At 19:35 06-01-01 -0500, you wrote:
>Hello all! I've been living a non Internet life for a month now. It's nice
>to check your mail and see 5k+ messages. (I also came into my house today
>after being away for ten days and found about 15 cm of water in my basement.
>It looks like it was twice as deep from the water stains. And I can't see
>where the water came from. Oh well the joys of home ownership.)
Welcome to the club. Thankfully, I have no basement to flood. Plenty of
other things to deal with, though.
>I have some questions that someone here could probably answer quickly, with
>out me doing any real work ;-)
>
>With the new planets being discovered, have they all been in basically the
>same axis as our solar system? How is our system orientated with respect to
>the rotation of the galaxy? 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?
>
>Just having fun.
>
>Hope everyone has a happy new year.
>
>Kevin Tarr
>Trump high, lead low.
Yes, the Doppler shift method only detects the component of the motion that
is in the line of sight (directly toward or away from us, called "radial
velocity" by astronomers). If we see the planet's orbit exactly face-on,
there would be no line-of-sight component to the velocity, so such a planet
would be undetectable by this method.
As it is, we can't compute the mass of the unseen body (planet, brown
dwarf, small star) from the radial velocity variations of the star unless
we have other information that tells us what the inclination of the orbit
is (e.g. in the case of the planet that was observed to pass in front of
its star back in 1999, for more details see
<http://antwrp.gsfc.nasa.gov/apod/ap991115.html>). What we can do is to
calculate the mass of the unseen body assuming that the orbit is edge-on,
which gives us the smallest possible mass of the unseen body.[1] It's
possible, then, that a brown dwarf or even a low-mass red dwarf in a nearly
face-on orbit would create no more Doppler shift than a Jupiter-class
planet in an edge-or orbit. Or, we can get a kind-of "likely" value for
the mass by assuming a value of 45° for the tilt of the orbit (halfway
between edge-on and face-on), in which case we are assuming that the tilts
of the orbits are randomly distributed.
This may provide some sort of answer to your question -- that we don't have
enough information about the orientation of orbits to assume that there is
any pattern to them. This seems to be true for the orbits of binary stars
where we can observe both components and so figure out the inclination of
the orbit, and at the moment there is no particular reason to assume it
would not be true for planetary orbits.
(Another interesting but so-far unanswered question would be whether the
orbits of planets around a star or the orbit of a binary companion would be
more or less in the equatorial plane of the star's rotation: the average
plane of our solar system is tilted at an angle of about 7° to the solar
equator, IIRC, and the moons of most planets in our solar system tend to
orbit in or near the equatorial plane of the planet. While it is possible
in some cases to determine the direction of the rotational axis of another
star, hence it's equatorial plane, we don't have that information about
many stars, so again, as far as we know, the inclination of a star's axis
can take on any value from equator-on orientation to pole-on with equal
likelihood.)
[1] This situation also arises in the search for stellar-mass black holes
in binary systems: when we find a binary system in which the unseen
component has a mass of at least 3 solar masses, it's a possible black
hole, since the upper limit on the mass of a stable neutron star is
somewhere between 2 and 3 solar masses (the Oppenheimer-Volkloff limit,
corresponding to Chandrasekhar's limit of 1.4 solar masses for the maximum
mass of a stable white dwarf -- the exact value of the O-V limit is harder
to determine because a degenerate neutron gas is even further from what we
can study in the lab than the electron degenerate material in a white dwarf).
HTH.
-- Ronn! :)
----------------------------------------------------------------------------
-- Ronn Blankenship
Instructor of Astronomy/Planetary Science
University of Montevallo
Montevallo, AL
Standard Disclaimer: Unless specifically stated
otherwise, any opinions stated herein are the personal
opinions of the author and do not represent the
official position of the University of Montevallo.
-----------------------------------------------------------------------------
