The Universe is at least 10 billion trillion miles across. It contains, astronomers estimate, about a billion trillion stars. That means there are more stars than there are grains of sand on all the beaches in the world.

 

Around any one of these stars there could be planets. And on many planets there could, possibly, be alien life. Where, then, do we begin to search for life in space? (see Figure 1)

 

We begin close to home, with the planets of our own Solar System. Compared with the huge gaps between stars, the distances to other worlds within the Solar System are extremely small. The planet Venus, for example, can come as close to the Earth as 23½ million miles. That is a million times closer than the nearest star beyond the Sun.

 

As it happens, Venus is also one of the last places in the Solar System where we would expect to find life. The temperature at its surface is about 900°F, or twice as hot as a household oven. Its atmosphere is so crushingly heavy that the pressure at the planet's surface is the same as that half a mile beneath the ocean's surface. And worst of all, on Venus it rains deadly acids!

 

Mars, our next nearest neighbor among the planets, is much more likely to support life. In the late 1800s, astronomer Percival Lowell reported that he had discovered a network of canals on Mars. Lowell claimed that the canals must have been built by a race of intelligent beings. Few scientists accepted this idea. Because of false newspaper and radio stories, though, many people thought the Earth was about to be invaded!

 

By 1965, photographs sent back by the space probe Mariner 4 proved that Lowell was mistaken. Yet, there was still the chance that much simpler forms of life, such as germs or tiny plants, existed on Mars.

 

The Vikings Have Landed!

Slowed by parachutes as wide as football field, two robot spacecraft dropped gently onto the desert landscape of Mars in 1976. These were the Viking 1 and 2 landers. During their mission, they examined the weather and rock formations of Mars and searched for signs of life in the rust-red Martian soil (see Figure 2).

 

At a signal from mission control, each lander extended a small scoop to gather a soil sample. This was then placed into a miniature laboratory aboard the lander where a number of chemical experiments were performed.

 

To begin with, the results seemed promising. For a few weeks after the first tests were carried out, excited researchers thought they may have discovered tiny life forms on Mars. Something in the samples appeared to be breathing and feeding. But it soon turned out that the results could be explained in another way – by the presence of very active chemicals, called superoxides, in the Martian soil.

 

In the Land of the Ancient Giants

Picture a world so huge that more than 1,300 earths could fit inside it. Picture a world on which a single storm, that has been blowing for centuries, dwarfs our own globe. This is Jupiter, the biggest planet in the Solar System (see Figure 3).

 

Looking at Jupiter from the outside, we do not see a firm surface like that of the Earth. Instead, we see the orange, brown, and yellow cloud-tops of a wind-tossed atmosphere, hundreds of miles deep. Below this thick, gassy blanket may be a pitch-black ocean of liquid hydrogen, tens of thousands of miles thick. Only at Jupiter's center is there perhaps a rocky core, not much larger than the Earth.

 

Because Jupiter is five times farther from the Sun than the Earth is, it receives much less warmth. In fact the temperature at its topmost clouds is about -240°F, much colder than the chilliest winter night in Antarctica. Nothing, you might imagine, could possibly survive in such a strange and bitterly cold place.

 

Yet, some scientists have suggested that Jupiter may have life – or, at least, the beginnings of life. The mixture of gases in Jupiter's atmosphere, which includes hydrogen, methane, ammonia, and water vapor, is quite similar to that on our own planet billions of years ago. What is more, Jupiter's atmosphere is almost certainly warmer at great depths. Far below the cloud-tops, there may be a layer in which conditions are not greatly different from those that existed long ago on Earth. It is here, say some researchers, hat the chemicals needed for life may have been, and may still be, formed.

 

Four spacecraft have flown by Jupiter. But a probe called Galileo, which was launched in October 1989, actually went into orbit around the giant planet in 1995. It dropped a package of instruments, suspended from parachutes into Jupiter's atmosphere. As the instruments descended through the upper lays of clouds, they sent back pictures and other information back to Earth. In this way, scientists learned much more about what is going on inside Jupiter's mysterious clouds. <

 

Beyond Jupiter lie three more giant, gassy planets – Saturn, Uranus, and Neptune – and, farthest of all, tiny Pluto. If the chances of finding life on Jupiter are small, then they are even smaller on these more distant and cooler worlds.

 

Some scientists have suggested that life forms may exist on some of the moons of the outer, giant planets. According to one bold idea, there may be a deep, watery ocean beneath the icy crust of Jupiter's moon, Europa, inhabited by strange sea creatures. Other researchers point to Saturn's large moon, Titan, which is wrapped in a thick orange smog. Some of the complex substances that built up on the young Earth may drift among Titan's clouds or collect in swamps on the surface.

 

There are, then, a number of places in the Solar System where simple life forms, or the basic chemicals of life, might still be found. Future probes to Mars and the outer planets will help us decide one way or the other. But even if we fail to find life on any of our neighboring worlds, there are probably millions of other planets out there on which the search for aliens can continue.

 

The Search for New Worlds

Planets are much dimmer than stars and, in most cases, cannot be seen by even the largest telescopes presently on Earth. At the great distances they lie, their faint glow is lost in the stellar glare. In searching for new worlds, then, astronomers have turned to less direct methods (see Figure 4).

 

Often it is said that planets "go around" the Sun. But this is not exactly true. In fact, the Sun and the planets go around each other. They orbit a common center of gravity – the point where there gravitational pulls are equal in strength.

 

This affects the way the Sun moves through space. Though the center of gravity follows a straight path, the Sun itself "wobbles" slightly from side to side as it moves.

 

The more massive a planet is, the bigger the wobble it causes. The center of gravity of the Sun-Jupiter system, for instance, lies 28,000 miles above the Sun's surface. This makes the Sun wobble quite a bit. Even so, viewed from the nearest star, the effect of Jupiter on the Sun's motion would be very hard to spot.

 

What is true of the Sun is also true of any other star that has planets. And though the wobbles caused by planets are very small, astronomers have used this method, and others, to discover hundreds of planets around other stars.

 

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