by Michio Kaku
Searches for evidence of intelligent signals near the watering hole, however, have been disappointing. In 1960 Frank Drake initiated Project Ozma (named after the Queen of Oz) to search for signals using the 25-meter radio telescope in Green Bank, West Virginia. No signals were ever found, either in Project Ozma or in other projects that, in fits and starts, tried to scan the night sky over the years.
In 1971 an ambitious proposal was made by NASA to fund SETI research. Dubbed Project Cyclops, the effort involved fifteen hundred radio telescopes at a cost of $10 billion. Not surprisingly, the research never went anywhere. Funding did become available for a much more modest proposal, to send a carefully coded message to alien life in outer space. In 1974 a coded message of 1,679 bits was transmitted via the giant Arecibo radio telescope in Puerto Rico toward the Globular Cluster M13, about 25,100 light-years away. In this short message, scientists created a 23 × 73 dimensional grid pattern that plotted the location of our solar system, containing an illustration of human beings and some chemical formulae. (Because of the large distances involved, the earliest date for a reply from outer space would be 52,174 years from now.)
Congress has not been impressed with the significance of these projects, even after a mysterious radio signal, called the “Wow” signal, was received in 1977. It consisted of a series of letters and numbers that seemed to be nonrandom and seemed to be signaling the existence of intelligence. (Some who have seen the Wow signal have not been convinced.)
In 1995, frustrated by the lack of funding from the federal government, astronomers turned to private sources to start the nonprofit SETI Institute in Mountain View, California, to centralize SETI research and initiate Project Phoenix to study one thousand nearby sunlike stars in the 1,200-to 3,000-megahertz range. Dr. Jill Tarter (the model for the scientist played by Jodie Foster in the movie Contact) was named director. (The equipment used in the project was so sensitive that it could pick up the emissions from an airport radar system 200 light-years away.)
Since 1995 the SETI Institute has scanned more than one thousand stars at a cost of $5 million per year. But there have been no tangible results. Nevertheless, Seth Shostak, senior astronomer at SETI, optimistically believes that the 350-antenna Allen Telescope Array now being built 250 miles northeast of San Francisco “will trip across a signal by the year 2025.”
A more novel approach is the SETI@home project, initiated by astronomers at the University of California at Berkeley in 1999. They hit upon the idea of enlisting millions of PC owners whose computers sit idle most of the time. Those who participate download a software package that will help to decode some of the radio signals received by a radio telescope while the participant’s screen saver is activated, so there is no inconvenience to the PC user. So far the project has signed up 5 million users in more than two hundred countries, consuming over a billion dollars of electricity, all at little cost. It is the most ambitious collective computer project ever undertaken in history and could serve as a model for other projects that need vast computer resources to carry out computations. So far no signal from an intelligent source has been found by SETI@home.
After decades of hard work, the glaring lack of any progress in SETI research has forced its proponents to ask hard questions. One obvious defect might be the exclusive use of radio signals at certain frequency bands. Some have suggested that alien life might use laser signals instead of radio signals. Lasers have several advantages over radio, because a laser’s short wavelength means that you can pack more signals into one wave than you can with radio. But because laser light is highly directional and also contains just one frequency, it is exceptionally hard to tune into precisely the right laser frequency.
Another obvious defect might be SETI researchers’ reliance on certain radio frequency bands. If there is alien life, it may use compression techniques or might disperse messages via smaller packages, strategies that are used on the modern Internet today. Listening in on compressed messages that have been spread over many frequencies, we might hear only random noise.
But given all the formidable problems facing SETI, it is reasonable to assume that sometime in this century we should be able to detect some signal from an extraterrestrial civilization, assuming that such civilizations exist. And should that happen, it would represent a milestone in the history of the human race.
WHERE ARE THEY?
The fact that the SETI project so far has found no indication of signals from intelligent life in the universe has forced scientists to take a cold, hard look at the assumptions behind Frank Drake’s equations for intelligent life on other planets. Recently astronomical discoveries have led us to believe that the chance of finding intelligent life are much different than originally computed by Drake in the 1960s. The chance that intelligent life exists in the universe is both more optimistic and more pessimistic than originally believed.
First, new discoveries have led us to believe that life can flourish in ways not considered by Drake’s equations. Before, scientists believed that liquid water can exist only in the “Goldilocks zone” surrounding the sun. (The distance from Earth to the sun is “just right.” Not too close to the sun, because the oceans would boil, and not too far away, because the oceans would freeze, but “just right” to make life possible.)
So it came as a shock when astronomers found evidence that liquid water may exist beneath the ice cover on Europa, a frozen moon of Jupiter. Europa is well outside the Goldilocks zone, so it would appear not to fit the conditions of Drake’s equation. Yet tidal forces might be sufficient to melt the ice cover of Europa and produce a permanent liquid ocean. As Europa spins around Jupiter, the planet’s huge gravitational field squeezes the moon like a rubber ball, creating friction deep within its core, which in turn could cause the ice cover to melt. Since there are over one hundred moons in our solar system alone, this means that there could be an abundance of life-supporting moons in our solar system outside the Goldilocks zone. (And the 250 or so giant extrasolar planets so far discovered in space might also have frozen moons that can support life.)
