This chapter was copied with permission from Nick Strobel’s Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version. This chapter has been edited for content (some sections and review questions have been omitted).

Life Beyond the Earth

This chapter covers: life zones (habitable zones), types of stars to focus on in the search for suitable planets, characteristics of life, evolution by natural selection, working definitions of life, the kind of planet where we think life would likely arise, bio-markers in exoplanet spectra, and finally the frequencies we use in the Search for Extra-Terrestrial Intelligence (S.E.T.I.). Updates include expanded discussion on the characteristics of life, new material on working definitions of life, looking for habitable free floater exoplanets, updates on planets in multiple-star systems (including Alpha Centauri).

17.1 Introduction

This chapter covers the concept of a habitable zone 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 used in the Search for Extra-terrestrial Intelligence (S.E.T.I.) is discussed. The vocabulary terms in the text are in boldface.

17.2 Habitable Zones and Suitable Stars for E.T.

For reasons explained in the habitable planets and bio-markers sections below, our search for inhabited exoplanets is focusing on those that have water-based life existing on the surface of the exoplanet. The habitable zone, or life zone, is the distance from the star where the temperature on the surface 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 habitable 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 habitable zone outward so that the Earth is nearer the inside edge of the habitable zone. Climate research is still at the beginning stages of development, so the habitable zone boundaries are a bit uncertain. Note that the discussion in this section ignores the effect of internal heating that could create liquid water places at much greater distances from the star (e.g., tidal heating of a jovian planet’s moon).

The habitable 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 habitable 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 habitable zones for different luminosity stars. The boundary distance is

star boundary = Sun boundary × Sqrt[(star luminosity)/(Sun luminosity].

For example, if the Sun’s habitable zone boundaries are 0.9 and 1.5 A.U, the inner and outer bounds of the habitable 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 habitable zone stretches from only 0.056 to 0.095 A.U.

One of the first exoplanets discovered that orbits in its star’s habitable zone is “Gliese 581c”. Gliese 581c orbits in the habitable zone of an M3-type star, Gliese 581 about 20.4 light years from us. Gliese 581 has a luminosity = 0.013 solar luminosities (even though it is a cooler spectral type than Kapteyn’s star). That would put the habitable zone of Gliese 581 between 0.1 AU and 0.17 AU. What is even more intriguing is that Gliese 581c has a mass of just five times the Earth (though that is a minimum derived mass), so it should have a solid surface that liquid water could collect upon as well as enough gravity to hold onto an atmosphere. This planet and the another slightly more massive planet (at a minimum of 8 Earth masses) orbiting Gliese 581 will certainly be studied a lot over the coming years! A major problem with the planet’s habitability is its very close distance to the star as described in the next section. Since its discovery, many other exoplanets have been discovered in the habitable zones of their planetary systems, especially by the Kepler mission as described in exoplanet section elsewhere on this website.

17.2.1 Suitable Stars

Despite the fact that hotter, more massive stars have wider habitable 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 habitable zone in any consideration of suitable stars.

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 habitable zones get narrower and closer to the star as you consider less and less massive stars. At the outer boundary of the habitable 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 habitable 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 habitable 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 habitable 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). On such a planet the night side temperature could drop so much that the atmosphere froze out. 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. The “super-Earths” of Gliese 581 would be tidally-locked to their star (Gliese 581 has a mass of only 0.31 solar masses).

On the other hand, if the planet has a thick carbon-dioxide atmosphere, the atmosphere could circulate enough heat between the day and night sides to keep the surface temperatures uniform (like Venus that has a very slow rotation rate). Most small, cool M stars have frequent stellar flares with more energy than our Sun’s flares that could kill off any complex life. Perhaps a planet with thick enough atmosphere to keep the surface temperatures uniform could also provide enough of a shield from the flares. The very narrow habitable zone of the small, cool stars would mean a small chance of finding a nice planet in the habitable zone. On the other hand, the sheer number of M stars in the Galaxy (recall that the M stars make up the greatest proportion of stars) means that there could be many habitable worlds around M stars.

