Alert Notice 350: Exoplanet Transit Search For GJ 436

June 6, 2007

Object: 1137+27 GJ 436
    RA (2000)       DEC           V         B-V    V-Rc   Rc-Ic    V-Ic
11:42:11.58 +26:42:17.7  10.702  1.489  1.073  1.349  2.462


Dr. Greg Laughlin (UC Santa Cruz/Lick Observatory & has requested help in observing GJ 436 to look for transits of previously detected and undetected extrasolar planets.

A Neptune mass planet was recently discovered transiting this M2V class star with a photometric depth of 0.6% and a period of 2.64 days. This is the planet which has been mentioned in the popular press recently as a "hot water ice" planet because it likely contains water solidified by atmospheric pressure.

According to Dr. Laughlin: "The radial velocity data set for the star indicates that the transiting planet has a significant eccentricity. Given the fact that it has a Neptune-like composition, the tidal circularization timescale is quite short, and the presence of an eccentric orbit thus strongly indicates the presence of additional planets in the system. These can potentially be detected through transit, or alternately, and more provocatively through precise timing of the transits of the known transiting planet.

"This is probably the most important opportunity that has come around for small-telescope observers in a long time."


Since this is a red star and observations will have to both precise and accurate, careful photometry will be critical. Precision of less than 0.006 will not be easy. So please take your time and be as careful as possible. We recommend this project for experienced observers only. Consider this a challenge to push your system's capabilities.

Arne has proposed the following photometric procedures and guidelines fothis project:

"Since we are looking for extremely small transit depths, I'd suggest using ensemble techniques if your software permits. Use the set of 8  comparison stars for the field as your ensemble.

If you use the standard comparison and check scheme, the best stars look like two stars in the grouping just NNW of the target:

                                                       V        B-V    V-Rc   Rc-Ic   V-Ic
comp   11:42:28.04 +26:49:42.6  10.684  0.986  0.582  0.506  1.082
check  11:42:12.08 +26:46:07.4  11.370  1.071  0.562  0.503  1.060

These are about as close in color as possible to the target, but there will still be systematic differences between observers if data is not transformed. Beware that the 11.370 star has a fainter companion about 15arcsec to the NW - try to use an aperture small enough to exclude this companion.

GJ436 is near 11th magnitude, so exposures will be relatively long in comparison to some of the other transiting targets. Scintillation will not be as important. If you can do two filters, it is strongly advised to do so in order that your data can be transformed. Choosing B and V would be your best choice, as B is less influenced by molecular lines and, since GJ436 is fainter in the blue, will enable longer exposures so that scintillation is even less important. Remember that exoplanet transits are 'grey', meaning there is little advantage for any specific bandpass in maximizing transit depth. Instead, use filters to: standardize your observations; cut down the amount of light so that you can use longer exposures; and to remove the influence of spectral features like molecular absorption lines that will plague unfiltered measurements.

We will provide instructions as to how to determine your transformation coefficients in the near future."

A comparison star sequence w/BVRI photometry has been added to the Variable Star Plotter. Visit the URL below and use "GJ 436" as the star name. Note that GJ 436 is a high proper motion system, so the "dot" to the immediate northwest of the crosshairs is the position of GJ 436 when the NOMAD database was created. Thus the dot is not real and GJ 436 is not a double star.


An ephemeris for transits is located at the URL below. However, we need observations at all times, not just during transits. So please don't restrict your observing to transit windows alone.


It would be most useful if we had many observers working on the same evening. If you can, post your observing session dates and times to the AAVSO-Photometry Discussion Group and try to schedule together. We will keep an eye on the incoming data and post periodic updates and light curves to the same discussion group.


There is an opportunity for non-observers to also help out with this campaign. Dr. Laughlin and his team have developed a tool that does characterization and dynamical analysis of published datasets for known extrasolar planets. Volunteers are needed to work with the data of this and other extrasolar planetary systems. The software, called Systemic Console, runs on Windows, OS X and Linux with a java-based GUI. Info is here:


This campaign will be coordinated by: A. Price.



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