Furthermore, scientists believe the universe could be peppered with wandering planets that no longer circle around any star. Because of tidal forces, any moon orbiting a wandering planet might have liquid oceans under its ice cover and hence life, but such moons would be impossible to see by our instruments, which depend on detecting light from the mother star.
Given that the number of moons probably greatly outnumbers the number of planets in a solar system, and given that there could be millions of wandering planets in the galaxy, the number of astronomical bodies with life-forms in the universe might be much larger than previously believed.
On the other hand, other astronomers have concluded, for a variety of reasons, that the chances for life on planets within the Goldilocks zone are probably much lower than originally estimated by Drake.
First, computer programs show that the presence of a Jupiter-sized planet in a solar system is necessary to fling passing comets and meteors into space, thereby continually cleaning out a solar system and making life possible. If Jupiter did not exist in our solar system, Earth would be pelted with meteors and comets, making life impossible. Dr. George Wetherill, an astronomer at the Carnegie Institution in Washington, D.C., estimates that without the presence of Jupiter or Saturn in our solar system, the Earth would have suffered a thousand times more asteroid collisions, with a huge life-threatening impact (like the one that destroyed the dinosaurs 65 million years ago) occurring every ten thousand years. “It’s hard to imagine how life could survive that extreme onslaught,” he says.
Second, our planet is blessed with a large moon, which helps to stabilize the Earth’s spin. Extending Newton’s laws of gravity over millions of years, scientists can show that without a large moon, our Earth’s axis probably would have become unstable and the Earth might have tumbled, making life impossible. French astronomer Dr. Jacques Lasker estimates that without our moon the Earth’s axis could oscill
ate between 0 and 54 degrees, which would precipitate extreme weather conditions incompatible with life. So the presence of a large moon also has to be factored into conditions used for Drake’s equations. (The fact that Mars has two tiny moons, too small to stabilize its spin, means that Mars may have tumbled in the distant past, and may tumble again in the future.)
Third, recent geological evidence points to the fact that many times in the past, life on Earth was almost extinguished. About 2 billion years ago the Earth was probably completely covered in ice; it was a “snowball Earth” that could barely support life. At other times, volcanic eruptions and meteor impacts might have come close to destroying all life on Earth. So the creation and evolution of life is more fragile than we originally thought.
Fourth, intelligent life was also nearly extinguished in the past. About a hundred thousand years ago there were probably only a few hundred to a few thousand humans, based on the latest DNA evidence. Unlike most animals within a given species, which are separated by large genetic differences, humans are all nearly alike genetically. Compared to the animal kingdom, we are almost like clones of each other. This phenomenon can only be explained if there were “bottlenecks” in our history in which most of the human race was nearly wiped out. For example, a large volcanic eruption might have caused the weather to suddenly get cold, nearly killing off the entire human race.
There are still other fortuitous accidents that were necessary to spawn life on Earth, including
• A strong magnetic field. This is necessary in order to deflect cosmic rays and radiation that could destroy life on Earth.
• A moderate speed of planetary rotation. If the Earth rotated too slowly, the side facing the sun would be blisteringly hot, while the other side would be freezing cold for long periods of time; if the Earth rotated too quickly, there would be extremely violent weather conditions, such as monster winds and storms.
• A location that is the right distance from the center of the galaxy. If the Earth were too close to the center of the Milky Way galaxy, it would be hit with dangerous radiation; if it were too far from the center, our planet would not have enough higher elements to create DNA molecules and proteins.
For all these reasons astronomers now believe that life might exist outside the Goldilocks zone on moons or wandering planets, but that the chances of the existence of a planet like Earth capable of supporting life within the Goldilocks zone are much lower than previously believed. Overall most estimates of Drake’s equations show that the chances of finding civilization in the galaxy are probably smaller than he originally estimated.
As Professors Peter Ward and Donald Brownlee have written, “We believe that life in the form of microbes and their equivalents is very common in the universe, perhaps more common than even Drake and [Carl] Sagan envisioned. However, complex life—animals and higher plants—is likely to be far more rare than is commonly assumed.” In fact, Ward and Brownlee leave open the possibility that the Earth may be unique in the galaxy for harboring animal life. (Although this theory may dampen the search for intelligent life in our galaxy, it still leaves open the possibility of life existing in other distant galaxies.)
THE SEARCH FOR EARTH-LIKE PLANETS
Drake’s equation, of course, is purely hypothetical. That is why the search for life in outer space has gotten a boost from the discovery of extrasolar planets. What has hindered research into extrasolar planets is that they are invisible to any telescope since they give off no light of their own. They are in general a million to a billion times dimmer than the mother star.
To find them astronomers are forced to analyze tiny wobblings in the mother star, assuming that a large Jupiter-sized planet is capable of altering the orbit of the star. (Imagine a dog chasing its tail. In the same way, the mother star and its Jupiter-size planet “chase” each other by revolving around each other. A telescope cannot see the Jupiter-sized planet, which is dark, but the mother star is clearly visible and appears to wobble back and forth.)
