  Newly forming proto-planets are expected to create cavities and substructures
in young, gas-rich proto-planetary disks, but they are difficult to detect as
they could be confused with disk features affected by advanced image-analysis
techniques. Recently, a planet was discovered inside the gap of the
transitional disk of the T-Tauri star PDS 70. Here we report on the detection
of strong H-alpha emission from two distinct locations in the PDS 70 system,
one corresponding to the previously discovered planet PDS 70 b, which confirms
the earlier H$\alpha$ detection, and another located close to the outer-edge of
the gap, coinciding with a previously identified bright dust spot in the disk
and with a small opening in a ring of molecular emission. We identify this
second H$\alpha$ peak as a second proto-planet in the PDS 70 system. The
H$\alpha$ emission spectra of both proto-planets indicate ongoing accretion
onto the proto-planets, which appear to be near a 2:1 mean motion resonance.
Our observations show that adaptive-optics-assisted, medium-resolution,
integral-field spectroscopy with MUSE targeting accretion signatures will be a
powerful way to trace ongoing planet formation in transitional disks at
different stages of their evolution. Finding more young planetary systems in
mean motion resonance would give credibility to the Grand Tack hypothesis in
which Jupiter and Saturn migrated in a resonance orbit during the early
formation period of our Solar System.
