Life in the Universe
Are We Alone?

posted May 23, 2001, jeb

Outline

• Evolution in the Cosmos
  
• Life in the Solar System
  
• What is Life
  
• Is There Life on Earth - The ET Perspective
  
• Drake Equation
  
• Fermi Paradox - Where Are They?
  
• UFOs
  
• The Scientific Search
  
• We Discover Them - SETI
  
• They Discover US - CETI
  
• Carbon Chauvinism
  
• Search for Extrasolar Planets
  

Extraterrestrial Life

NO? YES? MAYBE?

We will now consider the question of Life in the Universe

Are we alone?
• Drake equation

The search for extraterrestrial life

• What are we looking for?

Attempts for us to discover them

• SETI

Attempts to help them discover us

• CETI

The UFO phenomenon

• Why do scientists generally find the evidence unconvincing?

Evolution in the Cosmos

Our understanding of life in the universe requires we understand how life evolved in the one place we know for sure it did - on the Earth

Life on Earth emerged as the end of an evolutionary sequence:
• Particulate phase
• Galactic phase
• Stellar phase
• Planetary phase
• Chemical phase
• Biological phase
• Cultural phase

Life appeared on Earth about 3.5 billion years ago

Was this a unique event in the Universe?

Life in the Solar System

Carbon based

Liquid water essential

Metereorites often contain organic molecules

Three large bodies in the solar system besides the Earth have possiblities:


• Titan, Saturn's largest moon
  • atmosphere of methane, ammonia and nitrogen
  • Cassini will drop a probe into the atmosphere of Titan
  
  
• Europa, Jupiter's moon
  • liquid water below the icy surface
  
  
• Mars
  • explored by Viking -> no life
  • meterorites from Mars reach Earth

What Is Life?

Living organisms
1. react to environment and heal
2. grow - produce energy from food
3. reproduce - pass along traits
4. evolve from generation to generation - adapt

These are guidelines
• Some nonliving entities might posses these properties
• Some living entities might not always display these properties

Case For Extraterrestrial Life

Assumption of mediocrity
1. Life on earth depends on a few basic molecules
2. elements making these molecules are common to all stars
3. if same laws of science apply throughout the universe - life must have originated elsewhere

Life started very early on earth and therefore, we assume, it is easy to begin

• earth cooled
• acquired water
• stage of volcanism
  • added chemicals to the sea
• then life
  3 1/2 billion years ago (About 1 billion years after formation of earth)

We expect other harbors of life have been around for many billions of years more

Case Against Extraterrestrial Life

Intelligent life on earth - product of series of extremely fortunate accidents

Scientific Assumptions

Earth was once viewed as "center of the universe" with special status

Now, science views earth as an insignificant speck in the vast universe

Origin Of Life - Can It Happen Spontaneously?

Life - as we know it - is based on complex molecules built as carbon chains


Can these molecules be created in the lab?
• 1953 - Harold Urey and Stanley Miller
  • Mixed H₂O (water), CH₄ (methane), NH₃ (ammonia), CO₂ (carbon dioxide), and applied sparks
  • They found amino acids were formed - the basic building blocks of life
• More recent experiments have produced amino acid, complex sugars, and nucleotide bases
  • DNA?
• Still no life
  • This step is a mystery
  • But research continues

Possible Alternate Origin Of Life On Earth

Not primordial soup

Initial organic material produced in interstellar space and came to Earth as interplanetary dust and with meteors

Last visit of Halley's Comet

large amounts of organic material detected

Did life arrive from Outer Space?



WE DON'T KNOW

Is There Life On Earth? - The ET Perspective

or how easy would it be to detect?

Mass of the earth = 6x10²⁷ grams
Mass of the atmosphere = 5x10²¹ grams
Biological material on surface of earth = few x 10¹⁷ grams

THEREFORE

Life = 0.01% of the atmosphere


(10^-10 of the Earth)

"Biological Rust"
• very small effect
  Ref: Shklovskii and Sagan - page 248

Drake Equation

1961 - A small group of astrophysicists (including Carl Sagan) gathered at the National Radio Astronomy Observatory, Green Bank, WV


• Frank Drake presented his equation
  
  
  • to estimate the number of technological civilizations that presently exist in the Milky Way
    
    
    

    N = R_*f_pn_ef_ef_if_cL

    
    (Note: This original form is a little different from the textbook)

Suppose we require conditions like the Earth, since this is the only example of life we know

Fermi Paradox - Where Are They?

