  The intergalactic medium was reionized before redshift z~6, most likely by
starlight which escaped from early galaxies. The very first stars formed when
hydrogen molecules (H2) cooled gas inside the smallest galaxies, minihalos of
mass between 10^5 and 10^8 solar masses. Although the very first stars began
forming inside these minihalos before redshift z~40, their contribution has, to
date, been ignored in large-scale simulations of this cosmic reionization. Here
we report results from the first reionization simulations to include these
first stars and the radiative feedback that limited their formation, in a
volume large enough to follow the crucial spatial variations that influenced
the process and its observability. We show that, while minihalo stars stopped
far short of fully ionizing the universe, reionization began much earlier with
minihalo sources than without, and was greatly extended, which boosts the
intergalactic electron-scattering optical depth and the large-angle
polarization fluctuations of the cosmic microwave background significantly.
Although within current WMAP uncertainties, this boost should be readily
detectable by Planck. If reionization ended as late as z_ov<~7, as suggested by
other observations, Planck will thereby see the signature of the first stars at
high redshift, currently undetectable by other probes.
