Re: Solar System around 55 Cancri

On Fri, 9 Nov 2007 16:04:19 -1000, "Alvin E. Toda" <aet@xxxxxxxx>
wrote:

On Fri, 9 Nov 2007, Rumpelstiltskin wrote:

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.

I agree. I seen more stuff on this in the news. Looks
like they have been measuring the wobble for many
years-- as much as 10. This with the doppler shift
data-- although I don't remember hearing about that--
would give them a pretty precise idea of the planets
esp if we were looking on the sun from the north or
south poles.


Actually there wouldn't be any doppler shift if we
were looking at the system pole-on, because the star
would be pulled in a plane orthogonal to our line of
sight, but it would stay the same distance from Earth
although it moved around in that plane. We probably
couldn't detect that sideways motion unless the star
was extremely close, since the motion is not very
much. If the star moves in and out along our line of
sight though, because we're looking at the system
edge on, then we can see variations in the
wavelengths emitted by the star as it's pulled
forwards and backwards by its planets along our line
of sight, as a planet orbits from in front of the
star to behind it. We can see that shift in the
fraunhofer lines in the stars radiation.

For an example of the above, according to:
http://exoplanets.org/doppframe.html
Jupiter pulls the sun at 12 miles per second.
The speed of light is 186,000 miles per
second. If we were on another star looking at
our solar system edge on, we could easily
detect the 24/186000 difference in the
spectral lines from the sun as Jupiter pulled the
sun first toward and then away from us. That's
regardless of how far away we are.

That's assuming the planets are all more or less
in the same plane, but theory says that's how they
should form and that is in fact the way ours are.
Pluto is tilted a lot more than the others, but it's
not much of a planet. Something happened to
Neptune, because Neptune is nearly tipped
over on its side, such that each hemisphere gets
six Neptune-months of daylight, which would be
82 earth-years of daylight followed by 82
earth-years of darkness, most latitudes, if you
could live on Neptune. Nevertheless, even
whatever happened to Neptune to tip it on its
side still didn't move it out of the plane of the
Solar System. So we can make a working
assumption that other solar systems are
probably in a plane, as ours is.

(Note: though Jupiter and the sun are pulling
each other, they don't crash because they're
in orbit, so the pull on the sun when Jupiter is in
a given position is balanced six Jupiter-months
later when Jupiter pulls the sun in the opposite
direction to "back where it was before".)

.

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