David Buck wrote:

> Um, yea :-).
> 
> The star (Delta Pavonis) is about 20 lightyears from our Sun.  I don't 
> think a planet could orbit a star at a distance of 20 light years.

v^2/r = Mg/r^2 relates orbital speed to distance and solar mass.  v = 2 pi r / tau,
where tau is the orbital period, so 4 pi^2 r / tau^2 = M g / r^2 =>

tau = 2 pi * sqrt(r^3 / M g)

The Earth's orbital radius is 1.58e-5 light years.  20 light years is a factor 1.26e6
larger.  For comparable solar mass, this would yield an orbital period of 1.4 billion
years.  The age of the universe is 13.7 billion years.  So classically there's been
time for at least a few such orbits...

The problem is there could be no massive objects of comparable mass to the star within
the neighborhood defined by the length scale of the orbital radius, for example within
100 light years, or the orbit wouldn't be stable.

Dan

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