Fermi Paradox

Enrico Fermi pondered extraterrestrial life


• He thought it would take about 10,000,000 years to colonize the Milky Way
• We haven't seen them - Why? - This is the Paradox of Fermi
  
  
  1. Perhaps civilizations are content to stay at home
  2. Perhaps it takes longer
  3. Perhaps it has happened but we are not aware of it

UFOs

Claims of sightings and journeys None strongly substantiated Occam's Razor Fundamental principle in science - "the simplest explanation that satisfies all the evidence is the best". Most sightings are explained by optical illusions Many sightings turn out to be fraud No sign that extraterrestrials want to contact authorities - like the President Occam's Razor leads to the conclusion that reported UFO's are not extraterrestrial life



Of course, we know some people take UFOs very seriously

The Scientific Search

1. We are looking for intelligent, advanced life since we assume primitive life is too difficult to detect
   
   
2. We assume that intelligent life wants to communicate
   
   
3. Two avenues for discovery
   
   
   1. We discover them
      SETI - search for extraterrestrial intelligence
      
      
   2. They discover us
      CETI - communication with extraterrestrial intelligence

   ---

   We Discover Them (SETI)

   SETI - search for extraterrestrial intelligence

   What should we look for?

   • advanced civilization is very likely to develop radio communication, as we have, so we should look for such signals

   Suppose they were trying to communicate - what frequency would they choose?

   • Popular guess among scientists
     - 21 cm spin-flip line of hydrogen
         (1420 MHz)
     because hydrogen is the simplest atom in the universe and the 21 cm wavelength is a simple emission of hydrogen
     The entire band of wavelengths around 21 cm is an interesting part of spectrum and is know as the water hole

   ---

   Searches at 21 cm (1420 MHz)

   1960 - Frank Drake
   1. tau Ceti G type 11 light years away
      • very much like the sun - No SIGNAL
   2. epsilon Eridani K type 11 light years away
      • BIG SIGNAL
      • turned out to be airplane
      • then nothing
   
   1973 - Zuckerman and Palmer
   • surveyed 600 nearest sun-like stars
     • no signal of life
   
   1980's-90's - people such as Horowitz at Harvard are using sophisticated computer controlled and analyzed scanners to investigate millions of potential sources
   • Still no sign of life

   ---

   They Discover Us (CETI)

   CETI - communication with extraterrestrial intelligence
   1. We have been beaming radio signals to outerspace now for about 50 years
      • Howdy Doody, I Love Lucy, etc.
      • (sign of intelligent life? - maybe)

      closest stars ( Centauri) are about 4 light years away and since radio signals travel at the speed of light, Centauri has been receiving these signals for nearly 50 years
      roughly 1,000 stars have now been exposed to this emission
      

   2. Maybe we need to signal them more directly
      Big Question - should we?
      No,
      they will be much more advanced and view us as primitive life
      they may choose to exploit us, enslave us, or EAT US
      Yes,
      if they are so advanced
      • they can teach us a lot
      • help solve problems
        • population explosion
        • energy shortage
        • world of wars
      besides their distance is probably so great that we need not worry about their coming here
   1974 - Arecibo - Puerto Rico
   message from people on earth beamed at globular cluster M13 (many stars) - distance 24,000 light years
   1679 bits of information (0000001010101000000000000101......
   only factors of 1679 are 23 and 73 (1679 = 23 x 73)
   
   Placed messages aboard space vehicles
   • Pioneer
   • Voyager
   
   We assume these messages will be interpreted correctly if received

Carbon Chauvinism

Usual assumption of SETI is that life is based on carbon chemistry, like on earth
• WHY?
  1. We know it works
     • example - earth
  2. Basis element must be plentiful
     • no exotic elements like Ytterbium or Gadolinium
  3. Chemistry must form long chains to pass information like with DNA
     only possibilities are Silicon and Carbon (ref: Rood and Trefil)

Why Not Silicon?

1. We don't have an existence proof - like for carbon
2. Life on earth is based on molecules containing millions of atoms (DNA)
   This is required to pass the enormity of information:
   • blue or brown eyes
   • male or female
   • short or tall

Silicon
• Room temperature
  30-40 atoms maximum
• At -200^oC, long chains form - but other problems like Nitrogen is a liquid and reactions slow down

Conclusion

• It is easy to imagine that life exists on other distant worlds
  
  
• Efforts have been underway on Earth for many years to detect extraterrestrial intelligence
  
  
• If we do someday "make contact," our view of the universe will be changed forever

Search for Extrasolar Planets

List of web sites on recent results

The SETI Institute