  Topological superconductivity holds promise for fault-tolerant quantum
computing. While planar Josephson junctions are attractive candidates to
realize this exotic state, direct phase-measurements as the fingerprint of the
topological transition are missing. By embedding two gate-tunable Al/InAs
Josephson junctions in a loop geometry, we measure a $\pi$-jump in the junction
phase with increasing in-plane magnetic field, ${\bf B}_\|$. This jump is
accompanied by a minimum of the critical current, indicating a closing and
reopening of the superconducting gap, strongly anisotropic in ${\bf B}_\|$. Our
theory confirms that these signatures of a topological transition are
compatible with the emergence of Majorana states.
