ARE WE ALONE?
by Art Hobson
ahobson@uark.edu
http://physics.uark.edu/hobson/
Are we alone? Humankind has speculated on this
question for at least 2000 years.
In the first century B.C., Roman poet Lucretius suggested that, just as
life originated by spontaneous chemical interactions on Earth, "we must
acknowledge that such combinations of other atoms happen elsewhere in the
universe to make worlds such as this one.
…So we must realize that there are other worlds in other parts of
the universe, with races of different men and different animals."
Nineteenth-century
essayist Thomas Carlyle has a darkly humorous take on the issue: "A sad spectable. If the stars be inhabited, what a scope
for misery and folly. If they be
not inhabited, what a waste of space." And for contemporary architect Buckminster Fuller,
"Sometimes I think we're alone.
Sometimes I think we're not.
In either case, the thought is staggering."
Today we can do much
more than speculate on extraterrestrial life. We've discovered planets orbiting 108 (and counting) other
stars. These "extra-solar
planets" are nearly impossible to spot visually amidst the glare from
their "parent" star, so we've detected them by observing the small back-and-forth
motions of their parent stars caused by the motion of the planet around the
star. To date we've managed to
detect only large Jupiter-sized planets, but there's every reason to expect
that these other solar systems also harbor smaller planets. There are theoretical reasons to
believe that about 10 percent of the stars similar to our sun have Earth-like
planets in orbit around them.
Three upcoming space
missions will bear on the existence of Earth-like planets and life on
them. If an extra-solar planet's
orbit happens to be oriented exactly edge-on as seen from Earth, then each
orbit of the planet will cross directly in front of the star. This causes a slight dip in the star's
light as seen from Earth. NASA
will launch a satellite in 2007 that will search for such dips in the light
from 100,000 stars in a small patch of sky. This method will tell us what fraction of the 100,000 stars
have Earth-sized planets.
If that fraction is
as high as 10 percent, it will bode well for further missions. In 2010, another satellite will monitor
2000 "nearby" stars for the kinds of subtle back-and-forth motions
that Earth-sized planets would cause.
This survey might reveal 200 or more nearby planets for the third
mission to zero in on.
That multibillion-dollar
mission might be launched jointly by NASA and the European Space Agency around
2015. In one design, four
free-flying but carefully synchronized mirrors in space will combine slightly
different views of each "parent" star so as to just cancel the star's
light and reveal the much dimmer reflected light from planets orbiting the
star. This method promises to
detect not only Earth-like planets, but also the basic ingredients of their
atmospheres. If there is another
pale blue dot out there harboring Earth-like life, such telescopes could detect
its chemical imprint.
There
is plenty of reason to think there's life out there. The carbon-based chemicals that build life on Earth are
widespread throughout our galaxy.
Liquid water appears abundant in our solar system and probably in
others. There is evidence for
watery oceans during the history of Mars and today on three of Jupiter's moons,
and for icy oceans on the planet Pluto, on Neptune's moon Triton, and on
Saturn's moon Titan (now being studied by NASA's Cassini spacecraft). Earthly life in extreme environments
indicates that life could exist at the bottom of ice sheets where the ice is
heated from below by underground radioactivity.
Much is known about
the formation of our solar system and Earth 4.56 billion years ago. Scientists agree that present life
started surprisingly early, 3.8 billion years ago. Many experiments indicate that life's chemical building
blocks --nucleic acids and amino acids--form surprisingly easily from gases similar
to the early Earth's atmosphere.
One plausible route for the "chemical evolution" from
non-living to living forms involves ribonucleic acid, which combines features
of both DNA and proteins and could have performed the functions of both in the
early biological world. Similar
processes could easily operate on other watery planets. Thus, nearly all scientists who have
studied the question believe that life is abundant in our galaxy and throughout
the universe.
But is there other
intelligent life out there? That
is a far harder question. Some 40
percent of our galaxy has now been scanned for intelligent radio signals from
other planets, searching in places and in radio frequencies that seem most
plausible for communication, with negative results. It is beginning to look as though we are alone, or nearly
alone, in our galaxy. Thus, we
might be the only form of matter in our galaxy capable of understanding such
things as galaxies--the only form through which the universe can begin to
understand itself.
But it's hard to
believe that intelligent life arose only here, when our solar system appears so
non-unique. What, then, explains
this absence of extraterrestrials?
One explanation is especially compelling: Perhaps intelligent life, once it develops technology, is
unable to overcome its own biological instinct to procreate and to amass power
and thus "grows" itself to extinction.
This "short lifetime
hypothesis," first suggested 50 years ago by the great physicist Enrico
Fermi, is a kind of lesson from the stars, urging us not to misuse technology.
The short lifetime hypothesis suggests that, in our present technological
period, we had better use our brains, rather than following our instincts.