Worlds of David Darling > Children's
Encyclopedia of Science > Could You Ever Meet an Alien? > 2. Where Are
COULD YOU EVER MEET AN ALIEN?
a book in the Could You Ever? series by David Darling
2. Where Are They?
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.
| Europa, a moon of Jupiter, is one of the
worlds in the Solar System on which there is a chance we might find
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?
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
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
|A camera aboard the Viking 1 lander
took this photograph of the surface of Mars
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.
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.
|An orbiting planet causes
a star to wobble in its path
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