Hunting For Other Planets

Out of curiousity in regards to the search for Extrasolar planets, why
are planet hunters studying worlds hundreds of light years away when we
have several star systems in our own back yard that don't seem to get
much attention, and in theory wouldn't their closer proximity make them
easier to search for and study possible planets?

Firstly we have the Alpha Centauri system a binary system and to my
knowledge planets have been already confirmed in binary systems, we
have Barnards star, Sirius, Fomalhaut, etc.

It's not that they focus on stars hundreds of light-years away as that
there are simply a lot more stars hundreds of light-years than there
are close by.

The fraction of stars that exhibit detectable symptoms (detectable with
current technology, at least) of being surrounded by planets is not a
large one. Suppose it's 5 percent. That means that out of every 20
stars, just one has a detectable planetary system. This figure is
roughly (though not exactly) the same for a nearby star as it is for
a distant star. The reason is that detection depends on detecting the
Doppler shift on a wobbling star, which only requires a good spectrum
(and one can get that from quite a ways away), or on detecting the
dimming of light as a planet transits the star, which only requires a
good photometry reading (and again, one can get that from quite a ways
away).

There are 20 stars within, oh, maybe 15 light-years of the Earth? I
forget what the right figure is. It doesn't matter too much. But let
us assume that there are 20 stars whose distance between from the Sun
is between 0 and 15 light-years.

Assuming the same density of stars, the number of stars whose distance
is between 100 and 115 light-years--the same range of distance, mind
you--would be about 150 times as much, or 3,000 stars. Of those,
therefore, about 150 would have detectable planetary systems. You can
work out the figures for both larger and smaller distances--it goes up
as about the square of the distance.

It's therefore not surprising that the vast majority of detected systems
are at large distances. If and when it comes to *imaging* a planet
directly that we've discovered by other means, we will of course prefer
nearby systems to distant ones. However, keep in mind that since
detectable systems are few and far between (relatively speaking), even
the nearby imageable ones may be a few tens of light-years away.

Brian Tung
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