MODERN TIMES
Art Hobson
ahobson@uark.edu
NWA Times 15 August
2009
On the trail of
life in the universe
For
at least 2000 years, humankind has speculated on the question "Are we
alone?" In the first century
BCE, the Roman poet and philosopher Lucretius suggested that, just as life
originated by blind, 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 Éwith races of different men and different
animals."
I
expect we will discover extraterrestrial life within 15 years, although it's
highly unlikely to involve intelligent life and will probably be single-celled
microbial life, like simple algae or bacteria. It will be a landmark in human history.
You
live in the heroic age of exoplanet (planets around stars other than the sun)
discovery. Since all other stars
can be seen only as a single point of light, even the discovery of a single such planet is an awsome
feat. The first exoplanet was
discovered in 1995, and 340 are now known. Encouragingly for those of us who would be thrilled to find
that the universe is filled with life, most of the discovered planets lie in
their star's "habitable zone" within which the star's radiation has
an intensity similar to the intensity of sunlight on Earth. So these planets are candidates for
having water in the liquid state rather than as steam or ice. Life on Earth came from the oceans and
is still made mostly of water.
Laboratory experiments suggest that, given an Earth-sized rocky planet
plus water, life is highly likely to develop through natural chemical
processes. This is probably how
life developed on Earth.
To
date, astronomers have discovered most exoplanets by detecting a star's subtle wobbling
in response to an orbiting planet.
A star that wobbles sends out light whose spectrum (or colors) wobble
first toward violet and then toward red, for the same reason (known as the
Doppler effect) that an ambulance's siren wobbles toward higher and then toward
lower pitches as it approaches and then recedes from you. Most such discoveries have been of
large, Jupiter-like planets, because high-mass planets cause big wobbles of the
central star and so are easiest to detect. These large planets are not so
likely to harbor life. But the
discoveries indicate that the actual number of (mostly still undetected)
planets rises steeply with decreasing planet mass, making Earth-like planets
highly likely. Most known
exoplanets orbit sun-like stars and smaller, cooler stars known as "red
dwarfs." Red dwarfs seem more
likely than sun-like stars to be hosts for life, since they comprise 80 percent
of the stars near Earth and they give off their dim red light steadily for far
longer than our sun's 10-billion year lifetime (of which 5 billion years have
now passed). Massachusetts Institute of Technology astronomer Sara Seeger
thinks that "infrared astronomy," based on invisible light whose
wavelength is a little longer than visible light's wavelength, might find signs
of life around a red dwarf within a few years.
A
second method for detecting exoplanets concentrates only on those planets that
happen to orbit their star in such a way as to cross directly in front of the
star as seen from Earth. These
"transiting" planets, while they are in front of their star, reveal
their presence by temporarily reducing slightly the light arriving at Earth
from the star. Careful monitoring
of the reduced starlight, combined with information from the star's Doppler
effect, tells astronomers both the radius and mass of the exoplanet and can
reveal small rocky planets similar to ours. Sixty transiting planets are known to date. A space-based transit survey by the
European Space Agency's Corot satellite
has detected planets as small as two Earth diameters. NASA's Kepler
mission, launched this past March and named for the great German astronomer who
deciphered our solar (sun) system's architecture, will monitor 100,000 sun-like
stars. About 1 percent of these
stars are expected to have transiting Earth-like planets in their star's
habitable zone.
A
third method is the most technically difficult but offers the greatest prospect
for revealing life. It is the
direct detection of an exoplanet's light--directly seeing the planet. It's not easy to detect a planet's dim
light in the many billion times more intense glare of light from the planet's
nearby star. To combat this glare,
astronomers must place a screen either inside or outside their telescope to
block the star's light while allowing the planet's light to enter the
telescope. This method will be
perfected before long and can, by studying the precise frequencies (or colors)
of light from the planet, reveal the chemical signs of life.
The
prospects are anybody's guess, but most of us who love to ponder this question
think that astronomers will find evidence that single-celled microbial life
exists around millions to billions of stars in our Milky Way galaxy alone. Because our galaxy is only a tiny part
of the universe, this would mean that the number of instances of life in the
universe is unimaginably large.
But the prospects for complex multi-cellular life are far more dim.
And
the prospects for intelligent life are dimmer still. Here's one of the reasons why: Judging from Earth's history, intelligent life requires
billions of years of evolution.
Animal instincts develop during this period, and these primitive
instincts can be destructive in intelligent lifeforms unless they are
rationally controlled. Judging
from humankind's destruction of our own environment, our superstitious belief
systems, and our uncontrolled violence, the prospects for such rational control
here and on other planets are not promising. Thus there might be few if any surviving intelligent civilizations
out there.