  GJ 436b is a warm-- approximately 800 K--extrasolar planet that periodically
eclipses its low-mass (half the mass of the Sun) host star, and is one of the
few Neptune-mass planets that is amenable to detailed characterization.
Previous observations have indicated that its atmosphere has a methane-to-CO
ratio that is 100,000 times smaller than predicted by models for
hydrogen-dominated atmospheres at these temperatures. A recent study proposed
that this unusual chemistry could be explained if the planet's atmosphere is
significantly enhanced in elements heavier than H and He. In this study we
present complementary observations of GJ 436b's atmosphere obtained during
transit. Our observations indicate that the planet's transmission spectrum is
effectively featureless, ruling out cloud-free, hydrogen-dominated atmosphere
models with an extremely high significance of 48 sigma. The measured spectrum
is consistent with either a high cloud or haze layer located at a pressure of
approximately 1 mbar or with a relatively hydrogen-poor (three percent hydrogen
and helium mass fraction) atmospheric composition.
