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This chapter covers the concept of a lifezone and the types of stars to focus on in the search for suitable planets. The basic definitions of life and the kind of planet where we think life would likely arise are covered next. At the end of the chapter the frequencies we use in the Search for Extra-terrestrial Intelligence (S.E.T.I.) is discussed. The vocabulary terms in the text are italicized.
The lifezone is the distance from the star where the temperature is between the freezing point (0° C) and boiling point (100° C) of water, 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. The lifezone 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 lifezone of a cooler main sequence star will be closer to the star and narrower. We can use the inverse square law of light brightness to determine the extent of the lifezones for different luminosity stars. The boundary distance is
where the boundaries of the Sun rsun = 0.7 and 1.5 A.U., and Lstar/Lsun is the luminosity of the star compared to the Sun. For example the inner and outer bounds of the lifezone for a star like Vega (an A0-type main sequence star with LVega/Lsun = 53) are 5.1 - 10.9 A.U, respectively. For a cool star like Kapteyn's Star (a M0 main sequence star with LKapt/Lsun = 0.004), the lifezone stretches from only 0.044 - 0.095 A.U.
Despite the fact that hotter, more massive stars have wider lifezones, astronomers are focusing their search on main sequence stars with masses of 0.5 - 1.4 Msun. 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. 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) × the Sun's lifetime = 1/M3 × (the Sun's lifetime) if the star's mass is in terms of solar masses. Multiply by the Sun's lifetime 1010 years to get the lifetime in years. The most massive star's (1.4 Msun) lifetime = 3.6 billion years (a 1.5 Msun with a lifetime = 3.0 billion years 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° C for all of the stars and the inner boundary is at 100° C for all of the stars. We can use the observed mass-luminosity relation L = M4 in the lifezone boundary relation given above to put everything in terms of just the mass. Substituting M4 for the luminosity L, we find the 1.4 Msun star's lifezone is from 1.37 A.U. to 2.94 A.U. from the star (plenty wide enough). The 0.5 Msun star's lifezone is only 0.18 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 Msun 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 Msun.
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.
From the biology textbook we can list these agreed upon characteristics:
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 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.
Now that we know what kinds of stars would be good to explore further and what criteria we should use 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, we 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:
The Drake Equation is a way to estimate the number of communicating advanced civilization (N) inhabiting our Galaxy. It breaks this big unknown, complex question into several smaller (hopefully manageable) parts. Once we know how to deal with each of the pieces, we can put them together to come up with a decent guess.
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 ly3. Solve for the average distance d = (Vol/N)1/3 light years.
The certainty we have of the values of the terms in the Drake Equation decrease substantially as we 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 - 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%. We have detected proto-planetary disks around some stars and are now just beginning to detect planets around solar-type stars (solar-type stars are discussed in the first section of this chapter).
The number of stars with detected planets and the details about them changes so rapidly that the best place to find up-to-date information on extra-solar planets is on the internet. Here are some WWW links:
The section title is a bit misleading---we're only trying to eavesdrop on conversations already going on. We use electromagnetic radiation (light) in our search for messages because it is the speediest way to send a message. It travels at about 300,000 km/sec or about 9.5 trillion km per year (remember that this is equal to one light year?). We use the radio band part of the electromagnetic radiation spectrum to search 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 - 20 Gigahertz. Our atmosphere narrows this range to 1 - 9 Gigahertz. The optimum range is 1 - 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''.
Commplicating 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.
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. 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).
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last update: 12 December 1997