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Abstract 2MASS J16120668–3010270 (hereafter 2MJ1612) is a young M0 star that hosts a protoplanetary disk in the Upper Scorpius star-forming region. Recent Atacama Large Millimeter/submillimeter Array (ALMA) observations of 2MJ1612 show a mildly inclined disk (i = 37°) with a large dust-depleted gap (R_cav ≈ 0 $\stackrel{\text{″}}{.}$4 or 53 au). We present high-contrast Hαobservations from MagAO-X on the 6.5 m Magellan telescope and new high-resolution submillimeter dust continuum observations with ALMA of 2MJ1612. On both 2025 April 13 and 16, we recovered a point source with Hαexcess with a signal-to-noise ratio ≳5 within the disk gap in our MagAO-X angular and spectral differential images at a separation of 141.96 ± 2.10 mas (23.45 ± 0.29 au deprojected) from the star and a position angle ​​​​​of 159 $\stackrel{\text{°}}{.}$00 ± 0 $\stackrel{\text{°}}{.}$55. Furthermore, this Hαsource is within close proximity to aK-band point source in the SPHERE/IRDIS observation taken on 2023 July 21. The astrometric offset between theKband and Hαsource can be explained by orbital motion of a bound companion. Thus, our observations can be best explained by the discovery of an accreting protoplanet, 2MJ1612 b, with an estimated mass of 4M_Jupand a Hαline flux ranging from (29.7 ± 7.5) × 10⁻¹⁶erg s cm²to (8.2 ± 3.4) × 10⁻¹⁶erg s cm². 2MJ1612 b is likely the third example of an accreting Hαprotoplanet responsible for carving the gap in its host disk, joining PDS 70 b and c. Further study is necessary to confirm and characterize this protoplanet candidate and to identify any additional protoplanets that may also play a role in shaping the gap. 

Abstract We present 3 yr of high-contrast imaging of the PDS 70 b and c accreting protoplanets with the new extreme AO system MagAO-X as part of the MaxProtoPlanetS survey of Hαprotoplanets. In 2023 and 2024, our sharp (25–27 mas FWHM), well-AO-corrected (20%–26% Strehl), deep (2–3.6 hr) images detected compact (r∼ 30 mas;r∼ 3 au) circumplanetary disks (CPDs) surrounding both protoplanets. Starlight scattering off the front edge of these dusty CPDs is the likely source of the bright compact continuum light detected within ∼30 mas of both planets in our simultaneously obtained continuum 668 nm filter images. After subtraction of contaminating continuum and point-spread function residuals withpyKLIPangular differential imaging and spectral differential imaging, we obtained high-contrast ASDI Hαimages of both planets in 2022, 2023, and 2024. We find the Hαline flux of planet b fell by (8.1 ± 1.6) × 10⁻¹⁶erg s⁻¹cm⁻², a factor of 4.6 drop in flux from 2022 to 2023. In 2024 March, planet b continued to be faint with just a slight 1.6× rise to an Hαline flux of (3.64 ± 0.87) × 10⁻¹⁶erg s⁻¹cm⁻². For c, we measure a significant increase of (2.74 ± 0.51) × 10⁻¹⁶erg s⁻¹cm⁻²from 2023 to 2024, which is a factor of 2.3 increase. So both protoplanets have recently experienced significant Hαvariability with ∼1 yr sampling. In 2024, planet c is brighter than b: as c is brightening and b generally fading. We also tentatively detect one new point source “CC3” inside the inner disk (∼49 mas; at PA ∼ 295°; 2024) with orbital motion roughly consistent with a ∼5.6 au orbit.