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A Closer Look

Extraterrestrials?
xtraterrestrial is a descriptive term, conditions necessary to support life (Note:
Emeaning a thing or event outside Earth If 1 percent have planetary systems, this
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or its atmosphere. The term is also used to means 3 × 10 stars have planets). Thus,
describe a being, a life form that originated it is a statistical observation that suitable
away from Earth. This reading is concerned planets for life are very likely to exist. In ad-
with the search for extraterrestrials, intelligent dition, radio astronomers have found that
life that might exist beyond Earth and outside many organic molecules exist, even in the
the solar system. space between the stars. Based on statistics
Why do people believe that extrater- alone, there should be life on other planets,
restrials might exist? The affirmative answer life that may have achieved intelligence and
comes from a mixture of current theories developed into a technological civilization.
about the origin and development of stars, If extraterrestrials exist, why have we
statistical odds, and faith. Considering the not detected them or why have they not
statistical odds, note that our Sun is one of contacted us? The answer to this question is
the some 300 billion stars that make up our found in the unbelievable distances involved
Milky Way galaxy. The Milky Way galaxy is in interstellar space. For example, a logical
one of some 10 billion galaxies in the ob- way to search for extraterrestrial intelligence
servable universe. Assuming an average of is to send radio signals to space and analyze BOX FIGURE 14.1 The 300 m
(984 ft) diameter radio telescope at
about 300 billion stars per galaxy, this means those coming from space. Modern radio
Arecibo, Puerto Rico, is the largest
there are some 300 billion times 10 billion, telescopes can send powerful radio beams,
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or 3 × 10 , stars in the observable universe. and present-day computers and data pro- fixed-dish radio telescope in the world.
There is nothing special or unusual about cessing techniques can now search through
our Sun, and astronomers believe it to be incoming radio signals for the patterns of
quite ordinary among all the other stars (all artificially generated radio signals (Box In addition to problems with distance
3,000,000,000,000,000,000,000 or so). Figure 14.1). Radio signals, however, travel and time, there are questions about in which
So the Sun is an ordinary star, but what through space at the speed of light. The diam- part of the sky you should send and look
about our planet? Not too long ago, most eter of our Milky Way galaxy is about 100,000 for radio messages, questions about which
people, including astronomers, thought our light-years, which means 100,000 years would radio frequency to use, and problems with
solar system with its life-supporting planet be required for a radio transmission to travel the power of present-day radio transmit-
(Earth) to be unique. Evidence collected across the galaxy. If we were to transmit a ters and detectors. Realistically, the hope
over the past decade or so, however, has super, super strong radio beam from Earth, for any exchange of radio-transmitted mes-
strongly suggested that this is not so. Planets it would travel at the speed of light and cross sages would be restricted to within several
are now believed to be formed as a natural the distance of our galaxy in 100,000 years. If hundred light-years of Earth.
part of the star-forming process. Evidence of some extraterrestrials on the other side of the Considering all the limitations, what
planets around other stars has also been de- Milky Way galaxy did detect the message and sort of signals should we expect from extra-
tected by astronomers. One of the stars with send a reply, it could not arrive at Earth un- terrestrials? Probably a series of pulses that
planets is “only” 53 light-years from Earth. til 200,000 years after the message was sent. somehow indicate counting, such as 1, 2, 3,
Even with a very low probability of Now consider the fact that of all the 10 billion 4, and so on, repeated at regular intervals.
planetary systems forming with the de- other galaxies in the observable universe, our This is the most abstract, yet simplest, con-
velopment of stars, a population of 3 × nearest galactic neighbor similar to the Milky cept that an intelligent being would have. It
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10 stars means there are plenty of plan- Way is Andromeda. Andromeda is 2 million could provide the foundation for communi-
etary systems in existence, some with the light-years from the Milky Way galaxy. cations between the stars.

(Figure 14.16). Other galaxies have other shapes and other might exist in the shapes. Finding underlying order is impor-
characteristics. The American astronomer Edwin Hubble devel- tant because it leads to the discovery of the physical laws that
oped a classification scheme for the structure of galaxies based on govern the universe. Soon after Hubble published his classifi-
his study of some 600 different galaxies. The basic galactic struc- cation results in 1926, two models of galactic evolution were
tures were identified as elliptical, spiral, barred, and irregular. proposed. One model, which was suggested by Hubble, had
extremely slowly spinning spherical galaxies forming first,
which gradually flattened out as their rate of spin increased
THE LIFE OF A GALAXY while they condensed. This is a model of spherical galaxies flat-
Hubble’s classification of galaxies into distinctly different cat- tening out to increasingly elliptical shapes, eventually spinning
egories of shape was an exciting accomplishment because it off spirals until they finally broke up into irregular shapes over
suggested that some relationship or hidden underlying order a long period of time.

364 CHAPTER 14 The Universe 14-14