  Whether molecular states indeed exist in nature has been disputed for a long
time. Several new resonances have been observed in the recent experiments and
they seem to be of exotic structures and some of them have been proposed to be
molecular states. The very recent observation of $Z_b(10610)[(10608.4\pm 2.0)$
MeV] and $Z_b(10650)[(10653.2\pm 1.5)$ MeV] encourages the interpretation of
multi-quark states. In the Beter-Salpeter (BS) approach, we study the
possibility if two heavy mesons can form a molecular state by exchanging light
mesons. Our results indicate that two heavy mesons can form an isospin singlet
(I=0) bound state but cannot form an isospin triplet (I=1) when the
contribution of $\sigma-$ exchange is reasonably small, i.e. as the coupling of
$\sigma$ with mesons $g_{\sigma}$ takes the value given in previous
literatures. Thus we conclude that the newly observed $Z_b(10610)$ should not
be a molecular state, but a tetraquark state instead, at most, the fraction of
the molecular state in the physical resonance $Z_b(10610)$ is tiny.
