Life Zones and Suitable Stars for E.T.
The life zone, or habitable zone, is the distance from the star where the temperature is
between the freezing point (0° C) and boiling point (100° C) of water. If you consider a
planet with the same reflectivity (clouds and surface material) as the Earth, reradiates the
solar energy it absorbed as efficiently as the Earth does, and rotates as quickly as the
Earth does, then the life zone for the Sun (a G2 main sequence star) is between
approximately 0.63 and 1.15 A.U. Calculations that include the effects of the greenhouse
effect and whether or not there is a runaway process and ultraviolet dissociation of water
like what happened on Venus shift the Sun's life zone outward so that the Earth is nearer
the inside edge of the life zone. Climate research is still at the beginning stages of
development, so the life zone boundaries are a bit uncertain.
The life zone of a hotter main sequence star will be farther out and wider because of the
hotter star's greater luminosity. Using the same line of reasoning, the life zone of a cooler
main sequence star will be closer to the star and narrower. You can use the inverse square
law of light brightness to determine the extent of the life zones for different luminosity
stars. The boundary distance is
star boundary = Sun boundary × Sqrt[(star luminosity)/(Sun luminosity].
For example, if the Sun's life zone boundaries are 0.9 and 1.5 A.U, the inner and outer
bounds of the life zone for a star like Vega (an A0-type main sequence star with (Vega
luminosity/Sun luminosity = 53) are 6.6 to 10.9 A.U., respectively. For a cool star like
Kapteyn's Star (a M0 main sequence star with Kapteyn's star luminosity/Sun luminosity =
0.004), the life zone stretches from only 0.056 to 0.095 A.U.
Suitable Stars
Despite the fact that hotter, more massive stars have wider life zones, astronomers are
focusing their search on main sequence stars with masses of 0.5 to 1.4 solar masses. Why
are these types of stars 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. You will
need to include main sequence lifetime and the distance and width of the star's life zone
in our considerations.
First consider the lifetime of a star. The star must last at least 3 billion years! Use lifetime
= (mass/luminosity) × 10 billion years = 1/M3 × 10 billion years if the star's mass is in
units of solar masses. The most massive star's (1.4 solar masses) lifetime = 3.6 billion
years (a 1.5-solar mass star with a lifetime = 3.0 billion years would just barely work
too).
The less massive stars have longer lifetimes but the life zones get narrower and closer to
the star as you consider less and less massive stars. At the outer boundary of the life zone
the temperature is 0° C for all of the stars and the inner boundary is at 100° C for all of
the stars. You can use the observed mass-luminosity relation L = M4 in the life zone
boundary relation given above to put everything in terms of just the mass. Substituting
M4 for the luminosity L, the 1.4-solar mass star's life zone is between 1.76 A.U. and 2.94
A.U. from the star (plenty wide enough). The 0.5-solar mass star's life zone is only 0.23
A.U. to 0.38 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 0.7-solar mass
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 0.5 solar
masses.
Any life forms will need to use some of the elements heavier than helium (e.g., carbon,
nitrogen, oxygen, phosphorus, sulfur, chromium, iron, and nickel) for biochemical
reactions. This means that the gas cloud which forms the star and its planets will have to
be enriched with these heavy elements from previous generations of stars. If the star has a
metal-rich spectrum, then any planets forming around it will be enriched as well. This
narrows the stars to the ones of Population I---in the disk of the Galaxy.
Life Characteristics
From the biology textbooks there is this list of agreed upon characteristics for life:
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 (the tendency of a system to
become more disorganized and less complex), living things use energy to
maintain homeostasis (i.e., maintain their 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 varied 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 their environment.
