Kevin Tarr wrote:
>
>With the new planets being discovered, have they all been in basically the
>same axis as our solar system?
>
Uh, dunno.
>How is our system orientated with respect to
>the rotation of the galaxy?
>
IIRC, the double stars and our solar system are randomly oriented wrt the
galactic plane.
>I think that the way they 'discover' other
>planets is from its sun's wobble towards and away from us as that planets
>circles the sun. Can we detect stars that wobble up and down yet, or in some
>way?
>
A few figures.
When these planets are discovered using doppler shift, it means that
it's the *velocity* vector, projected over the Sun - Star axis, that is measured.
This component of the vector [first order, neglecting the eccentricity of
the orbit] is:
Vz = - a sin(inc) cos(omega + M) 2 Pi / Period
where inc is the orbital inclination, omega is a constant angle,
Period is the orbital period, M is an angle that varies with the
orbital period, and a is the semi-major axis of the star's orbit
[which is *small*, because the star is much more massive than
the planet, since it's the semi-major axis of the planet's orbit
multiplied by the ratio of the two masses].
The big problem is:
(a) it's impossible to separate _a_ from sin(inc), and so it's
impossible to estimate the planet's mass. The best that can
be given is the lower limit for the planet's mass, because
|sin(inc)| <= 1
Notice that, in the case of binary stars where both stars are
visible, we may know their masses [using the spectre, and
then applying some formulas that were derived from the
masses - yes, it's a circular logic :-)] so we can determine
the inclination. Even then, it's still impossible to *exactly*
determine the orientation of the orbit, because there is
still one parameter that can't be measured: *where* does the
orbital plane cross the x-y plane.
Alberto Monteiro
