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. =
+26°42'23" (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 Transitsearch.org). 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
Transitsearch.org prediction was made for a central
transit, which is not the case for GJ 436 b. Based
on my observations, I found following data :