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