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Life, as defined by humans, is not extremely rare in the total universe.…

- - - - Some agreement on life. A living thing must meet some form of each of these requirements
        
        Usually homeostasis, equilibrium within their internal environment
        
        Response and learning ability in advanced forms
        
        The ability to take in energy from their environments and transform it into growth and reproduction
        
        Reproduction and growth
        
        It is very complex and highly organized
    - - Merriam-Webster definition of life
        
        Definition of life in the dictionary- “the quality that separates plants and animals from such things as water or rock : the quality that plants and animals lose when they die” (page 228)
      - Merriam-Webster definition of rare
        
        Definition of rare in the dictionary- “very uncommon” (page 319)
  - - - evolution
      - structure and complexity
      - replication
      - products (waste or biomineral)
      - energy consumption
      - movement (internal or external)
    - - genes- the set of instructions that tell the system how to sustain and reproduce
      - metabolism- the mechanism that carries these instructions out.
    - - Share a carbon-based chemistry
      - Depend on water
      - Leave behind fossils w/ carbon or sulfur isotopes that point to present or past metabolism
    - - Adenine
      - Guanine
      - Cytosine
      - Thymine
  - - - Second most complex
    - - Oldest and least complex
      - Extremophiles
        
        Organisms that live in extreme environments.
        
        Possibly the origins of life on Earth started with extremophiles in the sea vents. They don't need the sun as they survive through chemosynthesis, the production of energy by chemical reactions. If they can live here, then life could exist in the least likely places in space.
    - - most complex organisms
      - Individual cells have a nucleus with info for reproduction
- - - - The theory that comets and other space objects rained upon Earth within the first couple hundred million years of its existence, bringing ice, ammonia, hydrogen cyanide, mineral rich rocks, and many more raw materials for life.
      - It is possible that life appeared very quickly after its birth and as the bombardment of Earth went on, there was a series of mass "extinctions" in which most of life on Earth was wiped out, but what was left continued that life on and continued to expand.
        
        It is actually possible that life has actually been wiped out several times on Earth with the conditions still being right for life to rearise after each one. This is unlikely, but it would mean that life can be reestablished out of "nothing."
      - The formation of Earth involved the gathering of rocky materials which came during a period of accumulation and accretion
    - - By Stanley Miller and Harold Urey. It was an experiment intended to create a mini-environment similar to that of Earth's when life first originated on Earth.
      - Results
        
        They discovered that, with as close to past-Earth's conditions as they could possibly create, the water of their environment turned out to be rich in "organic gunk." This meant that it was rich in a compound of numerous complex molecules like sugar and some of the simplest amino acids. It also produced some modestly complex molecules called nucleotides.
        
        Showed that these ancient Earth conditions could produce the necessary starting materials and building blocks for life but there is still a huge gap between these building blocks and full on life. That gap is made up almost entirely of time.
      - Established that the existence of life in the universe requires:
        
        A source of energy
        
        A type of atom that allows complex structures to exist
        
        A liquid solvent in which molecules can float and interact
        
        Sufficient time for life to arise and evolve
    - - Of the archaea branch of life
      - Many view them as potentially the beginnings of life on Earth. They can survive in extreme environments like the sea vents on Earth. If they can survive here, they can survive in unlikely places outside of Earth.
      - More on the sea vents. Around these sea vents appeared to be entirely separate ecosystems totally disconnected from the rest of the world outside of its reliance on the oxygen provided by the undersea plants undergoing photosynthesis.
      - They have also been found in the rims of Yellowstone springs.
  - - - Water is among the most common molecules in the universe
    - - A source of energy
        
        For life to prosper on a planet, a long-lasting energy source like a star is necessary.
        
        Even with a star, a planet must be within the circumstellar habitable zone: a range of distances within which the radiative output of a parent star allows for liquid water.
      - A type of atom that allows complex structures to exist
      - A liquid solvent in which molecules can float and interact
      - Sufficient time for life to arise and to evolve
- - - - Several protoplanetary nebular disks in star-forming regions strengthen the idea that planetary systems are actually abundant.
    - - This supports the Big Bang Theory which states that all matter of the universe was at one point in one tiny point of existence. Then, due to some sort of impetus, there was a sort of explosion in which the matter all burst out and occupied space.
      - This would mean that there was a beginning to the universe and thus, we can more effectively analyze the change of life in the universe over time. We can determine that the conditions of the universe have changed over time, possibly affecting the new arising of life across the universe. Possibly, the conditions for life in the universe were "better" within the first couple hundred million years of the big bang, if it actually occurred.
  - - - Named after Frank Drake, the American astronomer who created it in the early 1960s.
      - Organizes our knowledge and ignorance by separating the number of places where intelligent life exists in our galaxy into a set of terms.
      - He brought together the necessary conditions for life
        
        The number of stars in the Milky Way that survive sufficiently long for intelligent life to evolve on those planets near them
        
        The average number of planets around those stars
        
        The fraction of those planets that have the necessary conditions for life
        
        The probability that life may actually come about on those few planets
        
        The chance that the life that does arise may evolve to produce an intelligent civilization. (Meaning a form of life that can communicate with us).
      - When we multiply these terms together, it should give us our best estimate of the number of planets in the Milky Way that have an intelligent civilization at some point.
      - To get the number of intelligent civilizations that exist at any given time, we multiply that number by the ratio of the average lifetime of an intelligent civilization and the total lifetime of the Milky Way.
      - This equation is obviously not the end all be all as even a tiny error in any one of the terms can result in a vastly different outcome. Also, as of right now we only have the first two terms, with the third seemingly coming soon.
        
        However, based on what we know, there are billions of potential sites for potential life, meaning the potential sites just for any form of life are extremely high.
    - - provides a quantitative estimate of the relative probability that complex, macro-organismic life forms could have arisen on other planets outside of Earth.
      - As of now, only 11 (about 1.7%) of the extrasolar planets known to date have a BCI above that of Europa. However, by extrapolation, that total could be over 100 million in just our galaxy.
      - Many planets have BCIs close to that of Earth, and even Gl 581c is rated above Earth in this regard.
  - - - The orbital radius around a star of a given spectral type that would provide a surface temperature that would keep water in liquid form
      - Recently, NASA’s Kepler satellite provided the first clear evidence of rocky planets. This gives us quite a few candidates for potential life that are inside the habitable zone
      - Once a rocky planet within the habitable zone has been identified, there are two ways to approach whether it may have life or not: the "inside-out" perspective and the "outside-in" perspective.
        
        The “inside-out” perspective is to study the detailed history of life on Earth over the last 4.6 billion years.
        
        The “outside-in” perspective is reliant on extra-solar cues from planetary systems in formation, where we attempt to understand the earliest evolutionary stages provided an environment where liquid water and reactive molecules were allowed to react to create chemical complexity.
        
        This requires a focus on the beginning chemical and physical conditions of “proto planets” or younger planets.
      - Exoplanets
        
        planets that circle suns in other solar systems
        
        The Kepler space telescope has greatly expanded the discovery of exoplanets by confirming over 1000 planets and listing another 4000 as candidates.
      - The habitable zone changes gradually with time due to stellar evolution, whereby a star's luminosity increases over time. Ex. Look at Mars. Seems to have once had life, however it is no longer in the habitable zone.
        
        The range of radial distances within which liquid water could be found throughout a star's entire life is the continuously habitable zone.
      - Since 1995, astronomers have detected at least 22 planets orbiting other stars
- - - - Provides the backbone of life
      - Silicone could function as carbon does, but silicon mostly bonds to oxygen while carbon will bond with itself and others with no clear primary bonder.