Exoplanet detections have revolutionized astronomy, offering new insights into solar system architecture and planet demographics. While nearly 1900 exoplanets have now been discovered and confirmed, 1 none are still in the process of formation. Transition discs, protoplanetary disks with inner clearings 2-4 best explained by the influence of accreting planets 5 , are natural laboratories for the study of planet formation. Some transition discs show evidence for the presence of young planets in the form of disc asymmetries 6, 7 or infrared sources detected within their clearings, as in the case of LkCa 15. 8,9 Attempts to observe directly sig-Author Contributions: This work merged two independently acquired and analysed data sets. S.S. led preparation of the manuscript, the orbital fits, and the acquisition and analysis of the LBT data while K.B.F. led the acquisition and analysis of the MagAO data, development of the MagAO SDI pipeline, and drafted MagAO manuscript sections.

We present ALMA 850 µm continuum observations of the Orion Nebula Cluster that provide the highest angular resolution (∼ 0. 1 ≈ 40 AU) and deepest sensitivity (∼ 0.1 mJy) of the region to date. We mosaicked a field containing ∼ 225 optical or near-IR-identified young stars, ∼ 60 of which are also optically-identified "proplyds". We detect continuum emission at 850 µm towards ∼ 80% of the proplyd sample, and ∼ 50% of the larger sample of previously-identified cluster members. Detected objects have fluxes of ∼ 0.5-80 mJy. We remove sub-mm flux due to free-free emission in some objects, leaving a sample of sources detected in dust emission. Under standard assumptions of isothermal, optically thin disks, sub-mm fluxes correspond to dust masses of ∼ 0.5 to 80 Earth masses. We measure the distribution of disk sizes, and find that disks in this region are particularly compact. Such compact disks are likely to be significantly optically thick. The distributions of sub-mm flux and inferred disk size indicate smaller, lower-flux disks than in lower-density star-forming regions of similar age. Measured disk flux is correlated weakly with stellar mass, contrary to studies in other star forming regions that found steeper correlations. We find a correlation between disk flux and distance from the massive star θ 1 Ori C, suggesting that disk properties in this region are influenced strongly by the rich cluster environment.

Spatially resolved structures in protoplanetary disks hint at unseen planets. Previous imaging observations of the transitional disk around MWC 758 revealed an inner cavity, a ring-like outer disk, emission clumps, and spiral arms, all possibly generated by companions. We present ALMA dust continuum observations of MWC 758 at 0.87 millimeter (mm) wavelength with 43×39 mas angular resolution (6.9×6.2 AU) and 20 µJy beam −1 rms. The central sub-mm emission

Most massive stars form in dense clusters where gravitational interactions with otherstars may be common. The two nearest forming massive stars, the BN object and Source I, located behind the Orion Nebula, were ejected with velocities of ∼29 and ∼13 km s −1 about 500 years ago by such interactions. This event generated an explosion in the gas. New ALMA observations show in unprecedented detail, a roughly spherically symmetric distribution of over a hundred 12 CO J=2−1 streamers with velocities extending from V LSR =−150 to +145 km s −1 . The streamer radial velocities increase (or decrease) linearly with projected distance from the explosion center, forming a "Hubble Flow" confined to within 50″ of the explosion center. They point toward the high proper-motion, shockexcited H 2 and [Fe II] "fingertips" and lower-velocity CO in the H 2 wakes comprising Orionʼs "fingers." In some directions, the H 2 "fingers" extend more than a factor of two farther from the ejection center than the CO streamers. Such deviations from spherical symmetry may be caused by ejecta running into dense gas or the dynamics of the N-body interaction that ejected the stars and produced the explosion. This ∼10 48 erg event may have been powered by the release of gravitational potential energy associated with the formation of a compact binary or a protostellar merger. Orion may be the prototype for a new class of stellar explosiozn responsible for luminous infrared transients in nearby galaxies.

We imaged a 2 ′ × 2 ′ region of the Orion Nebula cluster in 1.3 mm wavelength continuum emission with the recently commissioned Combined Array for Research in Millimeter Astronomy (CARMA) and with the Submillimeter Array 2 Here and throughout the text, "massive disks" refer to disks with mass comparable to or greater than 0.01 M ⊙ , the lower range of estimates for the minimum-mass solar nebula.