
Methane Concentration
0 10 20 30
parts per billion
Martian Methane
Evidence of geological or biological activity!
CH4 doesn’t last long in Mars’s atmosphere — must be released by an ongoing process.
Mars methane media mess
Life on in Europa?
Possibly twice as much liquid water as Earth’s oceans.
Underwater volcanic vents
Underwater vents could be environments for life!
But . . . O2 produced on surface may poison interior.
3. OTHER STARS
a. Systems Containing Habitable Planets
b. Searching for Extraterrestrial Intelligence
c. Prospects for Interstellar Travel
Habitable Planets
A habitable planet is one with conditions suitable for life.
Water on Mars
Liquid surface water is probably necessary to support a real ecosystem.
Habitable planets don’t necessarily have life.
What Kinds of Stars?
1. Stars with masses M > 2M⊙ burn out in a Gyr or less; that’s probably not enough time for life to get going.
Sunset on Tatooine
2. Binary stars may be OK if they are much closer to each other than to any planets.
How Far From the Star?
Low-mass stars are less luminous, so their habitable zones are smaller and narrower than the Sun’s.
Too close is too hot, while too far is too cool; there’s a habitable zone where liquid water can exist.
1 10 100 10000.10.011
100.
10.
0110000
P (yr)
M (
MJ)
• direct detection
• doppler method
• transit method
Wikipedia: Extrasolar planet
What Kinds of Systems?Hot Jupiters
‘Hot Jupiter’ systems are not hospitable because orbits of other planets are disturbed as giant planets migrate in.
Giant planets at large distances are OK, as our solar system shows.
Giant planets within the habitable zone are probably not habitable, but may have habitable satellites.
Jupiter
Saturn
Other Considerations
1. The outer galactic disk (where metals are scarce) may have few habitable planets, while the inner galaxy may be dangerous for life.
2. Giant planets at large distances (eg, Jupiter) may be needed to deflect comets away from habitable planets.
3. Plate tectonics and a large moon may be necessary to regulate a habitable planet’s climate.
Conversely, life itself may help regulate climate!
How Many Habitable Planets in the Galaxy?
Between 5% and 50% of the ~1011 or more stars in our galaxy could have habitable planets.
— most stars are low-mass — about half are single
Plate tectonics and large moons may or may not be rare — and may or may not matter. . .
109 to 3×1010 potentially habitable planets in MW!
Systems like ours are hard to detect; perhaps 20% to 60% of these stars have terrestrial planets.
— hot jupiters not common — dust as by-product
Detection
Time
Brig
htne
ss
Transit method can find earth-sized planets with current technology.
Kepler spacecraft is monitoring 105 stars for transits.
Eventually, spectra of terres-trial planets orbiting other stars will allow detection of H2O and O2.
Free O2 is evidence of life!
How Many Civilizations in the Galaxy?
Drake equation: Nciv = Nhp × flife × fciv × fnow
Nhp = number of habitable planets
flife = fraction of planets with life
fciv = fraction with life which develop civilizations
fnow = fraction of civilizations still around today
This is really just a way to organize our ignorance!
(109 to 3×1010)
(very uncertain)
(took us half Sun’s life-span; say ~0.5)
(depends on lifetime; 10−8 to 1)
Brain Size and Intelligence
We have big brains for our bodies, but not the biggest brains on Earth.
We are higher above the line than other animals.
Other big-brained animals probably evolved before us, but didn’t build technological civilizations (no hands?).
Lifetimes of Civilizations
fnow, the fraction of civilizations still around today, may be the most uncertain term in the Drake equation.
lifetime of civilizationage of galaxy
fnow ≈
A rough estimate is
Our technological civilization has lasted ~100 yr, while the galaxy is ~1010 yr old; this gives fnow ≈ 10−8.
If civilizations last forever, fnow ≈ 1.
We need more data to reduce the uncertainty.
Signaling the Universe
We have sent accidental and intentional radio signals.
Accidental signals (since ~1950):
Intentional signal (1974):
• TV broadcasts
• Early-warning radar } current range ~60 ly
• 73×23 bits; designed to be easily decoded
No answer expected any time soon!
Arecibo message
Searching for Extraterrestrial Intelligence (SETI)
The first searches used ‘obvious’ radio wavelengths (eg, 21cm hydrogen line) and targeted nearby stars.
Wow! signal
No plausible signals were found — although there were some interesting false alarms.
More recent searches scan wider ranges of wavelength and survey large swaths of the sky. Analyzing the data to find possible signals takes lots of computing power.
So far, no convincing signals have been found. SETI appears to be a long-shot project; we can try, but should be prepared to fail.
Interstellar Travel
The main problem is the huge distances involved; we need very fast ships and very patient explorers.
— top speed to date: ~16 km/s (New Horizons)
at 0.00005c, takes ~105 year to reach αCen!
No point launching interstellar probes with present technology; better to wait until faster rockets exist.
Speeds of ~0.1c (10% light-speed) make robotic probes to nearby stars much more interesting. . .
Destinations
Wikipedia: Nearest stars
At 0.1c there are a number of interesting stars we can reach in ~1 century.
4.3 ly
8.6 ly
10.5 ly11.4 ly
11.9 ly
11.8 ly
11.4 ly
Starship Designs
1. ‘Starwisp’ light-sail (robot):— total mass 1 kg; carbon wire mesh 100 m across— microwave beam power; reach 0.1c in 2 weeks
2. ‘Project Daedalus’ fusion drive (robot):— total mass 54,000 mt; 2-stage ship 190 m long— 2H/3He fuel (50,000 mt); reach 0.12c in 4 years
3. ‘Project Orion’ bomb drive (space ark):— total mass 40,000,000 mt; ship 20 km diameter— 30,000,000 fusion bombs; reach 0.0033c in 100 year
Fermi’s Paradox
A technological civilization can send robots to nearby stars in a few centuries, and colonies in a few millennia.
Colonies can launch missions to more distant stars; self-replicating robots can do the same.
In a few million years, a single civilization could colonize or explore the entire Milky Way.
“Where are they?” — Fermi
“They call themselves Hungarians” — Szilard