The first true extrasolar planet was found in 1994 by Dr. Alexandr Wolszczan of Pennsylvania State University, who observed planets revolving around a dead star, a rotating pulsar. Because the mother star had probably exploded as a supernova, it seemed likely that these planets were dead, scorched planets. The following year two Swiss astronomers, Michel Mayor and Didier Queloz of Geneva, announced that they had found a more promising planet with a mass similar to Jupiter orbiting the star 51 Pegasi. Soon after that the floodgates were opened.
In the last ten years there has been a spectacular acceleration in the number of extrasolar planets being found. Geologist Bruce Jakosky of the University of Colorado at Boulder says, “This is a special time in the history of humanity. We’re the first generation that has a realistic chance of discovering life on another planet.”
None of the solar systems found so far resemble our own. In fact, they are all quite dissimilar to our solar system. Once, astronomers thought that our solar system was typical of others throughout the universe, with circular orbits and three rings of planets surrounding the mother star: a rocky belt of planets closest to the star, next a belt of gas giants, and finally a comet belt of frozen icebergs.
Much to their surprise, astronomers found that none of the planets in other solar systems followed that simple rule. In particular, Jupiter-sized planets were expected to be found far from the mother star, but instead many of them orbited either extremely close to the mother star (even closer than the orbit of Mercury) or in extremely elliptical orbits. Either way the existence of a small, Earth-like planet orbiting in the Goldilocks zone would be impossible in either condition. If the Jupiter-sized planet orbited too close to the mother star, it meant that the Jupiter-sized planet had migrated from a great distance and gradually spiraled into the center of the solar system (probably due to friction caused by dust). In that case, the Jupiter-size planet would eventually cross the orbit of the smaller, Earth-like planet, flinging it into outer space. And if the Jupiter-sized planet followed a highly elliptical orbit, it would mean that the Jupiter-sized planet would pass regularly through the Goldilocks zone, again causing any Earth-like planet to be flung into space.
These findings were disappointing to planet hunters and astronomers hoping to discover other Earth-like planets, but in hindsight these findings were to be expected. Our instruments are so crude that they can detect only the largest, fastest-moving Jupiter-sized planet that can have a measurable effect on the mother star. Hence it is not surprising that today’s telescopes can detect only monster planets that are moving rapidly in space. If an exact twin of our own solar system exists in outer space, our instruments are probably too crude to find it.
All this may change, with the launching of Corot, Kepler, and the Terrestrial Planet Finder, three satellites that are designed to locate several hundred Earth-like planets in space. The Corot and Kepler satellites, for example, will examine the faint shadow that would be cast by an Earth-like planet as it crosses the face of the mother star, slightly reducing its sunlight. Although the Earth-like planet would not be visible, the reduction in sunlight from the mother star could be detected by satellite.
The French Corot satellite (which in French stands for Convection, Stellar Rotation, and Planetary Transits) was successfully launched on December 2006 and represents a milestone, the first space-based probe to search for extrasolar planets. Scientists hope to find between ten and forty Earth-like planets. If they do, the planets will probably be rocky, not gas giants, and will be just a few times bigger than the Earth. Corot will also probably add to the many Jupiter-sized planets already found in space. “Corot will be able to find extrasolar planets of all sizes and natures, contrary to what we can do from the ground at the moment,” says astronomer Claude Catala. Altogether scientists hope the satellite will scan up to 120,000 stars.
Any day, the Corot may find evidence of the first Earth-like planet in space, which will be a turning point in the history of astronomy. In the future people
may have an existential shock gazing at the night sky and realizing that there are planets out there that could harbor intelligent life. When we look into the heavens in the future, we might find ourselves wondering if anyone is looking back.
The Kepler satellite is tentatively scheduled for launch in late 2008 by NASA. It is so sensitive that it may be able to detect up to hundreds of Earth-like planets in outer space. It will measure the brightness of 100,000 stars to detect the motion of any planet as it crosses the face of the star. During the four years it will be in operation, Kepler will analyze and monitor thousands of distant stars up to 1,950 light-years from Earth. In its first year in orbit, scientists expect the satellite to find roughly
• 50 planets about the same size as Earth,
• 185 planets about 30 percent larger than the Earth, and
• 640 planets about 2.2 times the size of the Earth.
The Terrestrial Planet Finder may have an even better chance of finding Earth-like planets. After several delays, it is tentatively scheduled for launch in 2014; it will analyze as many as one hundred stars up to 45 light-years away with great accuracy. It will be equipped with two separate devices to search for distant planets. The first is a coronagraph, a special telescope that blocks out the sunlight from the mother star, reducing its light by a factor of a billion. The telescope will be three to four times bigger than the Hubble Space Telescope and ten times more precise. The second device on the Finder is an interferometer, which uses the interference of light waves to cancel the light from the mother star by a factor of a million.