Lecture 26

How Rare is Earth? (Ward & Brownlee)

• long grocery list of ingredients necessary to make planet teeming with life -- material, time, chance events
• Rare Earth Equation (revision of Drake Equation)

N = N^* x fp x fpm x ne x ng x fi x fc x fl x fm x fj x fme

where N^* = stars in Milky Way, 
fp = fraction of stars with planets, 
fpm = fraction of metal-rich planets, 
ne = planets in star's habitable zone, 
ng = stars in Galactic habitable zone, 
fi = fraction of habitable planets where life arises, 
fc = fraction of planets with life where complex life (eukaryotic-type cells) arises, 
fl = percentage of a lifetime of planet that is marked by presence of complex life, 
fm = fraction of planets with large Moon, 
fj = fraction of planetary systems with Jupiter-sized planets, 
fme = fraction of planets with critically low mass-extinction events

• as any term in equation approaches zero, so does final result
• Some of the terms may be interdependent -- a Moon may be necessary for the development of complex life, or the presence of a Jupiter may make mass extinctions less common.  This form of the equation assumes no such connections exist!
• Remember that all our conclusions are based on one life-bearing planet; a Moon may turn out not to matter at all.
• M stars are most common, but fainter than Sun ---> to be warmed, planet must be too close ---> tidal lock

 

 



• only 5-6% of stars have detectable planets (only gas giants can be detected with current technology, but a Jupiter might be necessary for an Earth to develop intelligent life)
• Some of the known planetary systems cannot have an Earth; they have a gas giant too close to the star

• if take abundance of planets and location, lifetime of habitable zone into account, 1-0.001% of stars could have Earths (assuming that Galaxy is same everywhere)
• Astronomers still know relatively little about extrasolar planets -- the first were discovered less than ten years ago. Most extrasolar planets now known are found by using the Doppler effect -- the gravitational attraction of a planet makes the star wobble very slightly, which can be detected by obtaining a very detailed spectrum of the star.
• Other planetary detection techniques include careful measurement of the position of the star to detect wobbles, observation of planetary transits, and blocking out the light of the star to search for faint planetary companions. None of these techniques have been successful yet in detecting new planets, but they are promising and can potentially see objects as small as Earth using new telescopes currently in the planning stages. NASA's webpage for one such telescope (which it hopes to launch in 2011) is here.
   

   
   
   
   

• microbial life could arise fast on an Earth, and did on our Earth
• but what about rise of complex life? may need many additional factors, including plate tectonics, large Moon, critically low number of mass extinctions, long surviving oceans, other factors (see summary below)
• These factors are all speculative!
  • plate tectonics

  

  • may be dependent on planet's composition and position in solar system
  • Earth only planet that has plate tectonics now -- must have thin lithosphere so that continent-sized blocks (plates) can move on surface (Venus lithosphere is too thick now), hot liquid core (Mars's core is too cold)
  • rate of continental growth critical
  • majority of Earth's biodiversity found on continents today, though this was not always the case
  • 2/3's of all animal species live on land (mostly insects), majority of marine species live in shallow-water regions affected by plate tectonics
  • north-south coastline promote diversity (greater temperature, climate gradients)
  • smaller continents that have evolved from one land mass promote diversity (more barriers to dispersal)
  • growth of continents over time affects planet's overall albedo (its reflectivity to sunlight), occurrence of glaciation events (like Snowball Earth), oceanic circulation patterns, and amount of nutrients in sea
  • plate tectonics provides global thermostat by recycling chemicals necessary to keeping volume of CO₂ in atmosphere relatively uniform ---> single most important mechanism for allowing liquid water for more than 4 billion years
  • plate tectonics is dominant force behind changes in sea level, which are vital to formation of minerals like limestone that keep global CO₂ (and hence temperature) in check
  
   
   
  • if Moon-forming collisions are rare, Earth lucky to have Moon

   
   

  

