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Extraterrestrials: inhabitants of other planets.

Exobiology is the (so far purely theoretical) study of extraterrestrial life in general. The possibility of intelligent, civilized life forms living elsewhere than the planet Earth was already seriously entertained by ancient and medieval thinkers, including Nicholas of Cusa. In the sixteenth century, the philosopher ►Giordano Bruno was the first to realize that the assumption of an infinite ►universe logically implies that there should also be an infinite number of such civilizations. However, the number of extraterrestrial civilizations in our immediate vicinity -- that is, within the Milky Way -- is finite. It can be calculated by means of an equation formulated by the astronomer Frank Drake:

N = R · f_h· f_p· n_e· nj · f_l· f_i· f_c· L (Drake equation)

The value N to be calculated stands for the number of technical civilizations inhabiting our galaxy. It is calculated by multiplying the following factors:

R = the average rate of star formation in our galaxy;

f_h = the fraction of yellow stars similar to our sun;

f_p = the fraction of those stars that have planets;

n_e = the average number of roughly Earth-sized planets that belong to planetary systems and orbit their respective suns at a distance capable of supporting biological life;

n_j = the fraction of planetary systems with planets the size of Jupiter moving on stable external orbits;

f_l = the fraction of Earth-like planets on which life actually does arise;

f_i = the fraction of planets inhabited by intelligent beings;

f_c = the fraction of species developing technical civilizations;

L = the average lifetime of a technical civilization.

Unfortunately, we do not know most of these factors. Thus, we can only furnish fairly speculative estimates of their respective values. This restricts the usefulness of the Drake equation, for depending on the values inserted for the factors humanity could turn out to be the only civilization in the vast universe or one among millions of them. The first three factors can be determined relatively reliably. The average rate of star formation in our galaxy is approximately thirty per year*. Approximately ten percent of all stars are sun-like and have a so-called "life zone" within which an orbiting planet can reach the right temperature for the creation of life. At least twenty percent of all stars possess a planetary system.

Life zones surrounding stars; in the center, the solar system (ESA/Medialab)

Factor n_j was not included in Drake's original equation but became necessary once the first Jupiter-sized planets were discovered in other solar systems in 1995. For reasons still unknown, the orbits of such planets tend to draw gradually in toward their suns, affecting the orbits of Earth-like planets closer to the system's center. The fraction of planetary systems in which this does not happen is estimated to be at least ten percent.

If we assume a value of ten percent for all other factors, the only remaining open variable is the lifetime L of a technical civilization. Pessimists will here assume some hundreds of years (after which every civilization destroys itself), and optimists 12 billion years (the age of our Galaxis). If we assume a compromise value of 5 billion years (about half the time since habitable planets can exists), we obtain the result that there are more than 30,000 technical civilizations in our galaxy, living at an average distance of about 365 ►light-years** from one another.

Planets With Possible Extraterrestrial Life

The seven closest planet systems with the highest probability*** of intelligent life are:

Kepler 22b - a planet in 600 light-years distance orbiting a sun-like star in 290 days within the habitable life zone.
Gliese 581 - a planet system only 20.5 light-years away from us, orbited by a planet with a diameter 1.5 times that of the Earth and possible occurrences of water.
Beta CVn - a star similar to our sun, located 26 light-years away in the constellation Canes Venatici.
HD 10307 - a star whose size, temperature, and iron content are almost equal to those of our sun. Its iron content testifies to the presence of heavy elements in the planetary system, which is important for the development of life.
HD 211 415 - a candidate with roughly half of the metal content and a slightly lower temperature than our sun.
18 Sco - a star in the constellation Scorpius that is virtually identical to our sun.
51 Pegasus - a solar system with a Jupiter-like external planet that may also contain Earth-like planets.

It is reasonable to assume that civilizations that are millions or even billions of years old would have developed pretty advanced technology by now. Then why haven't they visited us yet with their spacecraft or space probes? Perhaps our estimates were completely off. Or perhaps they haven't noticed us yet; after all, it's been no more than eighty years since we started sending radio waves into space. Or perhaps the extraterrestrials have already arrived but prefer to hide. Or the CIA has hidden them away. Each of these hypotheses has its followers; the supposition that they've already been visiting us (albeit secretively) for a while, which forms the basis of ►ufology, seems to have the most popular appeal.

Eavesdropping on the Extraterrestrials

Given the lack of direct contact, it may be worthwhile to try to eavesdrop on the radio transmissions of extraterrestrial civilizations. This is the goal of the so-called SETI project (Search for Extraterrestrial Intelligence). In 1960 Frank Drake began to examine the two stars Tau Ceti and Epsilon Eridani by means of a radio telescope 25 meters in diameter. However, he did not detect any interesting signals in the frequency bands he examined. In 1964, the Soviet Union launched its own search program. The difficulty in any search for alien radio transmissions lies in finding not only the right star system but also the right wavelength range. The more frequency channels are examined the greater is the effort of analyzing the received data. To conduct an exhaustive search of the universe we would have to process an enormous quantity of data. This is why to date only random samples have been examined, which makes success contingent upon sheer luck.

