Other Discovery Techniques
The Doppler technique is by no means the final word on how to find and
study extrasolar planets. The human
imagination is fertile, and people have come up with a wide variety of other
ways to approach the problem, some new and some old. I will not mention photometric searches for transits by planets,
or photometric surveys for microlensing events by planets, or the ambitious
talk about using a nulling interferometer in space to cancel out the glare of
the parent star in order to make a one-pixel image of the orbiting planet, so
that its light can be fed into a spectrograph to see what kind of an atmosphere
it has. However, I do want to
acknowledge the several plans for space astrometry missions, because they show
great promise for planet searches starting just a few years from now.
The inspiration for several of these missions came from the remarkable
success of Hipparcos, a European mission to survey more than 100,000 stars with
an astrometric accuracy of 1 or 2 milli-arcseconds. Although this performance is impressive, it is barely adequate to
detect the presence of brown-dwarf companions, failed stars just below the
substellar limit. Two follow-up
missions have been proposed for the near future, DIVA in Germany and FAME in
the United States. These missions would
use modern CCD detectors to extend the Hipparcos survey to much fainter stars,
in much larger numbers, and with much better astrometric precision. These missions would be able to explore the
transition region between brown dwarfs and giant planets.
GAIA is a much more ambitious European successor to Hipparcos, and aims
to reach the astonishing astrometric accuracy of 4 micro-arcseconds, even for
very faint stars. That angle is about
the size subtended by a dime seen at the distance of our moon. GAIA should be able to explore extensively
the realm of giant planets orbiting a wide range of stars. GAIA is a candidate for the European Space
Agency's 2009 Cornerstone Mission.
In the United States, NASA is well along in its planning for the Space
Interferometry Mission (SIM), which adopts a rather different strategy of
pointing at specific targets rather than scanning the entire sky. This should allow SIM to achieve better
astrometric accuracy and to reach fainter stars than any of the other proposed
missions, but at the cost of a much smaller target list. SIM may even be able to achieve the
exquisite accuracy needed to detect the reflex motion of the nearest stars in
reponse to the pull of terrestrial-sized planets. Thus there is a reasonable prospect that in my lifetime we will
discover the first extrasolar planet like the earth.
It is an exciting time for extrasolar planet
research, both observationally and theoretically, and inevitably this will have
an impact on our thinking about the prospects that intelligent life may have
arisen elsewhere in the universe.
Contributed by: Dr. David Latham