  • Moon stabilizes tilt of Earth's spin axis (obliquity would vary from 0 to 85 degrees over 10 million years otherwise, just like other terrestrial planets) ---> stable seasonal changes)
  • Moon causes lunar tides ---> inland tidal pools may have concentrated first amino acids
  
   

   

  

  

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  • old, liquid water oceans
  
   

   

  

  • retained for more than 4 billion years
  • for last 2 billion years, average temperatures less than 50 degrees C, salinity and pH favorable to formation and maintenance of proteins ---> cradle of animal life
  • most original water consumed in oxidation process of iron and nickel oxides
  • perhaps residual water enhanced by comets
  
   

   

  

  • with evolution of continents, large shallow regions created, nutrient influx from continental regions increased, amount of plant material on shallow sea surfaces and beds skyrocketed, and oxygen production began in earnest
  • volume of water just right -- large enough to buffer global temperatures, small enough to form seas when continental uplift occurred
• only 1-in-1000 Earth-like planets may have habitable zone that evolves like Earth's
• intelligence only emerged after 3.5 billion years of evolution on Earth, required oxygen-rich atmosphere and atmospheric cooling (maximum temperature of about 45 degrees C), affected by chance events like mass extinctions and continental configurations produced by continental drift

 

 



• but, note that minimum time possible could be as low as 10 million years -- many unknowns!

Summary: Many Factors Make Earth Ideal for Intelligent Life as We Know It

• right distance from star
• habitat for complex life
• liquid water near surface
• far enough to avoid tidal lock

 
• right mass of star
• long enough lifetime
• not too much ultraviolet
• stable planetary orbits
• giant planets do not create orbital chaos
• right planetary mass
• retain atmosphere and ocean
• enough heat for plate tectonics
• solid/molten core
• Jupiter-like neighbor
• clear out comets and asteroids
• not too close, not too far

 

 

• plate tectonics
• CO₂-silicate thermostat
• build up land mass
• enhance biotic diversity
• enable magnetic field

 
• ocean
• not too much
• not too little
• large Moon
• right distance
• stabilizes tilt

 

 
 

• the right tilt
• seasons not too severe
• giant impacts
• few giant impacts
• no global sterilizing impacts after an initial period

 

 
 

• right amount of carbon
• enough for life
• not enough for Runaway Greenhouse Effect

 

 
 

• atmospheric properties
• maintenance of adequate temperature, composition, and pressure for plants and animals

 

 
 

• evolution of oxygen
• development of photosynthesis

 

 



• not too much or too little
• evolves at right time

 

 
 

• right kind of galaxy
• enough heavy elements
• not small, elliptical, or irregular

 

 

• right position in galaxy
• not in center, edge, or halo
• wild cards
• Snowball Earth
• Cambrian explosion
• inertial interchange event

But Is There a Lack of Imagination Here?

• as Ward & Brownlee point out themselves, Rare Earth hypothesis assumes that animal life will be somehow Earth-like in that it has some form of DNA (only molecule capable of replication and evolution on Earth)
• Ward & Brownlee titled their book "Rare Earth", not "Unique Earth" for a reason -- even they think it is likely that complex life exists somewhere else in the universe, and they're fairly extreme
• even on Earth, DNA-based life may be only type of life that formed or just sole survivor -- perhaps ammonia, rather than water, could be solvent for life
• is Earth-life every-life? are lessons from Earth not only guides, but rules?
• perhaps complex life is as widely distributed as bacterial life and as varied in its makeup
• Even on Earth there are extremophiles including some complex life based on a sulfur ecosystem, not on sunlight!
• 
• These large marine worms have a symbiotic relationship with bacteria at oceanic vents where hot acidic water is released into the ocean
• 
• A "black smoker" releases hot water laden with hydrogen sulfide into the ocean bottom
• This is complex multicellular life on our own planet that we know very little about, that lives without sunlight, under high pressure and with an ecosystem based not on photosynthesis but on chemosynthetic bacteria; how much wierder might complex alien life be?

To read some more about extraterrestrial life, go to the Hand-Outs and Reference Materials page.