  The axion could be used as a probe for extra dimensions. In large extra
dimensions, besides the QCD axion one obtains an infinite tower of massive
Kaluza-Klein (KK) states. We describe the processes of KK axions production in
the Sun via the axion-photon coupling, $g_{a\gamma\gamma}$, and we derive the
number density of KK axions that get trapped into the solar gravitational field
and then accumulate over cosmic times. The large multiplicity of states, as
well as their masses in the keV-range, deeply alter the phenomenology of the
axion. This scenario leads us to propose the presence of KK axions as an
interpretation of the non-thermal distribution of the solar X-rays. In this
work, we dedicate special attention on the astrophysical and cosmological
bounds that apply on the model. In particular, we show how the KK axions may
escape the EBL limit that constrains standard ALPs in the same mass range.
Present searches for KK axions make use of the decay channel,
$a\rightarrow\gamma\gamma$, for which we revise the event rate; our value lies
orders of magnitude below the rate usually quoted in the literature. This major
conclusion stems from recent measurements of the luminosity of the quiet Sun
which acts as an irreducible limit. The revised model remains a viable and an
attractive explanation for multiple astrophysical observations, and we propose
several approaches to search for solar KK axions in the near future.
