The Search for Life in the Universe
Excerpts from the written testimony submitted by Neil deGrasse Tyson, Department of Astrophysics & Hayden Planetarium, American Museum of Natural History, to the "Life in the Universe" hearings held by the House Subcommitee on Space and Aeronautics
The discovery of what is now more than seventy planets around stars other than the Sun continues to stimulate tremendous public and media interest. It seems to me that this attention is driven not so much by the discovery of the extrasolar planets themselves, but by the prospect of intelligent alien life.
|Astronomers have announced findings that imply Gliese 876 has a planet at least 1.6 times as massive as Jupiter – making this now one of the closest suspected planetary systems.|
Credit: Digitized Sky Survey
People really care about whether or not we are alone in the Universe. When the person next to me on a long airplane flight finds out that I am an astrophysicist, nine times out of ten they ask, with wide eyes, about life in the Universe. Only later do they ask me about the big bang and black holes. I know of no other discipline that triggers such a consistent and reliable reaction in public sentiment. The questions "Are we alone?" and "What is our place in the Universe" just might be genetically encoded in our species.
At the moment, life on Earth is the only known life in the Universe, but there are compelling arguments to suggest we are not alone. Indeed, most astrophysicists accept a high probability of there being life elsewhere. If the count of planets in our solar system is not unusual, then there are more planets in the Universe than the sum of all sounds and words ever uttered by every human who has ever lived. The numbers are, well, astronomical. To declare that Earth must be the only planet in the Universe with life would be inexcusably egocentric of us.
Many generations of thinkers, both religious and scientific, have been led astray by anthropocentric assumptions. In spite of a third century B.C. account of a sun-centered Universe proposed by the Greek philosopher Aristarchus, the Earth-centered Universe was the standard view for most of the last 2000 years. Codified by the teachings of Aristotle and Ptolemy, and by the preachings of the Roman Catholic Church, people generally accepted Earth as the center of all astrophysical motion. When the sixteenth century Italian monk Giordano Bruno publicly suggested that the Universe was filled with planets that harbor life, he was burned at the stake. Fortunately, today we live in somewhat more tolerant times.
In the absence of dogma and data, history tells us that it is prudent to be guided by the notion that we are not special. This is generally known as the Copernican principle, named for the Polish astronomer Nicholas Copernicus who, in the mid-1500s, put the Sun back in the middle of our solar system where it belongs. While there is no guarantee that the Copernican principle will guide us correctly for all scientific discoveries, it has humbled our egos with the realization that Earth is not in the center of the solar system, the solar system is not in the center of the Milky Way galaxy, and the Milky Way galaxy is not in the center of the Universe.
|Copernicus gave new currency to the ancient Pythagorean hypothesis that the Sun was at the center of the Universe and that the planets (including the Earth) revolved around the Sun.|
If life itself is not immune to the Copernican principle, then how does life on Earth provide clues to what life might be like elsewhere in the Universe?
I do not know whether biologists walk around every day awestruck by the diversity of life. I certainly do. On this single planet called Earth, there co-exist (among countless other life forms), algae, beetles, sponges, jellyfish, snakes, condors, and giant sequoias. Imagine these seven living organisms lined up next to each other in order of size. If you didn’t know better, you would be hard-pressed to believe that they all came from the same Universe, much less the same planet.
Try describing a snake to somebody who has never seen one: "You gotta believe me. There is this animal on Earth that (1) can stalk its prey with infrared detectors, (2) swallows whole live animals up to five times bigger than its head, (3) has no arms or legs or any other appendage, yet (4) can slide along level ground at a speed of two feet per second!"
Given the diversity of life on Earth, one might expect to see a similar diversity in the aliens depicted by Hollywood. But I am consistently amazed by the film industry’s lack of creativity. With a few notable exceptions, Hollywood aliens look remarkably humanoid. No matter how ugly (or cute) they are, nearly all of them have two eyes, a nose, a mouth, two ears, a head, a neck, shoulders, arms, hands, fingers, a torso, two legs, two feet — and they can walk. From an anatomical view, these creatures are practically indistinguishable from humans, yet they are supposed to have come from another planet. If anything is certain, it is that life elsewhere in the Universe, intelligent or otherwise, will look at least as exotic as some of Earth’s own life forms.
Is life chemically special? The Copernican principle suggests that it probably isn’t. Aliens need not look like us to resemble us in more fundamental ways. Consider that the four most common elements in the Universe are hydrogen, helium, carbon, and oxygen. Helium is inert. So the three most abundant, chemically active ingredients in the cosmos are also the top three ingredients in life on Earth. For this reason, you can bet that if life is found on another planet, it will be made of a similar mix of elements. Conversely, if life on Earth were composed primarily of, for example, molybdenum, bismuth, and plutonium, then we would have excellent reason to suspect that we were something special in the Universe.
How about intelligence? Let us assume, for the sake of argument, that humans are the only species in the history of life on Earth to evolve high-level intelligence. By some estimates, there have been more than ten billion species in the history of life on Earth. If life on Earth offers any measure of life elsewhere in the Universe, then intelligence must be rare – we might expect that no better than about one in ten billion alien life forms are as intelligent as we are.
The discovery of simple, unintelligent life forms elsewhere in the Universe (or evidence that they once existed) would be far more likely and, for me, only slightly less exciting than the discovery of intelligent life.
|The Arecibo radio telescope is currently the largest single-dish telescope in the world used in radio astronomy. In 1974, Arecibo was used to broadcast a message from Earth to the globular star cluster M13.|
Credit: NAIC – Arecibo Observatory, David Parker / Science Photo Library
But if intelligent alien civilizations do exist, and if they share our desire to communicate through the vast distances of interstellar space, then radio waves would be the communication band of choice. Radio waves can traverse the galaxy unimpeded by interstellar gas and dust clouds. But earthlings have only understood the electromagnetic spectrum for less than a century.
More depressingly put, for most of human history, had aliens tried to send radio signals to earthlings we would have been incapable of receiving them. For all we know, the aliens have already done this and unwittingly concluded that there was no intelligent life on Earth. They would now be looking elsewhere. A more humbling possibility would be if aliens had become aware of the technologically proficient species that now inhabits Earth, yet they had drawn the same conclusion!
If we consider the possibility that we may rank as primitive among the Universe’s technologically competent life forms — however rare they may be — then the best we can do is keep alert for signals sent by others because it is far more expensive to send rather than receive them. Presumably, an advanced civilization would have easy access to an abundant source of energy. These are the civilizations that would be more likely to send rather than receive signals.
The search for extraterrestrial intelligence (affectionately known by its acronym "SETI") has taken many forms. The most advanced efforts today use a cleverly designed electronic detector that monitors, in its latest version, billions of radio channels in search of a signal that might rise above the cosmic noise. The "SETI At Home" screen saver analyzes real data (downloaded from the Internet) for this type of an intelligent signal. This software has been downloaded by more than 3 million PCs users around the world, which actively taps an astonishing level of computing power from these plugged-in PCs that otherwise would be doing nothing. Indeed, "SETI At Home" is, by far, the largest computational project in the history of the world. Public support for this enterprise is real and it is deep.
The discovery of extraterrestrial intelligence, if and when it happens, will impart a change in human self-perception that may be impossible to anticipate. If we don’t soon find life elsewhere, what will matter most is that we had not stopped looking. Our species demands that we keep looking. Deep in our souls, we are intellectually curious nomads we search for other places and other life forms because we derive almost as much fulfillment from the search as we do from the discovery.