Scientists known as exobiologists have been searching for evidence of extraterrestrial life for years. Many of these specialists believe that life exists—or may once have existed—on other planets. Let’s look at the evidence.
The elements of life
Most scientists believe that life on Earth developed gradually through physical and chemica reactions that occurred during Earth’s formation and early development. Exobiologists use this theory as a basis in their search for life on other planets. They believe the chances are high that the chemistry of life elsewhere in the universe would resemble the chemistry of life on Earth. However, while they may have similar organic molecules (molecules that contar carbon atoms), extraterrestrial life and life on Earth would have developed in response to different environmental conditions. As a result, extraterrestrial life would almost certainly differ greatly in structure and appearance from life forms on Earth.
So far, scientists have found that organic molecules are surprisingly common in the sola’ system—in some comets and meteorites, on some planets and moons of the outer solar sys terns, and in the gases and dust of interstellar space. However, these locations are unlikely habitats for living things. Life appears to need liquid water, which in turn seems to need a planet or moon.
The search for life
Equipped with instruments that could detect signs of life, spacecraft have already explored more than 70 planets, satellites, asteroids, and comets. None has shown compelling indications of extraterrestrial life.
Beginning in the 1960’s, for example, the United States and the Soviet Union sent missions to Venus. They found that environmental conditions on the surface of Venus could not support life as we know it.
Mars, the nearest planet whose surface scientists can see, has an atmosphere, polar icecaps, seasonal changes, and a 24-hour-and-37-minute day. Mars seemed to be the most likely place to support life. In 1976, two U.S. space probes, Viking 1 and Viking 2, landed on Mars and performed experiments to test for life. These experiments found no likely signs of life in Martian soil, and a highly sensitive chemical detector failed to detect any organic compounds.
The Vikings’ findings suggest that Mars, today at least, is lifeless. But some scientists suggest that about 4 billion years ago, conditions on Mars may have favored life. The planet’s surface is covered with evidence of ancient rivers and lakes. In addition, the Martian climate was much warmer and wetter about 4 billion years ago. These facts suggest that life may have once existed on Mars. As a result, a number of scientists agree that any future exploration of Mars should include a search for fossils—evidence of ancient life—in addition to existing life.
A 746-pound probe from the Galileo spacecraft, which was launched in 1989, plunged into Jupiter on December 7, ‘995. The probe sent back to Galileo 75 minutes of weather and chemical data. After receiving the data, Galileo entered orbit around Jupiter for two years of study. The mission could give the best view ever of the planet’s composition. Scientists believe that Jupiter is primarily made up of the main elements of the primordial mix. Could there be life on Jupiter? Scientists will have to stay tuned.
While scientists wait to find out about Jupiter, they are nappy to report that Galileo has made other important contra-outions. Along its journey, the spacecraft made close-up observations of our own planet. And in December 1990, it detected life on Earth. While this may not seen like news, t is a tremendously significant observation. Galileo’s findings □roved that remote-sensing spacecraft can detect life at various stages of evolutionary development. These results help assure scientists that their methods would effectively spot signs of life in other parts of the universe. And, since scion-tists have not yet found such evidence, they can say with some certainty that widespread biological activity, at least today, exists on Earth—and nowhere else.
Other planetary systems
The findings of the Hubble Space Telescope have suggested 😮 exobiologists that many of the stars in the Milky Way may nave planetary systems. And some of those planets may have extraterrestrial life. In January 1996, two scientists revealed that they had evidence of only the second and third planets •ound to exist outside the solar system. And noth seem to be temperate enough to allow liquid water to exist. In addition, the scientists said, the two planets would be close enough to their suns to soak up the rays needed for life. The scientists are looking forward to the installation of a new infrared camera on the Hubble Space Telescope. Planned for 1997, the camera could take pictures of at least one of the newly unveiled worlds.
The first of the two giant planets orbits a star known as 47 Ursae Majoris, 200 trillion miles from Earth in the Big Dipper. It is about twice the size of Jupiter. The second circles the star 70 Virginis in the constellation Virgo, and it is more than six times the mass of Jupiter.
Given today’s technology, there is no way to say for sure that life exists on these planets. But if our solar system is an example, giant, volatile planets are likely to be accompanied by small, amicable ones. Giant planets also tend to keep company with giant moons, which too could be well suited for life. Probably the most important conclusion that can be made from the planets’ discoveries is that the Milky Way is probably filled with other worlds waiting to be discovered. And NASA has unveiled plans to further search forthose worlds. Known as the Origins project, the plan is to build a new generation of space telescopes.
Some scientists are now using radio telescopes, sophisticated receivers, and modern data analysis to detect communication signals from distant civilizations. These methods will only prove successful, however, if our extraterrestrial neighbors include intelligent beings capable of communicating
with humans across the vast reaches of space. Known as the Search for Extraterrestrial Intelligence (SETI), this ongoing effort has taken many forms. The most recent is BETA (Billionchannel Extra-Terrestrial Assay), which consists of an 84-foot, dish-shaped antenna. The antenna sweeps across the sky, capturing radio waves. Funded by NASA until 1993, SETI is now privately funded.
A few unmanned spacecraft carry greetings from planet Earth, so that if one is found some day by another civilization, its origin will be more obvious. Pioneer 10 and Pioneer 11 each carry a small metal. plaque that shows a man and woman offering greetings and also describes graphically the location of Earth.
Voyager 1 and Voyager 2 each carry a 12-inch disk containing digitized images and sounds specially selected to portray the diversity of life on Earth.