Everyone would like to see an image of an extrasolar planet. So far, the radial velocity detection method has only detected giant planets indirectly by measuring the wobbleof the planet's parent star toward or away from us. Astrometry also measures the wobble of the star in the other twodirections not covered by the radial velocity method. The pulsar and eclipsing binary methods also measure the offsetof stars by timing the "clocks" of pulsar pulses or binary eclipsesat minimum, respectively. While thephase variation method measures light directly from a planet, the light ismixed in with the much greater light contribution from the planets star. Finally, though the photometric transitmethod also is an indirect detection technique, it actually measures the shadowof the planet.

There are plans for two missions within the next decade: the Stellar InterferometryMission (SIM) and the Terrestrial Planet Finder (TPF). These missions propose using interferometryto very accurately measure the positions of stars, and find planets aroundthem. They plan to fly multipletelescopes at different distances and, like a pair of binoculars, combine thelight incident on each telescope to synthesize an equivalent aperture the sizeof their distances. While the mirrorarea of a telescope determines how much light it gathers, it is the baselinesize of the telescope that determines its resolving power (i.e. ability torender visible objects that are far away).

Extensions of present plans for such systems include concepts for a series of 25 40-metertelescopes that would be separated in orbit by hundreds of kilometers. They would nevertheless have to"know" each others positions within microns to re-construct imagesof even giant planets around other stars. But such second-generation systems should be able to image an"Earth" around another star system within 15 parsecs (about 50 lightyears away).

Suchsystems could thus allow the spectrum of extrasolar planets to be examined forsigns of exobiology. Biology on Earthproduces a number of signatures in the atmosphere, but many are highlyambiguous. Early observations of Marssaw seasonal variations in the darker patches. These were thought to be signs of vegetation, similar to seasonalforests or plants growing on Earth. However, the seasonal migrations of these dark patches turned out to besands shifting with the seasonal windstorms.

Aless ambiguous sign of exobiology on remote extrasolar planets would be thepresence of free oxygen in the atmosphere. Oxygen is so reactive that it should not last long free in theatmosphere. It is a major constituentin our atmosphere because plants produce it as a by-product ofphotosynthesis. The strongest oxygenline is actually caused by ozone, the triple-atom state of free oxygen. Such detection would be very strong evidencethat forests of some sort (kelp, trees, bacteriawhatever wasphotosynthesizing) were present.

Inthe next article, which will be the last in the extrasolar planets detectionseries, we will discuss the method of radio detection.