Alert Notice 370: GJ 436 Extrasolar Planet Transit Campaign

February 12, 2008


Observations of the extrasolar planetary system around GJ 436 are requested to help explain anomalous changes in the transiting properties of a "hot Neptune" type planet (GJ 436b). These perturbations manifest themselves in changes in the larger planet's transit properties. Specifically, the transit time, duration and amplitude may be affected. So far, only transit timing variations (changes in the transit  midpoint) may have been detected.

A recent paper published to ArXiv by Ribas, Font-Ribera and Beaulieu suggests that a previously undetected 5-Earth mass planet may exist elsewhere in the GJ 436 system and that its gravity may be the cause of the anomalies. If the smaller planet does exist, it would be the smallest planet yet detected in a solar system around a main sequence star.

However, that is still a big *if* as there are possibly other astrophysical explanations as well. The theory is provocative enough that it should be checked. The key will be to get large amounts of quality data to confirm or negate this dual planet hypothesis.

This system is not as bright (V=~10.7) and the expected transit length (~1h) is not as long as other extrasolar transit campaigns we have conducted. So this may be a good entry point for those who have not participated in an extrasolar planet campaign before. However, the transit depth is still very low (<.01 mag) so it will be a challenging system. Please follow our photometric guidelines (below) carefully and take your time when reducing the photometry.

Dr. Greg Laughlin ( and University of California, Santa Cruz) has analyzed recent data and measurements from both observers and the literature to come up with the following transit period and ephemeris:

 P = 2.643901 days
T_mid = 2454222.6157 HJD (02:46:36.48 UT May 2, 2007)

Dr. Laughlin has agreed to list anyone who contributes quality data of a transit as a co-author of any paper that may result from this campaign.


Photometric observations of future transits of the system are requested. 

The next transit is scheduled for February 14, 2008 from 06:49-07:47 UT. The actual transit may occur 10 to 15 minutes later. Transits occur about every 2.64385d. A list of subsequent transit dates can be found here:

GJ 436 is located at R.A. = 11:42:11.1, Dec. = +26:42:24 (2000). Finder charts can be made using the Variable Star Plotter ( and using "GJ 436" as the star's "name". Please submit your data to the AAVSO using the same name.

The GJ 436 M3V parent star is very red. As a result, it will be very important to submit your airmass values. They can be computed with our Airmass calculator at .    [broken link]

A spectra of the object from Maness, H. L., Marcy, G. W., Ford, E. B. et al. (2007PASP..119...90M) has been posted at this URL:

GJ436 itself has these magnitude and colors:
        V     B-V    V-Rc   Rc-Ic   V-Ic
     10.702  1.489  1.073   1.349   2.462

Note that it is intrinsicly very red, so the spectrum is full of molecular bands, and the star is *much* brighter redwards of V (Ic=8.240). The comparison stars in the field are typically much bluer than the target star (the reddest has V-Ic = 1.08). This is good, in that the comparison stars are unlikely to be variable; it is bad, in that everyone's photometry will be on a different "system" unless transformed, and second-order extinction will be important.

The bottom line is that you are probably better off not transforming your data, but be sure that you submit long time series so that we can detrend the photometry for airmass effects, and offset your results so that your data will be on the same scale as other observers. Luckily, the transits are short so you are likely to cover an entire transit and data from two or more observers will not be necessary to get ingress and egress timings. Don't just observe the hour-long transit; be sure you have a long lead-in and exit segment for our detrending needs.

Your best bet is to observe filtered, and with B as your first preference and V as your second preference. B is less affected by molecular absorption, and the star will be fainter at B. There are two advantages to this. First, being fainter means you can take longer exposures, which will reduce the effect of scintillation. Second, and perhaps more important, there will be a smaller magnitude difference between GJ436 and its nearby comparison stars. This means that an ensemble of these comparison stars will yield better results. V-band is a good second choice, but do not go any redder than this.

With a 0.006 magnitude transit depth, every small error gets magnified. Even at 8 micron (SST) depth is 7.3 +/- 0.3 mmag, with no dependence upon filter (so far). When possible, observe when the transit is expected to occur near meridinal crossing so that airmass changes are minimized. Use large apertures so that centroiding of stars does not cause a measurement error.  Get lots of signal/noise in your flats (1 million electrons per pixel is the minimum you should have, usually from a large stack of flats). Try to keep the field in the same spot on your chip (autoguiding really helps for this project). Use ensemble techniques for the photometry if at all possible. Keep accurate time, and make sure your submitted photometry uses the exposure midpoint for its time. I would suggest binning your data in chunks no larger than 30 seconds elapsed time to get good time resolution on your light curve.


Info on the system and on a recent, previous campaign we have run on GJ 436 is here:

Bruce Gary has created a web page with more detailed information on this system and some recent light curves:

The Ribas, Font-Ribera, and Beaulieu ArXiv paper is available at:

Please consult the AAVSO-Photometry Discussion Group for questions, advice and updates regarding photometry of this system. The URL for archives and subscription is:   [broken link]

This Alert Notice was written by A. Price & A. Henden.



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