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Optical
image of the Keyhole Nebula (bottom) within the larger Carnia
Nebula.
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Of
all the issues that the cosmos poses for humanity, one of the greatest
and most resonant focuses on the search for Earth's cousins, planets
that offer a world on which life can both originate and flourish.
The past half decade has taken us from almost total ignorance of
planets orbiting other stars to knowledge of the existence of more
than 70 such planets. All of these extrasolar planets have masses
comparable to the masses of Jupiter and Saturn, more than 80 times
Earth’s. Their large masses allow the planets to produce detectable
gravitational pulls on their starry masters. Only by observing these
tugs have astronomers been able to deduce the planets' existence
and their masses. Finding planets with masses comparable to Earth's,
either by indirect methods or by direct observation, will require
observational systems far superior to those so far devoted to the
search for other worlds.
Most
of those efforts will require space-borne interferometer systems.
Such systems combine the light and infrared waves collected by an
array of individual small telescopes to achieve the resolving power
of a single giant telescope whose size equals the size of the array.
Radio astronomy has long employed interferometer systems to achieve
high resolution. Today astronomers are making rapid progress to
create optical interferometers for the infrared and visible-light
spectral domains. The Space Interferometry Mission (SIM), now under
development for launch in the second half of the decade, will demonstrate
the feasibility of space-borne interferometer missions to observe
visible-light and infrared radiation. But SIM will still enable
discovery of planets not by direct observation but indirectly, through
highly accurate measurements of the displacement of stars' positions
on the sky.
To
make direct observations of Earth-sized planets around other stars
will require an interferometer system with more powerful telescopes,
capable of maintaining their separations at distances sufficiently
large to provide much finer angular resolution than SIM's. By the
end of this decade scientists should be poised to develop an interferometer
array capable of direct detection of Earth-like planets around nearby
stars — the Terrestrial Planet Finder (TPF).
Analysis
of the infrared radiation and visible light from any such planets
can reveal telltale signs of atmospheric gases. Oxygen or ozone
would signal the possible existence of conditions favorable to aerobic
life forms. Methane in the presence of oxygen would suggest the
actual presence of life on these faraway worlds.
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