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 (called “metals” by astronomers as a catch-all term) 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. Now that the Kepler mission has built up enough planet detections, researchers have been able to separate out enough stars with small planets from those with larger jovian planets to have decent statistics from which to draw valid conclusions. They find that smaller planets can form around stars with metallicities as low as just a quarter that of the Sun while the jovian planets require more metal-enriched environments. This result means that small terrestrial-sized planets could be more common in the galaxy (and the rest of the universe) than previously thought. Most searches are focusing on the stars more like the Sun that are not too hot nor too cool—those with masses between 0.5 and 1.4 solar masses. Some searches are including the M stars but they will need to look at a large number of M stars to improve their chances of finding the ones with habitable planets.

Most stars in the Galaxy have at least one stellar companion—binary or multiple star systems. Stars like our Sun with no stellar companion are in the minority. It would probably be difficult for there to be stable, only slightly elliptical planet orbits in a binary or multiple star system. Complex life (multi-cellular) will need to have a stable temperature regime to form so the planet orbit cannot be too eccentric. Simple life like bacteria might be able to withstand large temperature changes on a planet with a significantly elliptical orbit but complex life is the much more interesting case. Suitable binary stars would be those systems where either the binary stars orbit very close to each other with the planet(s) orbiting both of them at a large distance (called a “circumbinary planet”) or the binary stars orbit very far from each other so the planet(s) could reside in stable orbits near each of the stars—the one star’s gravity acting on a planet would be much stronger than that of the other star. The first discovery of a binary star system with a planet is the first case (orbiting both of them—circumbinary planet): the Kepler-16 stars are a K-type star with 69% the mass of the Sun and a M-type star with 20% the mass of the Sun orbiting each other every 41 days and the planet, Kepler-16b, about the mass of Saturn, orbits them both every 229 days, well outside the habitable zone of the combined stars. The planet was discovered using the transit technique (select the link to find out about that planet detection technique). Since Kepler-16’s discovery in 2011, several other circumbinary planets have been discovered, so we now know that Kepler-16 is not an unusual or rare case. Kepler 47 is a circumbinary planet system announced in August 2012 that has at least two planets, one of which is in the habitable zone. In October 2012, a circumbinary planet in a quadruple star system was announced. The planet named PH1 is slightly larger than Neptune and has less than half a Jupiter mass. It orbits the two inner stars every 138.5 days and well outside PH1’s orbit is a binary star system that orbits the first binary (primary pair) at about 1000 AU. The habitable zone of the primary pair is outside the orbit of PH1. Might there be other planets in this system?

Also in October 2012 was the announcement of a planet in the Alpha Centauri trinary system, the closest star system to us. The planet with a minimum mass of just 1.13 times the Earth orbits Alpha Centauri B at only 6 million kilometers with a period of 3.24 days. Alpha Centauri B is only slightly cooler than the Sun, so the planet has a surface temperature high enough to melt most rocks. The slightly brighter star, Alpha Centauri A, is slightly hotter than the Sun. Alpha Cen A and B orbit each other in an elliptical orbit that brings them as close as 11.2 AU and as far as 35.6 AU, so even at closest approach, Alpha Cen A is hundreds of times farther away from the planet and Alpha Cen B. The third star of the system, Proxima Centauri is a small red dwarf M-class star that orbits very far from the central pair at a distance of about 15,000 AU. The planet Alpha Centauri Bb was discovered using the Doppler shift technique and it produces the smallest doppler star wobble detected so far, just 0.51 meter per second or about the speed of a baby crawling. Because this size of wobble lies at the limit of detection, there is the need to independently confirm the finding as required in the standard process of science but the discovery has certainly made a number of science fiction fans (including those who are also astronomers!) very excited since the Alpha Centauri system has been the subject of numerous stories. There will undoubtedly be even more vigorous work on seeing if there are planets in Alpha Cen B’s or Alpha Cen A’s habitable zones. Stay tuned!

Finally, what about all of the possible “free floater” planets—those that probably formed around a star but were later ejected through gravitational interactions with other planets in the system? There may be more free floaters than there are normal stars in the Galaxy according to a recent estimate. It is possible that the free floater planets could harbor life but such planets would be too hard to investigate because they are too dim and small. Such planets are found only when by chance they pass nearly in front of some distant star and they distort the light of the distant star in a microlensing effect. The technique allows us to find the mass of the free floater planets but not much else. Therefore, we will focus our attention on the planets orbiting stars because those planets can be repeatedly observed over an extended length of time and probed with various analytical techniques.

