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How do astronomers find exoplanets ?
The most common method to date for finding exoplanets is by the Doppler method,
which uses the Doppler shift of light. As a planet orbits its star, it moves the center
of gravity of the star system back and forth. By measuring that motion in the spec-
tra of stars, it is possible to deduce whether or not a planet is orbiting that star, how
massive it is, and what its orbital distance is.
Another method for finding exoplanets is looking for the shadow of a planet
moving across our line of sight to its star. This method will find fewer planets,
because such eclipsing binary exoplanets are very rare, but when they are found
astronomers can learn much more about the exoplanet than with only the Doppler
method. Parameters including size, temperature, chemical composition, and
atmospheric density are measurable using the shadow method.
Most astronomers feel that the best way to find exoplanets would be to take a
direct image of them. Unfortunately, that method is impossible with current astro-
nomical instrumentation because the host stars outshine the planets by such a huge
factor that it would be harder than finding a firefly in the beam of a searchlight. Sci-
entists are working diligently, though, to develop the technology that will allow us
to compensate for the effects of such a huge contrast level. Perhaps within a few
years, we will be able to look at a picture of an exoplanet in a distant star system.
How can interferometry be used to find extrasolar planets ?
Just as it is possible to use interferometry to obtain very detailed images of objects
in space, the interference patterns of light can be analyzed to take very detailed
spectra. The resulting measurements of Doppler shifts in the spectra—and hence,
the motions of the objects producing the shifts—can be extremely precise. With
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By analyzing changes in spectra around a star, astronomers can detect whether a Jupiter-sized planet is orbiting close by.
( NASA/JPL-Caltech/R. Hurt )
LIFE IN THE UNIVERSE

current technology, for example, it is actually possible to measure changes of speed
as little as a person running or jogging at distances of hundreds of trillions of miles!
As it turns out, this is the level of speed changes that large planets exert on the
stars they orbit. If, for example, the gas giant planet Jupiter orbited the Sun at the
distance of Mercury, the Sun would wobble back and forth, changing the direction
of its motion every few weeks by an amount similar to a running person. By meas-
uring the spectra of nearby Sun-like stars, and using interferometry to detect the
minuscule changes in their speed, it is possible to detect and confirm the existence
of planets orbiting around them. Hundreds of extrasolar planets have been detect-
ed in exactly this way, all of them orbiting faraway stars.