Observing Campaign: Exoplanet Transit Search for GJ 436



June 6, 2007


Artist Conception
An artist's illustration of GJ 436b (NASA)

From AAVSO Alert Notice 350:

Dr. Greg Laughlin (UC Santa Cruz/Lick Observatory & Transitsearch.org) 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."

The full AAVSO Alert Notice 350 announcement


Geir Klingenberg (KGE) observed a transit on May 31, 2007.Full report and details here (updated!)  [broken link]   . This observation has been confirmed by Dr. Laughlin.

Charts & Additional Info:

Photometry Guidelines

Since we are looking for extremely small transit depths, please use 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      V-B   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.

Light Curves

Our first data set, from Bill Goff (GFB)

We will post light curves as data comes in.