17.3 Life Characteristics

As part of our search for life beyond the Earth, we try to understand how life began on the Earth, i.e. the origins of life on Earth, so that we can figure out how life might arise on another planet or moon. There is also the question of what will be the future of life on the Earth and the universe. However, whether you are looking for now-existing life or for how life began or at how the environment will affect life in the far future, you need to know what life is. Unfortunately, we don’t have an universally, agreed-upon definition of life. A couple of approaches that complement each other try to answer what is alive vs. not alive in the origins of life research: the “top-down” and “bottom-up” approaches. The top-down approach looks at all sorts of living and fossil life forms to figure out the most primitive forms that are or were alive. Unfortunately, all the life forms we know of are already very sophisticated. There is a big gap between the life forms we know of (either current or fossilized) and the never living material and reactions. The bottom-up approach uses laboratory experiments to mimic the emergent chemical processes of environments of ancient Earth or of current conditions on another planet (e.g., Mars) or moon (e.g., Europa or Titan) in order to create a self-reproducing chemical system. But debate still rages on what self-reproducing system would be considered truly alive.

We are limited by a sample size of one—the Earth—in figuring out what life is. All of the life forms on Earth share many common processes and features and therefore, seem to have evolved from a common ancestor. These common processes and features include such things as: (a) all life on Earth uses the “left-handed” version of amino acids to build the proteins it needs for carrying out the processes of life (look up “chirality” in a search engine for more on this); (b) all living cells use adenosine triphosphate (ATP) to store and release energy; and (c) the genetic code is used by nearly all cells in reading the information stored in the deoxyribonucleic acid (DNA) to build the proteins. These commonalities among all living things on Earth presents a challenge to us in looking for Life without an Earth bias.

Although an universally, agreed-upon definition of life does not yet exist, there are some necessary characteristics of life (life as we know it). The following list of characteristics are necessary but not sufficient to define life. Many counter examples of non-living things can be given for the 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. Possible counter examples could be rock crystals, machines, and electronics. However, it is a nice characteristic because it is visible in a short amount of time and it can also be used with past life.
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. Although non-living things such as electrical or gas appliances use energy too, this characteristic is easy to observe in a short amount of time and the reactions could leave residues or changes in the environment that would tell us of past life.
3. Growth. Living things grow. The size and shape of an individual are determined by its genetic makeup and by the 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. Possible counter examples could be fire and crystals. Looking for growth in living things might be hard to do if the life form grows slowly or has stopped growing and this characteristic cannot be used on past life.
4. Response to Stimuli. Living things react to information that comes from outside or inside themselves. Counter examples include rivers, clouds forming, or thermostats. While it is possible to see responses on a short time scale, we would need to know what the particular response is ahead of time to look for it and this characteristic would not be helpful in looking for past life.
5. Reproduction. Individuals reproduce themselves. Life also reproduces itself at the sub-cellular and cellular levels. In some instances, genetic information is altered. These mutations and genetic recombinations give rise to variations in a species. Some possible counter examples include some types of robots or computer codes or definite living things like mules that are the sterile offspring of a mated female horse and male donkey. “In between” cases like viruses and prions cause debates among biologists about whether they are truly alive. Viruses can reproduce only by infesting other life forms. Prions are infectious proteins that make copies of themselves by causing existing, properly formed proteins to change into the prion form rather than by actually replicating themselves (examples include “mad cow disease” BSE, and CJD in humans). The act of reproduction in truly living things is not always convenient to observe at a given time and it certainly will not work for past life.
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 (heredity). Looking for heredity, though, requires a whole series of the organisms and perhaps a long time to see what trait is passed on vs. random chance, as well as, the inherited trait might not be readily visible.
7. Adaptation. Living things adapt to changes in their environment. 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. On the positive side, it is possible to use this characteristic when examining past life but on the negative side, it would take a long observation time to look for adaptation and evolution by natural selection.

