  Planar Josephson junctions provide a platform to host topological
superconductivity which, through manipulating Majorana bound states (MBS),
could enable fault-tolerant quantum computing. However, what constitutes
experimental signatures of topological superconductivity and how MBS can be
detected remains strongly debated. In addition to spurious effects that mimic
MBS, there is a challenge to discern the inherent topological signals in
realistic systems with many topologically-trivial Andreev bound states,
determining the transport properties of Josephson junctions. Guided by the
advances in microwave spectroscopy, we theoretically study Al/InAs-based planar
Josephson junction embedded into a radio-frequency superconducting quantum
interference device to identify microwave signatures of topological
superconductivity. Remarkably, by exploring the closing and reopening of a
topological gap, we show that even in a wide planar Josephson junction with
many Andreev bound states, such a topological signature is distinguishable in
the resonance frequency shift of a microwave drive and the ``half-slope''
feature of the microwave absorption spectrum. Our findings provide an important
step towards experimental detection of non-Abelian statistics and implementing
scalable topological quantum computing.