Items 2 and 3 are related. Life grows by creating more and more order. Since entropy is
decreased (the amount of structure and complexity is increased), life requires an input of
energy. 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
Now that you know what kinds of stars would be good to explore further and what
criteria should be used for distinguishing lifeforms from other physical processes, let us
hone in on the right kind of planet to support life. Unfortunately, our information about
life is limited to one planet, the Earth, so the Earth-bias is there. However, scientists do
know of the basics of what life needs and what sort of conditions would probably destroy
life. With these cautionary notes, let's move forward. The habitable planet should have:
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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 more concentrated the solution of water and
molecules is, the more likely the molecules will react with each other. If the
molecules were fixed in a solid, they would not be able to get close to each other
and react with each other. If the molecules were in a gaseous state, they would be
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too far apart from each other to react efficiently. Though the reactions could
conceivably take place, they would be rare!
The planet should also have enough gravity to keep an atmosphere. An
atmosphere would shield lifeforms on the surface from harmful radiation (charged
particles and high-energy photons) and moderate the changes in temperatures
between night and day to maintain a stable temperature regime.
A relatively large moon nearby may be needed to keep the planet's rotation axis
from tilting too much and too quickly. This prevents large differences in
temperatures over short timescales (life needs sufficient time to adapt to
temperature changes).
Drake Equation: How Many Of Them Are Out There?
The Drake Equation is a way to estimate the number of communicating advanced
civilizations (N) inhabiting the Galaxy. It is named after Frank Drake who first
summarized the things we need to know to answer the question, ``how many of them are
out there?'' It breaks this big unknown, complex question into several smaller (hopefully
manageable) parts. Once you know how to deal with each of the pieces, you can put them
together to come up with a decent guess.
N = R* × fp × nE × fl × fi × fc × L
R* = average star formation rate (number of stars formed each year). Roughly 200 billion
stars in the Galaxy / 10 billion years of Galaxy's lifetime = 20 stars/year.
fp = average fraction of stars with planets. Astronomers are focussing on single star
systems (so planets would have stable orbits), where the star is not too hot (hence, short
life) or too cold (hence, narrow life zone and tidal locking of rotation). Also, look at stars
that have signatures of ``metals'' (elements heavier than helium) in their spectra---stars in
the galactic disk and bulge. Leftover ``metal'' material from the gas/dust cloud that
formed the star may have formed Earth-like planets.
nE = average number of Earth-like planets per suitable star system. The planet has a solid
surface and liquid medium on top to get the chemical elements together for biochemical
reactions. The planet has strong enough gravity to hold onto an atmosphere.
fl = average fraction of Earth-like planets with life. Extrasolar life will probably be
carbon-based because carbon can bond in so many different ways and even with itself.
Therefore, carbon can make the large and complex molecules needed for any sort of
biological processes. Also, carbon is common in the galaxy. Many complex organic
molecules are naturally made in the depths of space and are found in molecular clouds
throughout the Galaxy. The rarer element silicon is often quoted as another possible base,
but there are problems with its chemical reactions. When silicon reacts with oxygen, it
forms a solid called silica. Carbon oxidizes to form a gas. Silicon has a much lesser
ability to form the complex molecules needed to store and release energy. See Raymond
Dessy's article at Scientific American's “Ask the Experts – Astronomy” web site for
further discussion of the limitations of silicon chemistry.
fi = average fraction of life-bearing planets evolving at least one intelligent species. Is
intelligence necessary for survival? Will life on a planet naturally develop toward more
complexity and intelligence? Those are questions that must be answered before
“reasonable” guesses can be put in for fi. Take note that on the Earth, there is only one
intelligent (self-aware) species among millions of other species. (Perhaps, whales,
dolphins, and some apes should be considered intelligent too, but even still, the number
of intelligent species is extremely small among the other inhabitants of our planet.)
Sharks have done very well for hundreds of millions of years and they are stupid enough
to eat tires! Bacteria have thrived on the Earth for billions of years. Being intelligent
enough to read an astronomy textbook is very nice but it is not essential for the mandates
of life.
fc = average fraction of intelligent-bearing planets capable of interstellar communication.
Will intelligent life want to communicate to beings of a different species? The
anthropologists, psychologists, philosophers, and theologians will have a lot of input on
this term in the Drake Equation.