17.3.2 Working Definition of Life

With the understanding that an universally, agreed-upon definition of life does not exist, we can at least come up with a working definition to guide current research. One working definition from Gerald Joyce that guides NASA’s research in astrobiology is “life is a self-contained chemical system capable of undergoing Darwinian evolution”. Another similar definition from Max Coleman at NASA/JPL states “Life is a self-organized system capable of processing energy sources to its advantage.” The focus on chemical systems and chemical energy rules out computer programs and robots or other electronic entities and it is more easily measurable. The first definition includes evolution because of the primacy of evolution in modern biology but it is very difficult (at best) to measure evolution in the time length of a typical space mission or grant. The second working definition focuses on the characteristics that can be easily measured quickly. Coleman notes that life usually processes energy at a different rate than its surrounding environment; that life must process energy more efficiently than its environment does or it will be out-competed by non-biological processes; and that self-organization separates life from its surroundings, isolating its chemical processing abilities from its environment.

Although we have always been aware that our knowledge of life is limited, the discovery of life surviving, even thriving, under conditions not usually studied in high school or undergraduate biology classes, made us confront our biases of what life should be like. Organisms that survive in extreme environments are called extremophiles. There are life forms that live in the “dry valleys” of Antarctica when the ice thaws for a few hours a day in the summer and in the sub-surface waters of Lake Vostok miles below the ice of Antarctica. There are life forms growing in boiling hot springs on the surface and miles beneath the ocean surface in pitch blackness near submarine volcanic vents. Bacteria have been found living in rock miles below the surface in deep mines where water has percolated into pores or cracks in the rock. In fact, the total mass of all the microbes living in the rocks miles underground (called endoliths) could be greater than the total mass of life forms on the surface!

A hot springs at Yellowstone National Park. Different colors around the edge of the pool come from different types of bacteria living at different temperatures and acidity. Select the image to view a larger version. Other hot spring pictures are available in the Yellowstone photo album.

— Life is abundant near deep sea volcanic vents far from any sunlight.

Very hot conditions (up to 120º C), very cold conditions, very salty conditions, very acidic (low pH) conditions, very basic (high pH) conditions, and very high pressures: life is possible under all of these conditions as long as there is some liquid water. Other forms of life might be able to use another liquid but liquid water does have some advantages over other liquids as discussed in the next section. Current searches for extra-terrestrial life are focussing on places where liquid water could exist but our experience with extremophiles has taught us that life can be more creative than we can imagine with our biases.

17.4 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:

• a stable temperature regime provided by an energy source external to the life forms such as the star the planet orbits or planetary heating from some sort of geological activity and
• a liquid milieu. Liquid water is best for biochemical reactions and could be very abundant but liquid methane and/or ethane, like what is found on Saturn’s moon Titan might work. Since liquid water dissolves other compounds better than liquid methane/ethane and biochemical sort of reactions work better in liquid water than liquid methane/ethane, liquid water will probably be a requirement for a habitable planet. Water is liquid at a wide temperature range. Bio-chemical reactions will not happen in solids and they would be very inefficient in a gas. Water is liquid at a higher temperature than methane, ethane, and ammonia so chemical reactions will happen more quickly in the liquid water than in the other liquids. Also, frozen water floats! The hydrogen bonds of water make water less dense when it freezes. The frozen water ice could form a protective layer insulating the liquid water below it. The other types of liquids sink when they freeze and could lead to a runaway freezing process where all of the liquid freezes. Finally, water in some form (mostly either gas or solid) is actually quite abundant in the Galaxy so we are not limiting ourselves too much with the water bias. The liquid mileau is needed 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. Carbon will probably be the base of life because its great versatility to form compounds with other elements and even with itself. Carbon is more likely to share its electrons with other atoms rather than donate its electrons to other atoms or steal electrons from other atoms. Carbon has the highest degree of “catenation” (ability to form chemical bonds to itself) of all the elements. There are far more types of organic compounds (molecules containing carbon and usually also hydrogen) known than all the other types of compounds combined. On a planet with carbon-based life and life using carbon’s closest competitor, silicon, as a base, the carbon-based chemical reactions would be far more efficient than the silicon-based ones, so the carbon-based life would quickly overrun any silicon-based life present on the planet. For more on silicon as a base for life, see the Scientific American “Ask the Experts” answer written by Raymond Dessy (link appears in a new window).
• 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 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. The atmosphere would also provide the surface pressure needed for the liquid (most likely liquid water) to exist on the surface.
• 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).
• Plate tectonics may be needed to: 1) regulate the surface temperature of the planet via its crucial role in the carbon cycle; 2) create a magnetic field to shield the planet from the deadly stellar winds; 3) create dry land on a water-covered world; and 4) promote a high level of biodiversity across the planet by creating new environments that organisms would have to adapt to.

