Newly forming protoplanets are expected to create cavities and substructures in young, gas-rich protoplanetary disks1,2,3, but they are difficult to detect as they could be confused with disk features affected by advanced image analysis techniques4,5. Recently, a planet was discovered inside the gap of the transitional disk of the T Tauri star PDS 706,7. Here, we report on the detection of strong Hα emission from two distinct locations in the PDS 70 system, one corresponding to the previously discovered planet PDS 70 b, which confirms the earlier Hα detection8, 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 emission6,7,9. We identify this second Hα peak as a second protoplanet in the PDS 70 system. The Hα emission spectra of both protoplanets indicate ongoing accretion onto the protoplanets10,11, which appear to be near a 2:1 mean motion resonance. Our observations show that adaptive-optics-assisted, medium-resolution integral field spectroscopy with MUSE12 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 System13.
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The data used in this work are publicly available on the ESO archive (http://archive.eso.org/) under programmes 60.A-9100(K), 095.C-0298(A), 097.C-0206(A) and 097.C-1001(A).
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This work is based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 60.A-9100(K), 095.C-0298(A), 097.C-0206(A) and 097.C-1001(A). A.J.B. and J.d.B. acknowledge support from the European Research Council (ERC) Starting Grant 678194 (FALCONER). I.A.G.S. acknowledges funding from the ERC under the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 694513. R.B. acknowledges support from the ERC advanced grant 339659-MUSICOS. J.B. acknowledges support by FCT/MCTES through national funds by the grant UID/FIS/04434/2019 and through the Investigador FCT contract no. IF/01654/2014/CP1215/CT0003. The authors acknowledge the ESO AOF and Paranal teams for their expertise and support during the commissioning activities.