Wasn't it Eric Chomko who wrote:
Steve Willner ) wrote:
: In article ,
: (Eric Chomko) writes:
: My understanding is that when Jupiter and Saturn are near conjunction,
: the barycenter between them and the sun is about 100K miles away from the
: solar corona in the direction of the planets. I believe that Asimov
: discussed this in his book, "Jupiter".

: That's about 0.35 of a solar radius, which sounds about right. I
: haven't done the calculation.

Does that create a wobble similar to what we see from other stars?

Yes. The mechanism is exactly the same.



: Also, don't we know about the existence of planets outside the solar
: system due to the relationship between the barycenter of the star and its
: apparent motion related to it?

: Well, sort of. The thing that is actually measured is the radial
: velocity of the star. If we imagined that the whole effect above was
: due to Jupiter, the Sun would move 2.7 times its radius in about 6
: years (half Jupiter's sidereal period). (The barycenter stays
: "fixed," while the Sun moves from one side of it to the other.) For
: a distant observer located in the orbital plane, the average velocity
: seen would be 1.9E9 m/1.8E8 s = 10 m/s. (Of course the radial
: velocity won't be constant; it will follow a sine wave in the case of
: a single planet.) Measurement errors these days are of order 1 m/s
: (Any experts want to correct me on this?), so in the simplest case,
: Jupiter's effect on the Sun ought to be detectable by a distant
: observer after monitoring for a dozen years.

The smallest radial velocity I've seen for confirmed extrasolar planets
is about 8 metres/sec. It's one of the two "Neptune sized" planets
announced on 31-Aug-2004. The light curve graph showing the amount
of radial velocity is at
http://planetquest.jpl.nasa.gov/images/per2phasebl_CHART-simple_500x375.jpg

Before Tuesday's announcement, the smallest RV was about 20 m/s.



: There are, however, complications. On average, a distant observer
: will not be exactly in the orbital plane, and Saturn, with its 29
: year period, will make the motion more complex.

: You might want to plug in all the numbers and do a little simulation.
: Any spreadsheet program should suffice. Calculate the velocity for,
: say, every tenth of a year for 100 years, and plot the results. If
: you do this, you should check whether Uranus and Neptune will produce
: a noticeable effect. (Once you have done one planet, adding in more
: columns for more planets should be easy.)

Depending on the distance is key. 100s of LY away won't indicate anything
noticable.

For this purpose, the distance doesn't make much of a difference. All you need
is to have an apparent magnitude bright enough to be able to get a good
spectrum, and a planet that causes a wobble in the star that gives a change in
the star's radial velocity of a few tens of metres per second.

For example, HD330075, HD68988 and HD76700 are all over 160 light years away and
we've detected the wobbles caused by their planets.

--
Mike Williams
Gentleman of Leisure