Amateur Observations of exoplanet GJ 436 b

The discovery of transits by exoplanet GJ 436 b was announced yesterday, by a team of Swiss astronomers, in an Astronomy & Astrophysics manuscript by M. Gillon et al. The planet has a mass of 23 Earth-masses and an orbital period of 2.64385 days. It orbits a red dwarf star 33 light years away. The temperature on the planet is around 600K. The transit depth is 0.6%, implying a radius of approx. 25,000 km, almost the same as the radius of Neptune. 

GJ 436 is located at R.A. = 11h42311s and Decl. = +2642'23" (J 2000.0). This is a star of mag V = 10.6. Photometrists are quite lucky by the presence of V = 10.61 and V = 10.54 comparison stars, in the immediate vicinity of GJ 436.

Rather unexpectedly, the skies cleared out on May 17/18, 2007 over CBA Belgium Observatory, shortly before the start of a transit of exoplanet GJ 436 b. Photometric conditions were not ideal (bit of haze), but I started a photometry session using a 0.35-m f/6.3 telescope and SBIG ST-7XME CCD camera with V filter. Below is the resulting light curve, probably the first one of GJ 436 b by an amateur astronomer.

The times of predicted ingress and egress are indicated (taken from The actual transit started a bit later than predicted and its duration was also a bit shorter than predicted. This is in agreement with the fact that the prediction was made for a central transit, which is not the case for GJ 436 b. Based on my observations, I found following data :

- ingress : 2007 May 17 at 23h01m UT
- egress : 2007 May 17 at 23h53m UT
- mid-transit : 2007 May 17 at 23h28m UT
- duration : 52 m
- depth : 0.007 mag 

Note also the strange "dip" in the lightcurve near JD +0.41, for which I have no direct explanation.





Copyright © 2007 - Tonny Vanmunster.