  Helium is the second-most abundant element in the Universe after hydrogen and
is one of the main constituents of gas-giant planets in our Solar System. Early
theoretical models predicted helium to be among the most readily detectable
species in the atmospheres of exoplanets, especially in extended and escaping
atmospheres. Searches for helium, however, have hitherto been unsuccessful.
Here we report observations of helium on an exoplanet, at a confidence level of
4.5 standard deviations. We measured the near- infrared transmission spectrum
of the warm gas giant WASP-107b and identified the narrow absorption feature of
excited metastable helium at 10,833 angstroms. The amplitude of the feature, in
transit depth, is 0.049 +/- 0.011 per cent in a bandpass of 98 angstroms, which
is more than five times greater than what could be caused by nominal stellar
chromospheric activity. This large absorption signal suggests that WASP-107b
has an extended atmosphere that is eroding at a total rate of 10^10 to 3 x
10^11 grams per second (0.1-4 per cent of its total mass per billion years),
and may have a comet-like tail of gas shaped by radiation pressure.
