| Is there life existing outside our solar system?
Additionally, one should keep in mind that we are only one planet around a very ordinary star. There are roughly 400 billion other stars in our Galaxy, and nearly 100 billion other galaxies. It would be extraordinary if we were the only thinking beings in all these enormous realms.
These factors include the rate of formation of stars like the Sun, the fraction of those with planets, the fraction of Earth-like planets, the fraction of such planets where life develops, the fraction of those where the life becomes intelligent, the fraction of intelligent species who can communicate in a way we would detect, and the lifetime of the communicating civilizations. Unfortunately, many of the factors are poorly known, so estimates of N range from one (we are alone in the Galaxy) to thousands or even millions. As you may imagine, there is a lot of debate about reasonable values for most of these factors. As we learn more about the likelihood of planets around other stars, we are able to better estimate one of these parameters. For the other parameters, the estimates vary widely. The Drake Equation
N = R* fp ne fl fi
fc L
Frank Drake's own current solution to the Drake Equation estimates 10,000 communicative civilizations in the Milky Way. Where would we start our search?
1) A comprehensive sky survey. To cover the entire sky, you'll have to use telescopes with a wide field of view, which means they won't be very sensitive to weak signals. 2) A targeted search. Using very sensitive telescopes, you can focus on a select number of stars which seem most likely to harbor intelligent life. But the universe is a big place. Do you want to start looking inside our Milky Way, and if so, where exactly? Or do you want to search in another galaxy, such as Andromeda?
But something strange is happening in the nucleus that could make it hard for life as we know it to evolve. The core is very dense and may contain a giant black hole�the remains of millions of sun-sized stars all collapsed together. As more stars, dust and gas are pulled into this huge black hole, intense bursts of x-rays and gamma rays erupt from the center of the galaxy. It would be very difficult for life as we know it to evolve in such a strange and hostile place. Perhaps you should concentrate your targeted search on a friendlier region of space!
In our neighborhood (that is, within 80 light-years), there are about 800 stars similar to the sun. We can see some of these stars in our nighttime sky�Alpha Centauri, Tau Ceti, Epsilon Eridani, 61 Cygni, and Epsilon Indi. These stars are our neighbors. They are good places to start our search.
The Andromeda Galaxy is the nearest big spiral galaxy to ours, but it's still 2.2 million light years away! Any signals coming from extraterrestrials in Andromeda would be extremely faint by the time they reached us. They would also be extremely old (2.2 million years old, to be exact!). There may be extraterrestrial civilizations in Andromeda. However, our telescopes are not sensitive enough to detect signals coming from so far away. It's probably best to focus our search on stars closer at hand.
How would we communicate? Why can't we just send a spacecraft out to look for other planets and life orbiting other stars? The stars are simply too far away. With today's technology, our best rockets travel at about 10 miles per second. Even to reach the nearest other star system, Alpha Centauri, at about 4.2 light-years� distance, would take such a rocket 60,000 years. There are about a thousand stars like the Sun within 100 light-years of us. To investigate them all with spacecraft would take millions of years and vast amounts of money. An extraterrestrial civilization may use technology to communicate at the fastest speed possible, the speed of light, just like we do. All of our broadcast signals�radio, television, microwave, radar, and light�travel at the speed of light. So they are the fastest and most effective way to send messages through space. How fast is that? About 300,000 kilometers per second (9,500 billion km per year), the ultimate speed limit in the universe. But the universe is a very big place. Light from Proxima Centauri, the nearest star to our sun, takes 4.2 years to reach us. In comparison, a probe would take 75,000 years to travel the same distance! These signals can be visible, like the light waves we see every day. They can also be nonvisible, like x-rays and radio waves. What kind of signals would an extraterrestrial civilization be most likely to use? If an extraterrestrial species also evolved with a similar sense of vision�sensitive to the same light waves as we are�they might send a powerful beam of light toward other star systems. But there are problems with this approach. Vast clouds of dust and gas in space absorb light waves, blocking our view of much of our galaxy. Also, powerful beams of light must be very narrow, like a laser. They must be aimed precisely at the viewer. Radio waves, however, can easily pass through space dust. Radio waves can also be very wide. One radio beam can cover a lot of sky. So visible light may not be the best medium for interstellar communication. Perhaps you should investigate radio waves.
Extraterrestrials might use radio waves instead of light waves, for several reasons. First, vast clouds of dust and gas in space absorb light waves, blocking our view of much of our galaxy. Also, powerful beams of light must be very narrow, like a laser. They must be aimed precisely at the viewer. But nonvisible waves, like radio waves, can easily pass through space dust. Radio waves can also be very wide. One radio beam can cover a lot of sky. What's more, visible light waves are only a small fraction of the entire spectrum of electromagnetic radiation. Our eyes can only see this narrow band of light. Using the proper instruments, however, we can detect signals in other parts of the spectrum, including radio waves, x-rays, gamma rays, ultraviolet and infrared light.