In 1979 the University of California at Berkeley launched the SETI project SERENDIP using a 100-channel spectrum analyzer and a sequence of radio telescopes with mirror diameters of 25 to 65 meters. In 1982 the SENTINEL project was started at Harvard University with a 131,000-channel analyzer and a 25-meter-diameter radio telescope. It was followed in 1985 by the project META, supported by director Steven Spielberg, with 8 million channels. In 1986 Berkeley launched SERENDIP II with 65,536 channels and a 90-meter-diameter radio telescope in West Virginia. Its successor project SERENDIP III with 4 million channels was conducted with the Arecibo Telescope, the largest radio telescope on Earth. In 1992, NASA received federal funding for a rough SETI examination of the entire sky and a targeted search around approximately 800 nearby stars. However, shortly after its launch, funding for the program was canceled by Congress; it was eventually taken on by the privately-funded ►SETI Institute in California. For this purpose the institute rented a 64-meter-diameter Parkes telescope in Australia.

These projects have yielded many false positives and some odd signals that were archived, but not, so far, any definite indication of alien radio transmissions. Recently, a new radio telescope was built primarily for the search for extraterrestrial intelligence, the so-called Allen Telescope Array in Northern California. It was funded by Microsoft co-founder Paul Allen, the SETI Institute, and the University of California, Berkeley. It consists of 350 relatively small and inexpensive 6-meter-diameter single telescopes and began operations in October 2007.

If you are interested in launching your own personal search for aliens, you should take advantage of the SETI@home project at the University of California, Berkeley. This project uses the processing power of private internet-connected computers to analyze the data of SERENDIP IV. You can download the SETI@home program, which performs background analysis of the data, from the ►Berkeley server whenever your computer has sufficient capacities available. A DSL broadband connection is recommended. Once you are enrolled as a volunteer data analyzer, a screensaver will indicate the progress of your work. Upon completion of the data package, the results are automatically returned to Berkeley. If your computer encounters a signal by an extraterrestrial intelligence then your name will receive a permanent place on the SETI@home website honors board.

The Arecibo Message

To alert extraterrestrials to our existence, space probes have for some time been furnished with a message for them (see ►Pioneer). However, we will probably have to wait a very long time for an answer. Radio signals may be more promising. To date, only one direct attempt has been made in this direction. On November 16, 1974, the 305-meter-diameter telescope at the Arecibo Observatory broadcast a 1679-bit radio message*** into space -- or more precisely, toward the globular star cluster Messier 13, which is 23,000 light-years away from us. The number 1679 is the product of the two ►primes 23 and 73, so that the message can be unambiguously interpreted as a pictogram consisting of 23 x 73 black-and-white pixels:

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From top to bottom, the pictogram represents the binary ►numbering system, the elements hydrogen, oxygen, nitrogen, carbon, and phosphor, of which our bodies primarily consist, our DNA, a human figure, and the Arecibo telescope.

We do not even need to employ our radio telescopes, however, to transmit messages to aliens out there in space. Even our regular TV programs should be quite interesting to the aliens. TV transmitters bundle their signals toward the horizon in order to reach as many receivers as possible. A good part of the transmission energy is thus beamed into space, traveling away from Earth at the speed of light on a spiral orbit aligned with the equator. Theoretically, these TV signals can be received by kilometer-large radio telescopes even at a distance of thousands of light-years -- though, of course, it will take thousands of years for the signals to reach such telescopes if they are there. This is why we will probably need to wait for quite a while longer until the extraterrestrial space fleet finally arrives. And the aliens might think twice about visiting us once they have decoded our TV programs ...

* The number of stars (400 billion) divided by the age of the galaxy (13 billion years). At present, the rate is somewhat lower -- about 1 per year -- but the equation uses the average rate during the average lifetime of a civilization.

** N = 30· 10%· 20%· 0,1· 0,1· 10%· 10%· 10%· 5 · 10⁹ = 30,000

With 30,000 technical civilizations and a radius r of the Milky Way of 50,000 light-years, the average distance E to the next civilization

The above estimate is based on the model of the Milky Way as an even, flat disk. In reality, the distance to the next civilization in the vicinity of our sun, at the outer margin of a spiral arm, is clearly much greater, while it is smaller in the center of the galaxy.

*** Publication by Margaret Turnbull, Carnegie Institution, Washington DC, February 2006

**** A radio telescope can also be used for transmission, if the receiver located in the focus of the antenna mirror is replaced by a transmitter.