What is an interferometer? How are they used to find planets?

Hi Chris,

There are several large optical astronomy interferometers in
the planning stages around the world, though none is working yet. The
idea of an interferometer is that the bigger the telescope, the finer
the detail you can make out. If you can't afford a big enough
telescope, gain limited ability to make out fine detail by making it
big only in one direction: combine two smaller telescopes spaced far
apart. Many radio telescopes, such as the Very Large Array in
Socorro, New Mexico, use this principle.

The problem with a telescope like this when you want to find
planets is that compared with the stars they're orbiting, the planets
are very, very faint. Instead of looking for the planet directly, you
might use an interferometer to measure the star's position against
background stars with minute accuracy, and see whether it moves from
side to side under the influence of the planet's gravity. (This is
different from the recent discoveries, where Marcy's team detected the
Doppler shift due to the star's motion towards us and away). But the
side-to-side motion is very small even for planets the size of
Jupiter, so you need a huge interferometer.

My guess is that the NASA interferometer may start finding
planets in a decade or two.

All the best,
-Neal.

Neal Turner - Lick Observatory - Astronomy & Astrophysics - UC Santa Cruz
neal@ucolick.org W/408/459-2267 H/408/458-9235 Kerr 407
http://www.ucolick.org/~neal/home.html

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