  The frequency of Earth-size planets in the habitable zone of Sun-like stars,
hereafter $\eta_\oplus$, is a key parameter to evaluate the yield of nearby
Earth analogues that can be detected and characterized by future missions. Yet,
this value is poorly constrained as there are no reliable exoplanet candidates
in the habitable zone of Sun-like stars in the Kepler field. Here, we show that
extrapolations relying on the population of small ($< 1.8\,R_\oplus$)
short-period ($< 25\,$days) planets bias $\eta_\oplus$ to large values. As the
radius distribution at short orbital periods is strongly affected by
atmospheric loss, we re-evaluate $\eta_\oplus$ using exoplanets at larger
separations. We find that $\eta_\oplus$ drops considerably, to values of only
$\sim 5-10$%. Observations of young ($< 100$ Myr) clusters can probe
short-period sub-Neptunes that still retain most of their envelope mass. As
such, they can be used to quantify the contamination of sub-Neptunes to the
population of Kepler short-period small planets and aid in more reliable
estimates of $\eta_\oplus$.
