Download 04 Aug 2007

MODERN TIMES
Art Hobson
[email protected]
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 Earthdays, 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.