On planets or moons without an atmosphere and/or that are far from their parent star, it may be possible to have life existing below the surface if the planet or moon have a planetary heating source. An example of this would be Jupiter’s moon Europa. It has a water ice crust and a liquid water ocean below and is kept warm despite its great distance from the Sun because of tidal heating from Jupiter’s large gravity.

17.4.1 Methane-based Life

Although the rest of this chapter focuses on water-based life, the existence of methane lakes and rivers on Titan in our solar system compels us to consider life that could use liquid methane as the solvent to mix the organic chemicals about in its biochemistry. Another reason to consider methane-based life is that there are likely more very cold places where liquid methane could exist in our galaxy (and others as well) than liquid water places. For example, methane-based life could exist on exoplanets much further out from the very abundant cool K and M stars than what water-based life would be able to withstand—the habitable zone for methane-based life would be further out than that for water-based life. Planets in a “methane habitable zone” of a cool K and M star would not have their rotations tidally locked to the star.

With regard to Titan, methane-based life would have a ready supply of food from the acetylene and ethane raining down to the surface as a result of the photochemistry of ultraviolet light in sunlight breaking apart the methane vapor in Titan’s atmosphere. Using the hydrogen also present in Titan’s atmosphere, methanogens (organisms producing methane) would combine hydrogen with acetylene and ethane (and other hydrocarbons) to produce methane and energy. Titan life would need to develop special enzymes to extract oxygen from the water-ice rocks but the other essential elements such as carbon, hydrogen, and nitrogen would be easy to come by in the environment of Titan’s surface. See Chris McKay’s talk in the Silicon Valley Astronomers Lecture Series for more on the possibilities of life on Titan.

A recent study of the reflectivity of the surfaces of the lakes on Titan suggests that frozen methane ice might be able to float if the conditions are just right: if the temperatures were in a narrow range just below the freezing point of methane (like in Titan’s winters) and if the ice were composed of at least 5% nitrogen gas that is quite abundant in Titan’s atmosphere. However, if the temperature drops by a few more degrees, the ice will sink. An atmosphere of different composition on a cold exoplanet might get the frozen methane to float with a different temperature range. One last thing to note about methane-based life on a cold world is that the metabolic life cycles of an organism could be measured in time intervals of tens of thousands of years instead of the hours or days we are used on Earth, making it even more difficult to detect the metabolic processes. Also, it is very likely that any methane-based life is going to be microbial only. A complex, multi-cellular intelligent organism is much more likely to use oxygen in its metabolism with water as its liquid medium of choice.

17.5 Bio-Markers

While it may be possible for life to exist on a planet or moon below its surface, we will not be able to detect its presence from a great distance away (e.g., if it is another star system beyond our solar system). In our fastest rocket-propelled spacecraft, it would take us over 70,000 years to travel to the next star system (Alpha Centauri). The type of inhabited planet we will be able to detect outside of our solar system is life that has changed the chemistry of the planet’s atmosphere, i.e., the life will have to be on the surface. By analyzing the spectrum of the planet’s atmosphere, we may be able to detect bio-markers—spectral signatures of certain compounds in certain proportions that could not be produced by non-biological processes.