L = average lifetime (in years) that a civilization remains technologically active. How
long will the civilization use radio communication?
Another version of the Drake Equation (used by Carl Sagan, for example) replaces R*
with N*---the number of stars in the Milky Way Galaxy and L with fL---the fraction of a
planetary lifetime graced by a technological civilization. Once you have found N, the
average distance d between each civilization can be found from Nd3 = volume of Galaxy
= 5.65 × 1012 light years3. Solve for the average distance between each civilization =
(volume of the Galaxy/N)1/3 light years.
The certainty we have of the values of the terms in the Drake Equation decreases
substantially as you go from R* to L. Astronomical observations will enable us to get a
handle on R*, fp, nE, and fl. Our knowledge of biology and biochemistry will enable us to
make some decent estimates for fl and some rough estimates for fi. Our studies in
anthropology, social sciences, economics, politics, philosophy, and religion will enable
us to make some rough guesses for fi, fc, and L.
Some astronomy authors are so bold as to publish their guesses for all of the terms in the
Drake equation even though estimates of nE and fl are only rough and values quoted for
the last three, fi, fc, L, are just wild guesses. I will not publish my values for the last few
terms because I do not want to bias your efforts in trying come up with a value for N. We
do know enough astronomy to make some good estimates for the first two terms. The
current star formation rate is about 2 to 3 stars/year, but in the past it was much larger so I
quote the average value of 20 stars/year. The fraction of stars that are single, of medium
temperature, and that would have any chance of life-filled planets orbiting them is about
1/50 = 2%. Proto-planetary disks have been detected around some stars and astronomers
are now just beginning to detect planets around solar-type stars. See the end of the Solar
System Fluff chapter for a discussion of finding extrasolar planets and web links to up-todate information about them.
“Hailing Frequencies Open, Captain”
The section title is a bit misleading---astronomers are only trying to eavesdrop on
conversations already going on. Astronomers are searching for messages carried via
electromagnetic radiation (light) because it is the speediest way to send a message. It
travels at about 300,000 kilometers/second or about 9.5 trillion kilometers per year
(remember that this is equal to one light year?). In particular, the radio band part of the
electromagnetic radiation spectrum is searched for messages because radio can get
through all of the intervening gas and dust easily. The lowest interference from
background natural sources is between frequencies of 1 to 20 gigahertz. Our atmosphere
narrows this range to 1 to 9 gigahertz. The optimum range is 1 to 2 gigahertz. This is also
where the 21-cm line of neutral atomic hydrogen and the slightly smaller wavelength
lines of the hydroxide molecule (OH) are found. Because the water molecule H2O is
made of one hydrogen atom + one hydroxide molecule, the optimum range to use for our
searches is called the water hole.
Complicating the search is the doppler effect. Beings on planets orbiting stars will have
their transmissions doppler shifted by ever-changing amounts because of their planet's
orbital motion (and the Earth's motion around the Sun). Also, their star is moving with
respect to our solar system as they orbit the Galaxy. The radio astronomers must
therefore search many different frequency intervals to be sure to pick up the one interval
the other civilization happens to be at that time. Current searches scan several billion
frequency intervals at once.
A message was sent on November 16, 1974 to the globular cluster M 13. Unfortunately,
since M 13 is about 25,000 light-years away, we will have to wait about 50,000 years for
a reply. Messages have been attached to the Pioneer and Voyager spacecraft, but they
will take thousands of years to reach the nearest stars. Our main mode of communication
is the inadvertant messages we have been transmitting for several decades now: some of
the signal in television and radio broadcasts leaks out to space and rushes outward at the
speed of light. It takes many years for the radio and television signals to reach the nearest
stars because of the great distances to even the nearest stars. So perhaps radio
astronomers on other planets are watching the original broadcasts of ``Gilligan's Island''
or ``Three's Company'' and are seriously reconsidering their decision to say hello
(message for television legal department: that is a facetious statement and is not to be
taken as a serious statement about the quality of your boss' product).