PH206 topics

  Life in the universe

  • Definition of life
  • Assumption of mediocrity
  • Urey-Miller Experiment
  • Meteors, comets and life
  • Extremophiles
  • Mars
  • Europa
  • Extrasolar planets
    • Detection
  • Drake equation
    • Habitable zones
  • SETI
    • The water hole


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When you plant a seed in a flower pot, the system (dirt and seed) starts out in a very disordered state.

When you come back later to find the plant has grown to fill the pot, obviously life created order from a disordered system. The dirt has gone, and roots are there instead. The second law of thermodynamics states that the amount of disorder in a system cannot decrease. When you see something that looks like entropy (disorder) has decreased, it typically means that you have not considered the whole system. Somehow, the entropy in the universe as a whole has increased.


Even so, here, it is easy to tell that the plant is alive. But it is not so easy to define just what that means. Obviously, to us, a plant is alive but a rock is not. What exactly does it mean to be alive?

Definition of life: Living organisms

Even though, as astronomers, we often say that stars are born (when fusion starts) and live (on the main sequence) and die (via core-collapse supernova, for example) we are not implying that they are living beings. Stars do react to the environment. They can adjust pressure to stay in hydrostatic equilibrium. When a sunlike star begins fusion of helium, it happens catastrophically, blowing away a third of the star, but the star recovers and continues fusing helium in its core. They grow from by taking in matter from the fragment of the giant molecular cloud from which they are formed. Stars produce energy via fusion from the fuel in their cores. When stars die, the material from one generation goes into making the next generation of stars. This material contains more and more heavy elements from one generation to the next, so stars do evolve and change over time. It certainly does sound like they have fit the above criteria for life. But no one would say that stars are really alive.


The problem here is that life is hard to define. No human has ever created living organisms from completely inert materials. There is something fundamental about the process that we do not yet understand.

Ice crystals create self-assembled complex structures that fit many of the criteria for life, but are not alive. What is it about ice that determines that it is not alive?

Assumption of mediocrity

Life exists on Earth. To date, we have not found life anywhere else in the universe. Either we are alone in the universe, or there is life elsewhere and we have just not discovered it yet. In considering the possibility of finding extraterrestrial life, we can make some assumptions. The assumption of mediocrity is based on these simple statements. As far as we know, life is based on a few simple molecules that are commonly found throughout the universe. If this is true, and the basic laws of physics that we find here on Earth are the same everywhere in the universe, then whatever mechanism generated life on this planet could generate life elsewhere, and probably did.

The primordial sea that gave birth to life on Earth could be similar to environments on many planets in our galaxy and in the universe as a whole. If life exists elsewhere, it could exist in many places. Either our planet is unique or one of many. Since we don't yet understand what life is, and have not found it elsewhere, we really don't know which is true.

These stromatolite fossils found in Shark Bay in Australia are the oldest evidence of life on Earth. These mounds were created by microbes 3.4 billion years ago.