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T
he Copernican principle
states the working
assumption that Earth is
not special—it is no more than an
average planet, orbiting a medium-
sized star, in an unremarkable part
of an ordinary galaxy. If Earth is not
unique, then there is little reason to
think that other planets cannot also
harbor life. Given the number of
stars in the universe—in the order
of 10^23 —this might be a statistical
certainty. Over the centuries, many
thinkers, such as American Carl
Sagan, have pondered the possibility.Is Earth alone?
In the 16th century, the Italian monk
Giordano Bruno proposed that the
stars were other suns, each of which
could have its own solar system.
Life could even populate these other
earths. Believing that the universe
was infinite, Bruno also insisted
that it could have no center. Bruno
was tried by the Roman Inquisition
for these and other heretical beliefs
and burned at the stake in 1600.
Throughout history, various
astronomers have claimed to see
evidence for life on other planets
of the solar system. In the 1890s,
American astronomer PercivalLowell claimed to have mapped
artificial “canals” on Mars, while
the dense clouds of Venus were
imagined, by Swedish chemist
Svante Arrhenius in 1918, to hide
from view a lush surface blooming
with life. It is now known that the
clouds are acidic, while the surface
of Venus is an inhospitable 864°F
(462°C). However, these are just two
planets out of potentially billions.
The universe’s immensity and
the apparent universality of its
physical laws make it seem likely
that microbial life exists elsewhere.LIFE ON OTHER PLANETS
IN CONTEXT
KEY ASTRONOMER
Carl Sagan (1934 –1996)BEFORE
1865 German physician
Hermann Eberhard Richter
suggests that planets could
be seeded by simple life
carried by comets.AFTER
1973 Brandon Carter sets
out an early version of the
anthropic principle, which
states that the universe is
necessarily a certain way;
otherwise humans would
not be here to experience it.1977 Voyagers 1 and 2 are
launched, carrying images
and sounds from Earth.2009–2016 NASA space
telescope Kepler discovers
3,443 exoplanets in 2,571
planetary systems.2015 Kepler discovers the
first Earth-sized planet in a
habitable zone of another star.Do there exist many worlds,
or is there but a single world?
This is one of the most
noble and exalted questions
in the study of Nature.
Albertus Magnus
13th-century scholarCarl Sagan Carl Sagan is one of the most
widely known 20th-century
scientists. His deep, honeyed tones
are the instantly recognizable
voice of the documentary series
Cosmos. Sagan was raised in
a working-class Jewish area of
New York and as a boy was an
avid reader of science fiction.
A talented pupil, he went to the
University of Chicago in 1951 on
a full scholarship. Sagan received
his Ph.D. in 1960, showing that
the high surface temperatures
of Venus are due to a runaway
greenhouse effect. Sagan
conducted pioneering researchin planetary science and
exobiology (the biology of
extraterrestrial life), which many
in mainstream astronomy viewed
with suspicion. In 1985, he wrote
the sci-fi book Contact, which
was later turned into a movie.
With his visionary, positive, and
humanist outlook, the Cornell
University professor inspired a
new generation of astronomers.Key works1966 Intelligent Life in the
Universe (with Iosif Shklovsky)
1983 Cosmos