Re: Solar System around 55 Cancri

On Fri, 09 Nov 2007 13:23:36 GMT, Rumpelstiltskin
<PleaseDoNotReplyByEmail@xxxxxxxxxxx> wrote:

<snip>

We can tell the mass of the planets from the mass
of the star,


That should have been "We can tell the mass of the
planets from the mass of the star and the amount of
wobble,"

In fact, there is uncertainty because we don't
know the angle of the systems with respect to us.
We can't yet see the star move from side-to-side,
burt we can detect doppler shifts in the star due
to variations in motion toward or away from us.
Therefore the mass of the planets is an
approximation based on an assumption that we're
seeing the systems edge-on, which only gives us
a minimum mass for any planets. If a planetary
system was face-on with regard to us, we wouldn't
see any doppler shift in the star at all so we
wouldn't know there was a planetary system
there. Given a large enough sample, we can
assume that on average we would see the
systems at a 45 degree angle which would
mean we'd underestimated the size of the
planets by about 30%, but since we can't see
shifts in face-on systems at all, those systems
are excluded from the sample, so our average
is a little better than that.

There are, I think, a couple of cases with
small stars and huge planets where we actually
can "see" the planet, in the sense that if we
can add up a lot of observations, then the
planet will show up. I think police surveillance
can produce better pictures of criminals on
low-resolution videos than can be seen from
any one single frame, by adding up the
face of the criminal in a lot of individual
frames, in the same way.



.

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