Astronomers use radio telescopes to observe radio waves coming from stars, nebulae, and other galactic phenomenon. Radio telescopes can see farther than optical telescopes. They can see through clouds of interstellar dust, like the Horsehead nebula. They can also see things hidden from telescopes that look only at visible light. For example, by observing radio waves, astronomers can learn about aspects of galaxies that visible light does not reveal. Radio waves are also good ways for civilizations like ours to communicate. We use radio waves for television and radio broadcasts, among other uses. Finding similar radio signals coming from the stars would be strong evidence of an intelligent, technological civilization. So they're a good thing to look for! The main feature distinguishing signals produced by a transmitter from those produced by natural processes is their spectral width, i.e. how much room on the radio dial do they take up? Any signal less than about 300 Hz wide must be, as far as we know, artificially produced. Such narrow-band signals are what scientists look for. Other tell-tale characteristics include a signal that is completely polarized or the existence of coded information on the signal. Unfortunately, searches are burdened with confusion caused by narrow-band signals from our own planet. Military radar and telecommunications satellites produce such signals. A second telescope is used to sort out this unwanted interference. The second telescope is located hundreds of miles away from the main instrument - this would cause an extraterrestrial signal to have a slightly different frequency at the two sites. This is because of the Earth�s rotation and the effect of Doppler shift. Looking for the expected slight shift in frequency at the two telescopes is a good way to judge which signals are local, and which are truly extraterrestrial. Has Earth detected any extra-terrestrial signals yet? Not conclusively. There have been interesting "candidate" signals found - for example the famous "Wow" signal found at the Ohio State Radio Observatory. The Ohio State Big Ear radio telescope began listening in 1974 and, in 1977, it made history by detecting an extremely strong signal that, after ruling out terrestrial sources and known artificial satellites, it suggested had originated in outer space. The "Wow" signal - named for a note scribbled on the recorder tape - remains the most intrigueing clue, but it has not been heard again.
Because data collected in these searches were often processed long after the observation, candidate signals could not be immediately checked to see if they were of extraterrestrial origin. Subsequent observations conducted days to months after the original observations have never detected any of the candidate signals again. In order to be sure that a signal is from another civilization -- in order to have it accepted as real by the scientific community -- it must be independently verified and shown to originate from a point beyond the solar system. Therefore, no confirmed, artificially-produced extraterrestrial signal has ever been found. However, all previous searches have been limited in one respect or another. These include limits on sensitivity, frequency coverage, types of signals the equipment could detect, and the number of stars or the directions in the sky observed. For example, while there are hundreds of billions of stars in our galaxy, less than a thousand have been scrutinized with high sensitivity. Many searches have found unexplained signals, but unless a signal can be found repeatedly and confirmed by other telescopes, it won�t meet the stringent requirements set by scientists for a true detection. More and more scientists feel that contact with other civilizations is no longer something beyond our dreams but a natural event in the history of mankind that will perhaps occur in the lifetime of many of us. In the long run, this may be one of science's most important and most profound contributions to mankind and to our civilization - it is hard to imagine a more exciting astronomical discovery or one that would have greater impact on human perceptions than the detection of extraterrestrial intelligence." |
This is a question
that astronomers first started to quantify in the early 1960s. Over the
last half-century, scientists have developed a theory of cosmic evolution
that predicts that life is a natural phenomenon likely to develop on planets
with suitable environmental conditions. Scientific evidence shows that
life arose on Earth relatively quickly, suggesting that life will occur
on similar planets orbiting sun-like stars. With the recent discoveries
of extrasolar planetary systems, and the suggestive evidence that life
may once have existed on Mars, this scenario appears even more likely.
In 1961, a radio
astronomer named Frank Drake developed an equation to stimulate discussion
of the search for extraterrestrial intelligence and provide a way of estimating
the number of intelligent civilizations existing in our galaxy that might
be broadcasting signals. This equation, which is now called the Drake equation,
states that the number of communicating civilizations in our galaxy likely
depends on a number of factors which must combine to yield a habitable
planet where life has the chance develop to a certain level of technological
know-how. 
The universe
is a big place - by definition, the universe is everything there is! Our
Milky Way Galaxy is more than one billion billion kilometers, or 120,000
light years, in diameter. And it's only an average-size galaxy in a universe
containing billions of galaxies! We can take a two-prong approach to the
task of searching the skies for signals from extraterrestrials:
The center,
or nucleus, of the Milky Way is very different than the space around the
Earth. It's much denser with stars, so you might think it would be a good
place to search�after all, there are a lot more stars there than anywhere
else.
The Earth is not
in the center of the universe, or even the center of the galaxy. We live
in one of the Milky Way's spiral arms, about halfway between the nucleus
and the edge.