Spectral lines from water would say that a planet has a vital ingredient for life. If oxygen, particularly ozone (a molecule of three oxygen atoms), is found in the atmosphere, then it would be very likely that life is indeed on the planet. Recall from the solar system chapter that molecular oxygen quickly disappears if it is not continually replenished by the photosynthesis process of plants and algae. However, it is conceivably possible for a few non-biological processes (e.g., the runaway greenhouse effect with the photodissociation of carbon dioxide and water) to create an atmosphere rich in molecular oxygen and molecular oxygen does not produce absorption lines in the preferred infrared band that would be used in the future Terrestrial Planet Finder mission. Ozone does. Ozone existing along with nitrous oxide and methane in particular ratios with carbon dioxide and water, all of which produce absorption lines in the infrared, would be very strong evidence for an inhabited world.

One recent test of this concept was when the Venus Express spacecraft pointed its spectrometer at Earth in August 2007 while the spacecraft was orbiting Venus 78 million kilometers from the Earth. The near-infrared spectra of the Earth is shown for two different observing sessions. Earth was just the size of a single pixel in its camera. The part of the Earth facing the Venus Express spacecraft is shown in the simulated image above the spectra.

Could life exist on a planet without oxygen? Yes. Photosynthesis might be able to use another element such as sulfur instead of oxygen. The planet’s life might use another liquid besides water. Maybe the planet’s life would use a different element besides carbon as its base (such as silicon). The first missions that will hunt for life beyond the Earth will focus on biochemical processes that we are more familiar with (carbon-based life using liquid water) because it makes sense to start with what we know (or think we know) and then branch out to finding more exotic life after we have had some practice with the “ordinary” life. Detecting methane-based life on a cold world like Titan would require a lander to scoop up the organics in the soil to see if there are increased amounts of oxygen in the organics because the organisms would be scavenging the oxygen from the water-ice rocks.

17.6 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?” The equation 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_* × f_p × n_E × f_l × f_i × f_c × 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.

f_p = average fraction of stars with planets. Astronomers are currently focusing on single star systems so planets would more likely have stable orbits. With our current technology it is also easier to find planets around single star systems. They are looking at stars where the star is not too hot (hence, short life) nor too cold (hence, narrow habitable zone and tidal locking of rotation). Also, we should 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. 

n_E = 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. 

f_l = 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 — Space”web site for further discussion of the limitations of silicon chemistry. 

f_i = 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 f_i. 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.Bacteria and other simple forms of life have been found in some very extreme conditions on the Earth. Simple forms of life can even survive long passages through space. However, we will not be talking with such simple forms of life. We are more interested in complex life—multi-cellular animal and plant life. Complex life is more fragile than simple life, so while new research seems to increase the f_l term, the f_i term might be smaller than was initially thought. If the rise of intelligence is accidental, then the f_i term will be nearly zero. If intelligence is an emergent property of any biological system, then intelligence would be an expected result of complex life, boosting the f_i term. An emergent property is a property that is seen only when you look at a system of things as a whole rather than focusing on the individual parts.

f_c = average fraction of intelligent-bearing planets capable of interstellar communication. The intelligent life we will be able to talk to will have to use some sort of symbolic language. 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? Will they be around long enough to send messages and get a reply? Even if we manage to take better care of the Earth and each other, our technology is changing so we may not use radio communication. It used to be that our television and music/talk broadcasts were “over the air” using radio and microwaves. Some of that radio/microwave energy leaked out into interstellar space (that may be why all of the extraterrestrials are staying away). Those broadcasts are now happening mostly via cable. Now the voice communication that used to be via cable is now happening mostly with radio/microwaves (land lines vs. cellular phones). There is also the fact (yes, I use “fact”) that individual species have changed and died out as their environment changed. Humans will be very different in a million years from now (if we survive that long). Because of the huge interstellar distances in the Galaxy, the L term is the most significant constraint on communicating with an extraterrestrial.

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 f_L—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_*, f_p, n_E, and f_l. Our knowledge of biology and biochemistry will enable us to make some decent estimates for f_l and some rough estimates for f_i. Our studies in anthropology, social sciences, economics, politics, philosophy, and religion will enable us to make some rough guesses for f_i, f_c, 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 n_E and f_l are only rough and values quoted for the last three, f_i, f_c, 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-to-date information about them.

A nice interactive to try out your values in the Drake Equation is The Drake Equation interactive from NOVA’s Origins series that was broadcast on PBS (selecting the link will bring it up in a new window either in front of or behind this window).

