Curtis Johnson writes:

>MR> What is interesting about the Upsilon Andromedae is planets of great
> MR> mass apparently similar to Jupiter but much bigger are orbiting close
> MR> to the sun in the Upsilon Andromedae system. This defies conventional
> MR> wisdom that such gaseous planets with ice could only form and be in
> MR> orbit a great distance from a sun. Perhaps not all suns are the same.
>
>One should keep in mind that there still is a bit of a selection
>effect in that large planets close to a star are exactly the ones that
>would exert a large gravitational pull to give the "wobble"
detectability.

There's not a merely "a bit of selection" in the current
discovery mechanism
for extrasolar planets, there's an enormous selection coefficient inherent to
the process. Only those planets large enough and close enough into their suns
will produce a significant enough wobble on short enough time scales to be
readily discovered by present means.

At the moment, the detection of earth-scale planets at earth-like distances is
completely beyond our technology. The minimum doppler wobble shift we're able
to detect at the moment is about 3 m/s, although that might be improved to 1
m/s in the future. Nonetheless, the earth imposes only a 0.1 m/s wobble on our
sun -- and of course, that wobble is superimposed on all of the other planetary
induced wobbles. Detecting much lighter Mars-like planets at Mars-like
distances (1.4 AU) is simply all that much more difficult.

For more information concerning detection thresholds, see:

    http://star-www.st-and.ac.uk/~pja3/as4004/planets01.pdf

Doppler velocity curves are given at the back of the slide set. As you'll see,
it was no wonder that 51 Pegasi was the first extrasolar planet discovered with
a 50 m/s, 4-day period wobble.

Wirt Atmar
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