  Light new particles with masses below 10 keV, often considered as a plausible
extension of the Standard Model, will be emitted from the solar interior, and
can be detected on the Earth with a variety of experimental tools. Here we
analyze the new "dark" vector state V, a massive vector boson mixed with the
photon via an angle kappa, that in the limit of the small mass m_V has its
emission spectrum strongly peaked at low energies. Thus, we utilize the
constraints on the atomic ionization rate imposed by the results of the XENON10
experiment to set the limit on the parameters of this model: kappa times m_V< 3
times10^{-12} eV. This makes low-threshold Dark Matter experiments the most
sensitive dark vector helioscopes, as our result not only improves current
experimental bounds from other searches by several orders of magnitude, but
also surpasses even the most stringent astrophysical and cosmological limits in
a seven-decade-wide interval of m_V. We generalize this approach to other light
exotic particles, and set the most stringent direct constraints on
"mini-charged" particles.
