Figure 8.12. How nulling interferometry works.

composition is life. Plants generate 02 through photosynthesis, and animals generate CH4 as a by-product of digestion. As far as is known, there is no other natural way of generating such an atmosphere, and thus if a similar atmosphere could be detected about an extrasolar planet, it would provide strong evidence that life had independently evolved there. Such a conclusion would be profound indeed.

A precursor to the TPF is the Space Interferometry Mission (SIM), which will use optical interferometry to determine the positions and distances of stars several hundred times more accurately than has previously been possible. The predicted accuracy will allow SIM to determine the distances to stars throughout the galaxy and to probe nearby stars for Earth-sized planets by observing the "wobble" in the star's position as its planets orbit around it. SIM will use three co-linear interferometers mounted on a 10 m long boom, and will be placed in an Earth-trailing heliocentric orbit. The interferometer mirrors will be 0.3 m in diameter; SIM is designed to last for 5 years.

Unfortunately, the TPF mission is currently in an indefinitely deferred status and the launch of its precursor mission, SIM, has now been delayed until 2015-2016 at the earliest.

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