Some Requirements for Extra-terrestrial Life

This set of notes by Nick Strobel covers: life zones, types of stars to focus the search for suitable planets, basic definitions of life, the kind of planet where we think life would likely arise, and finally the frequencies we use in the Search for Extra-terrestrial Intelligence (S.E.T.I.).

Contents

• Life Zones and Suitable Stars for E.T.
• Life Characteristics
• Habitable Planets
• ``Hailing Frequencies Open, Captain''
• References and Web Links

Life Zones and Suitable Stars for E.T.

Index

I will define the lifezone as the distance from the star where the temperature is between C. The inner and outer bounds of the lifezone of the Sun (a G2 main sequence star) are 0.7 and 1.5 A.U., respectively. Let's determine the inner and outer bounds of the lifezone for a star like Vega (A0 main sequence star) and for a star like Kapteyn's star (M0 main sequence star). Some things we'll need to use: a) luminosity of a star = , where is the distance from the center of the star, is the temperature on the surface of the sphere of radius and are constants; b) and

At the outer boundary of the lifezone the temperature is 0 degrees C for both of the stars and the inner boundary is at 100 degrees C for both of the stars. So since the and cancel out. Now solve for the distance from the star () using the lifezone boundaries of the sun ( A.U.). For Vega we get: 5.1-10.9 A.U. and Kapteyn's star: 0.044-0.095 A.U..

Why are main sequence stars with masses more likely to have intelligent life evolve on planets around them? Let's assume that it takes 3 billion years for intelligence to evolve on a planet. We'll need to include main sequence lifetime and the distance and width of the star's lifezone in our considerations.

First consider the lifetime of a star. We'll want the star to last at least 3 billion years! Use lifetime = mass/luminosity so solar lifetimes = if the mass is in terms of solar masses. Multiply by years to get the lifetime in years. lifetime = 4.3 billion yr (a with a lifetime = 3.6 billion yr would just barely work too).

The lighter stars have longer lifetimes but the lifezones get narrower and closer to the star as you consider less and less massive stars. At the outer boundary of the lifezone the temperature is 0 degrees C for all of the stars and the inner boundary is at 100 degrees C for all of the stars. So since the and cancel out. As done above, solve for the distance from the star () using the lifezone boundaries of the sun ( A.U.). Use relation , where the luminosity and masses are relative to the solar values. The lifezone is from 1.26 A.U. to 2.70 A.U. from the star (plenty wide enough). The star's lifezone is only 0.21 A.U. to 0.46 A.U. from the star. Planets too close to the star will get their rotations tidally locked so one side of planet always faces the star (this is what has happened to the Moon's spin as it orbits the Earth, for example). This actually happens for stars but if the planet has a massive moon close by, then the tidal locking will happen between the planet and moon. This lowers the least massive star limit to around .

Life Characteristics

Index

From the biology textbook we can list these agreed upon characteristics:

1. Organization. All living things are organized and structured at the molecular, cellular, tissue, organ, system, and individual level. Organization also exists at levels beyond the individual, such as populations, communities, and ecosystems.

2. Maintenance/Metabolism. To overcome entropy, living things use energy to maintain homeostasis (maintain its sameness; a constant, structured internal environment). Metabolism is a collective term to describe the chemical and physical reactions that result in life.

3. Growth. Living things grow. The size and shape of an individual are determined by its genetic makeup and by the environment.

4. Response to Stimuli. Living things react to information that comes from outside or inside themselves.

5. Reproduction. Individuals reproduce themselves. Life also reproduces itself at the subcellular and cellular levels. In some instances, genetic information is altered. These mutations and genetic recombinations give rise to variations in a species.

6. Variation. Living things are vaired because of mutation and genetic recombinations. Variations may affect an individual's appearance or chemical makeup and many genetic variations are passed from one generation to the next.

7. Adaption. Living things adapt to changes in the their environment.

Items 2 and 3 are related. Life grows by creating more and more order. Since entropy is decreased, life requires energy input. Life gains local structure at the expense of seemingly chaotic surroundings on a large scale. Items 5, 6, and 7 are related. Life reproduces--complex structures reproduce themselves. Life changes itself in response to natural selection on the macroscopic level and to changes in DNA on the microscopic level.

Habitable Planets

Index

• a stable temperature regime and
• a liquid mileau to mix
• the essential building block elements together (Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, Sulfur, and transition metals like Iron, Chromium, and Nickel). Since the building block elements are only created in the stars, the best places to look for life is around stars formed from processed gas, ie., look at metal-rich stars.
• The planet should have a solid surface to concentrate the building block elements together in the liquid on top.
• The planet should also have enough gravity to keep an atmosphere.

``Hailing Frequencies Open, Captain''

Index

We did send a message on November 16, 1974 to the globular cluster M13. Unfortunately, since M13 is about 25,000 light-years away, we'll have to wait about 50,000 years for a reply. We have attached messages to the Pioneer and Voyager spacecraft, but they'll take thousands of years to reach the nearest stars.

References and Web Links

1. Life in the Universe edited by John Billinham (MIT Press: Cambridge, MA, 1982). Topics covered:
   • The Origin of Life---organic chemical evolution, role of sulfur and water.
   • Life-Supporting Environments---atmospheres, continents, oceans, climatic stability, stellar influences, planetary orbits (in single and multiple star systems), oxygen's role, detecting extrasolar planets.
   • Evolution of Complex Life in the Galaxy---protein synthesis, multicellular organisms, development of intelligence and technology, development of life elsewhere in the Galaxy, biochemical keys, development of land plants and role of gravity, how does rise of homo sapiens and our future fit the evolution of intelligent life theories.
   • Detectability of Technological Civilizations---finding suitable stars, manifestations of advanced civilizations, search strategies, eavesdropping and our radio leakage, plan for SETI.
2. Scientific American October 1994 issue. Entire issue devoted to extraterrestrial life. Topics covered:
   • The Evolution of the Universe
   • The Earth's Elements
   • The Evolution of the Earth
   • The Origin of Life on the Earth
   • The Evolution of Life on the Earth
   • The Search for Extraterrestrial Life
   • The Emergence of Intelligence
   • Future of Homo Sapiens---Merging of Nanocomputer circuitry With the Human Brain
   • Sustaining Life on the Earth
3. Paul Patton, The Three Suns of Centaurus in Astronomy Magazine January 1982, pp. 6--17. He talks about the stars themselves and also about stable planet orbits. He then discusses lifezones (``ecospheres''), possible types of intelligent life (very speculative!), and Project Daedalus and other starships.
4. The SETIQuest homepage talks about the current project to search for signals from extraterrestrial intelligent life.
5. The Berkeley SERENDIP homepage discusses U.C. Berkeley's contribution to the SETI project.

Index

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last updated 07 Dec 95