  GJ1214b stands out among the detected low-mass exoplanets, because it is, so
far, the only one amenable to transmission spectroscopy. Up to date there is no
consensus about the composition of its envelope although most studies suggest a
high molecular weight atmosphere. In particular, it is unclear if hydrogen and
helium are present or if the atmosphere is water dominated. Here, we present
results on the composition of the envelope obtained by using an internal
structure and evolutionary model to fit the mass and radius data. By examining
all possible mixtures of water and H/He, with the corresponding opacities, we
find that the bulk amount of H/He of GJ1214b is at most 7% by mass. In general,
we find the radius of warm sub-Neptunes to be most sensitive to the amount of
H/He. We note that all (Kepler-11b,c,d,f, Kepler-18b, Kepler-20b, 55Cnc-e,
Kepler-36c and Kepler-68b) but two (Kepler-11e and Kepler-30b) of the
discovered low-mass planets so far have less than 10% H/He. In fact, Kepler-11e
and Kepler-30b have 10-18% and 5-15% bulk H/He. Conversely, little can be
determined about the H2O or rocky content of sub-Neptune planets. We find that
although a 100% water composition fits the data for GJ1214b, based on formation
constraints the presence of heavier refractory material on this planet is
expected, and hence, so is a component lighter than water required. A robust
determination by transmission spectroscopy of the composition of the upper
atmosphere of GJ1214b will help determine the extent of compositional
segregation between the atmosphere and envelope.
