Paul Shankland has been visiting Santa Cruz this week, and everyone agrees that it’s about time to get the GJ 876 transit situation sewed up once and for all. Aquarius is still up in the early evening, and a planet “c” transit opportunity is bearing down with 30.1 day semi-clockwork precision. So out went the following alert to the transitsearch.org e-mail list:

Thursday Afternoon, Nov. 17, 2005

Dear Transitsearch Observers,

We’d like to alert you to an opportunity to check the GJ 876 system for planetary transits. Photometry is desired during a twelve-hour window centered on JD2453693.491 (Friday Nov. 18, 23:47 UT).

As you have likely heard, the GJ 876 system was recently found to harbor a low-mass (7.5 Earth Mass) planet on a 1.94 day orbit. The new planet is referred to (rather prosaically) as GJ 876 “d”, and is the third planet detected in the GJ 876 system. The discovery paper is scheduled for an upcoming issue of the Astrophysical Journal, and is also available on the astro-ph preprint server: http://arxiv.org/abs/astro-ph/0510508

Sadly, transits for planet “d” have been ruled out to high confidence.

As a result, however, of (1) inclusion of the third planet in the dynamical model for the system, and (2) a large number of new high-precision radial velocities, Eugenio Rivera has produced new transit ephemeris predictions for the outer two planets in the GJ 876 system. These differ by several hours from the dynamical predictions that are currently posted on the transitsearch.org candidates site, e.g.:

http://www.ucolick.org/~laugh/GJ876____c.transits.txt

We’re working through an extensive analysis which shows that neither “b” nor “c” is transiting, but this analysis is nevertheless in great need of observational verification. There is a conflict between dynamical fits to the radial velocities (which indicate that the system is inclined by 50 degrees to the plane of the sky) and the results of Benedict et al (2002, ApJL 581, 115), who used HST to get astrometric measurements that suggest a nearly edge-on configuration.

We’d thus like to request photometry of the star to six hours on either side of JD2453693.491 (Friday Nov. 18, 23:47 UT).

Information regarding observing GJ 876 and photometry submission instructions are at:

http://www.aavso.org/news/ilaqr.shtml

Additional background is on the transitsearch GJ 876 results page:

http://www.ucolick.org/%7elaugh/GJ876____c.results.html

Note that this page states that the photometric campaign is over, but the new dynamical model indicates that more photometry is desirable.

Other observing opportunities are (were) as follows:

For planet c (the middle one):

predicted central transit (UT)
————————————
2005 Aug. 20 15:40
2005 Sep. 19 18:41
2005 Oct. 19 20:53
2005 Nov. 18 23:47
2005 Dec. 19 01:36

For planet b (the outer one):
————————–
2005 Aug. 22 17:28
2005 Oct. 22 17:34
2005 Dec. 22 18:05

Finally, we’d like to thank everyone for being patient over the 8 months, during which we have not been running coordinated campaigns. With Shankland of USNO “on the bridge”, we’re now ramping up for a more active phase. Stay tuned!

After using the console for a while, you’ll notice that it’s often easy to find a reasonably good (say, chi-square of 3-5) multiple-planet fit to a given radial velocity data set. This rule of thumb tends to be especially true if you allow the planets to have large eccentricities. But how does one know whether the fit is likely to be correct?

This is one of the questions that the systemic simulation is designed to answer.

Most of the time, however, if a fit contains large enough eccentricities for the planet orbits to cross, then the trial system will be dynamically unstable. That is, the planets in the model will suffer a close encounter, which is generally followed (or directly accompanied) by a disaster. The planets collide, or one or more of them is ejected, or one of them is thrown into the central star.

While it is certainly true that such catastrophes have been reasonably common throughout galactic history, it is exceedingly unlikely that any particular planetary system that we observe will be on the verge of a dramatic instability. The stars that can be observed using the Doppler radial velocity method are billions of years old. If a star had an unstable planetary system, it is likely that the instability either occurred long ago, or that won’t happen for a long time to come.

As a result, an important requirement for any radial velocity fit is that it correspond to a dynamically stable system. Traditionally, this can be checked either by integrating the system forward in time, or by applying a technique which checks for the presence of chaos in the orbits. (Indeed, all of the planetary systems that underlie the systemic database have been integrated for one million orbits prior to being “observed”. These pre-integrations establish a strong likelihood of short-term dynamical stability for all the systemic systems.)

Here’s an idea that sounds possibly promising. If the radial velocity waveform of a planetary system is converted into an audio signal, is it possible for the human ear to rapidly detect whether a system is likely to be unstable? To test this, we’re working on bringing an audio generator into the systemic console.

More generally, what do the extrasolar planetary radial velocity reflex waveforms sound like? The short answer is, they sound terrible. There are interesting reasons for this, which we’ll pick up in a future post. For now, have a listen to these .wav’s (created by Aaron Wolf) of two of the best-known multiple planet systems: GJ 876 and Upsilon Andromedae

And try to listen for the (heavily processed voice of the better-voice-of-the-two GJ876 in the forthcoming James Alley Remix).