14. Life in the Universe

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 unive...
Author: Miles Ford
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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: • Growth:

ability to produce new individuals

increase size while maintaining structure

• Metabolism:

harvest energy to fuel activities

• Response:

react to changes in environment

• Evolution:

pass on favorable traits to offspring

Cells

Cell biology

Reproduction

Reproduction

Growth

Core Science Knowledge

Metabolism

Life

Response

Phototropism

Evolution

Evolution

Essential Characteristics of Life Life is process, not substance!

• Order:

structure composed of cells

• Reproduction: • Growth:

ability to produce new individuals

increase size while maintaining structure

• Metabolism:

harvest energy to fuel activities

• Response:

react to changes in environment

• Evolution:

pass on favorable traits to offspring

Essential Characteristics of Life Life is process, not substance!

• Order:

regular structure packed with information

• Reproduction:

ability to produce new individuals

• Growth or assembly in final form • Metabolism:

harvest energy to fuel activities

• Response:

essential for ‘interesting’ life

• Evolution:

pass on favorable traits to offspring

Alive, or not Alive? Viruses — reproduce and evolve — hijack host’s metabolism Bacteriophage

Digital Organisms — reproduce and evolve — compete for resources 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. yes B. no C. don’t know

Alive, or not Alive? Digital Organisms — reproduce and evolve — compete for resources Avida-ED

2. Are digital organisms alive? A. yes B. no C. 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. yes B. no C. 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.

Arbitrary Features of the Universe Relative strength of fundamental forces — gravity much weaker than others — no obvious reason for ratios Proton:electron mass ratio — exact value mp/me = 1836.152672... — separates atomic and nuclear scales Matter and Dark Energy Content — inflation explains ‘flat’ geometry — dark energy surprisingly small

e

p

Examples of Fine-Tuning Small changes in the basic parameters can have a big effect on the universe’s ability to support life: 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 Adams Biography of M.C. Escher

Is Fine-Tuning Necessary? 1. Universes very different from ours may support life. “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

2. Stars can exist in a wide variety of universes. 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.

Dark Energy Expected amount of dark energy (assuming it exists at 120 all) is 10 times observed value! 120 10

= 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

Why the tiny, but nonzero amount we observe?

Is Life Inevitable? In other words, is there some reason to think that the universe must be capable of supporting life?

• No — we’re just (very) lucky. • No — but nature tries all possible universes. • Yes — there’s some deep physical reason. • Yes — because the universe tends towards life. • Yes — because a creator designed our universe. • Yes — we’re living inside a virtual reality simulation.

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.

THE TOPOLOGY OF THE UNIVERSE

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., 40K (Potassium-40)

→ 40Ar + e+ (Argon-40)

(positron)

The rate of decay is fixed by the element’s half-life, the time for 50% to decay; for 40K, this time is 1.25 Gyr (1 Gyr = 1 billion years). 40Ar

Rocks contain no 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 Cyanobacteria (blue-green algae)

sediment trapped by microbes

3.5 Gyr-old fossil stromatolites

Shark Bay World Heritage Area

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

common ancestor

Comparing DNA shows how all the different organisms alive on earth today are related in a ‘family tree’. This tree hints at characteristics of a common ancestor.

genes

proteins Drawing Hands

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.

Pre-Biotic Chemistry 1. Take a Hydrogenrich atmosphere (like the early earth’s).

H2O

NH3

CH4

H2

Result: amino acids!

cool

heat

2. Cycle gas through simulated lightening (electric spark).

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.

1. Nucleic acids make RNA.

2. Proteins form pre-cells.

2. RNA replicates and evolves.

3. RNA made as by-product.

3. RNA uses proteins to help.

genes

Which came first, the protein or the gene?

proteins Drawing Hands

One Possible Scenario

Clay is a catalyst for formation of RNA and fatty membranes. RNA strands pre-cell membrane

“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). However, all known ecological systems require: 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? 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

Contains tiny rod-shaped objects resembling nanobacteria found on Earth. Biological origin not proved. ALH84001

Methane on Mars CH4 doesn’t last long in Mars’s atmosphere — must be released by an ongoing process. Evidence of geological or biological activity! Martian Methane

Methane Concentration 0

10

20 parts per billion

Mars methane media mess

30

Life on in Europa?

Underwater volcanic vents

Possibly twice as much liquid water as Earth’s oceans. 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

The Flake Equation

Habitable Planets A habitable planet is one with conditions suitable for life. Habitable planets don’t necessarily have life. Liquid surface water is probably necessary to support a real ecosystem.

Water on Mars

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. 2. Binary stars may be OK if they are much closer to each other than to any planets.

Sunset on Tatooine

How Far From the Star?

Too close is too hot, while too far is too cool; there’s a habitable zone where liquid water can exist. Low-mass stars are less luminous, so their habitable zones are smaller and narrower than the Sun’s.

Hot Jupiters 10

What Kinds of Systems?

1 0.1

Saturn

0.01

Giant planets at large distances are OK, as our solar system shows.

Jupiter

M (MJ)

‘Hot Jupiter’ systems are not hospitable because orbits of other planets are disturbed as giant planets migrate in.

• direct detection • doppler method • transit method 0.01

0.1

1

10

P (yr)

100

1000

10000

Wikipedia: Extrasolar planet

Giant planets within the habitable zone are probably not habitable, but may have habitable satellites.

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

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

109 to 3×1010 potentially habitable planets in MW! Plate tectonics and large moons may or may not be rare — and may or may not matter. . .

Transit method can find earth-sized planets with current technology.

Brightness

Detection

Kepler spacecraft is monitoring 105 stars for transits. Eventually, spectra of terrestrial planets orbiting other stars will allow detection of H2O and O2. Free O2 is evidence of life!

Time

How Many Civilizations in the Galaxy? Drake equation: Nciv = Nhp × flife × fciv × fnow Nhp = number of habitable planets (109 to 3×1010) flife = fraction of planets with life (very uncertain) fciv = fraction with life which develop civilizations (took us half Sun’s life-span; say ~0.5) fnow = fraction of civilizations still around today (depends on lifetime; 10−8 to 1) This is really just a way to organize our ignorance!

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. A rough estimate is lifetime of civilization fnow ≈ age of galaxy 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):

• TV broadcasts • Early-warning radar

} current range ~60 ly

Intentional signal (1974):

• 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. No plausible signals were found — although there were some interesting false alarms. Wow! signal

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.

SETI@home

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) 5 at 0.00005c, takes ~10 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

At 0.1c there are a number of interesting stars we can reach in ~1 century.

11.4 ly

11.4 ly 10.5 ly 11.9 ly

8.6 ly

4.3 ly

11.8 ly Wikipedia: Nearest stars

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