MODERN TIMES
Art Hobson
ahobson@uark.edu
NWA Times 4 August 2007
The search for Earth-like planets
My
guess is that we'll discover extraterrestrial life within 15 years. Here's why.
We've
discovered over 230 extraterrestrial planets ("exoplanets") so far.
The discovery techniques are mind-boggling, especially when one considers that
neither the star nor it's planet are "resolved" by the
telescope. The entire star plus
exoplanets appear in the telescope as only a single point of light.
One
technique is the measurement of the speed at which the planet's central star
(its "sun") wobbles in response to orbiting planets. This wobbling can be measured by
detecting the slight shift in frequency of the star's light as the star wobbles
first toward Earth and then away from it, similar to the shift you hear in the
frequency of a siren's sound as the siren passes you. It's also possible to detect these wobbles by visually
tracking the changing position of the star. A third technique works only for planets whose orbits pass
in front of their host star, as seen from Earth. The planet blocks part of the star's light, and this
decrease in the star's light is detectable. This technique can even determine the chemical structure of
the planet's atmosphere, because the atmosphere filters certain frequencies out
of the star's light, and we can detect this filtering effect in the
starlight. Yet a fourth technique
relies on the bending of light caused by gravity.
Astronomers,
confronted with a bonanza of newly-discovered exoplanets, report that "the
frequency of planetary systems is immensely larger than anyone would have
guessed." Much of the new
research focuses on stars called "red dwarfs" that are smaller the
sun, giving off dim red light steadily for tens of billions of years, far
longer than our sun's lifetime.
Red dwarfs seem more likely than sun-like stars to be "hosts"
for life; they constitute 80 percent of the stars near Earth.
Now
astronomers, using the frequency-shift technique, have discovered a planet with
conditions sufficiently similar to Earth that it could be habitable by
Earth-like life. It's orbiting
Gliese 581, a red dwarf only 120 trillion miles, or "20 light years"
(the distance traveled by light in 20 years), away. It's the third planet detected orbiting Gliese 581. The Earth-like planet is some five
times more massive (heavier) than Earth, is probably made of rock, orbits its
star in 13 Earth-days, and, most importantly, is orbiting in the "water
zone" of its star--the range of distances out from the star that will
allow liquid water (rather than ice or steam) to exist. At least for life resembling our own,
liquid water is essential. The
average temperature on Gliese 581 is estimated to be between 32 and 104 degrees
Fahrenheit. However, any number of
details could prevent life: The
planet might have no atmosphere, or too thick an atmosphere, or happen to have
no water, etc.
In
December 2006, France launched COROT, the first space mission dedicated
entirely to the search for exoplanets.
In May, it detected its first exoplanet--a Jupiter-like giant
planet--and demonstrated that it has the scope and accuracy to monitor
thousands of stars at a time for the telltale dips in intensity indicating that
a planet is passing directly in front of the star. Researchers hope to monitor
60,000 stars and discover hundreds of planets including tens of Earth-like
planets. NASA's Kepler mission in
2008 will monitor 100,000 sun-like stars and probably find dozens of Earth-like
planets in the habitable zone.
Europe's
Darwin mission in 2014 will comprise several space vehicles "flying"
in an accurately fixed formation so that the light received by their telescopes
can be combined in such a way that the light waves "interfere" with
each other, the way light reflecting from the top and the bottom of an oil
slick interferes to make different colors. Around 2020, NASA will launch its Terrestrial Planet Finder
mission that will find stars with Earth-like planets, block the light from the
parent star, and observe only the detailed "spectrum" (the
"rainbow" of light and other radiations) from the atmosphere of each
planet. This mission will be able
to detect the signatures of life in the atmospheres of these planets.
What
will it mean to find life out there?
That depends on what kind of life it is. It's highly likely to be single-celled life, similar to
bacteria. Earth's history
indicates that single-celled life arises easily, but multi-cellular forms evolve
with great difficulty.
Single-celled life formed 4 billion years ago, right after rocks stopped
raining down on the primitive Earth.
It probably started from random reactions among chemicals prevalent at
that time, reactions that easily form the building blocks of life (amino and
nucleic acids) in today's laboratories.
But then biological evolution had to invent photosynthesis, and complex
single cells with nuclei to house the genetic material, and sexual
reproduction, before multi-celled organisms could appear. This took another 3 billion years.
Most
observers think there's lots of life among the other stars, but that very
little of it is complex life and that intelligent life might be extremely
rare. One evidence of the rarity
of intelligent life is that it's never visited here (lurid headlines about UFOs
notwithstanding), and another evidence is that we've searched part of our Milky
Way galaxy for radio signals from extraterrestrials and found nothing so
far.
The
discovery of extraterrestrial life, which could occur soon, will be a landmark
in human history. The discovery of
intelligent extraterrestrial life might never occur, but if it does it will go
down as our most momentous discovery and will surely change us in momentous
ways.