For a sample of the scientific debates over the values in the Drake Equation (and perhaps the need to add more terms!), see the Complex Life Elsewhere in the Universe? debate that includes the “Rare Earth” authors, Don Brownlee and Peter Ward (selecting the link will bring it up in another window). Ward and Brownlee got the astrobiology/SETI community to re-examine its assumptions about extra-terrestrial life when they laid out their case for why complex life (life beyond the microbial level) may be very rare in the universe in their book “Rare Earth.” Needless to say there is disagreement, but it is a healthy debate in the determination of what it takes to make a habitable planet that can support complex, intelligent (self-aware) life.

17.7 “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. Radio also does not require that much energy to produce it so it should be easier for extraterrestrials to make. 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. While radio emission from natural objects (i.e., not from intelligent life technology) is over a wide range of frequencies, radio signals from an extra-terrestrial intelligence are expected to be in a very narrow frequency range, just like the narrow radio frequencies used by human technology because that would be a more energy efficient way to communicate. 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, each just 1 Hertz wide.

Another key feature of any signal we intercept is that it must be repeatable—it must be confirmed by others as being extra-terrestrial before any announcement to the press is made. With all of the various sources of radio interference possible, S.E.T.I. scientists want to be sure they have not picked up someone’s microwave oven, garage door opener, or one of our satellites, etc. The famous “Wow!” signal of August 15, 1977 in a S.E.T.I. project at Ohio State University was not repeatable. It was a narrowband radio signal that was potentially from beyond Earth but it lasted for only 72 seconds and it has not been detected again.

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 network legal department: that is a facetious statement and is not to be taken as a serious statement about the quality of your boss’ product). We are currently in a major technology switch-over between television/radio broadcasts (over the air to cable) and telephone communication (land-line to cellular) that does affect how much noise and the type of noise we are making.

Some searches for extraterrestrial signals are looking for very brief (nanosecond) optical (visible light) flashes because visible light can travel great distances out of the galactic plane and a tightly-focused beam of light (like a laser) can made to be many times brighter than a star (for very brief pulses just a nanosecond long). If the beam of light is pointed our way, we could see it over the glare of the star the extraterrestrials orbit. Also, the higher frequencies of visible light than radio means that a larger amount of information could be transmitted in a given amount of time than with radio. Perhaps the extraterrestrials would use radio signals to make their presence known and use lasers to beam information to us once we have started looking their direction.

Review Questions

1. Which stars have large habitable zones and which ones have small habitable zones?
2. Why should the search for life be narrowed to stars with masses 0.5 to 1.4 solar masses?
3. What spectral types of stars are excluded from S.E.T.I.?
4. What is the range of temperatures for stars included in S.E.T.I.?
5. Why are binary or multiple star systems usually excluded from S.E.T.I. searches? Which type of binary system might be good ones to check out?
6. What are the characteristics of life?
7. What is the definition of life? How do you know if something is living?
8. Where can life exist?
9. How will we be able to detect if life exists on an exoplanet?
10. Where in the Galaxy should the search be concentrated (bulge, stellar halo, disk, dark matter halo) and why?
11. Where is extraterrestrial intelligent life expected to be found?
12. How do we guess how many other communicating civilizations we expect to find? What parts of that guess are fairly well-known and what parts are much more uncertain?
13. What do you find when you plug in values for the Drake Equation?
14. What frequency or wavelength bands are the focus of current S.E.T.I. searches and why?

References and Web Links

1. NASA Astrobiology Program. NASA’s Astrobiology Program addresses three fundamental questions: How does life begin and evolve? Is there life beyond Earth and, if so, how can we detect it? What is the future of life on Earth and in the universe?
2. Life in the Universe edited by John Billingham (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, multi-cellular 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.
3. 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
4. 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 life zones (“ecospheres”), possible types of intelligent life (very speculative!), and Project Daedalus and other starships.
5. The Berkeley SERENDIP homepage discusses U.C. Berkeley’s contribution to the SETI project (will display in another window).
6. The SETI Institute’s homepage. Will display in another window.
7. My list of information about the detection of other planets.

Is this page a copy of Strobel’s Astronomy Notes?