While detecting low mass exoplanets at tens of au is beyond current instrumentation, debris discs provide a unique opportunity to study the outer regions of planetary systems. Here we report new ALMA observations of the 80-200 Myr old Solar analogue HD 107146 that reveal the radial structure of its exo-Kuiper belt at wavelengths of 1.1 and 0.86 mm. We find that the planetesimal disc is broad, extending from 40 to 140 au, and it is characterised by a circular gap extending from 60 to 100 au in which the continuum emission drops by about 50%. We also report the non-detection of the CO J=3-2 emission line, confirming that there is not enough gas to affect the dust distribution. To date, HD 107146 is the only gas-poor system showing multiple rings in the distribution of millimeter sized particles. These rings suggest a similar distribution of the planetesimals producing small dust grains that could be explained invoking the presence of one or more perturbing planets. Because the disk appears axisymmetric, such planets should be on circular orbits. By comparing N-body simulations with the observed visibilities we find that to explain the radial extent and depth of the gap, it would require the presence of multiple low mass planets or a single planet that migrated through the disc. Interior to HD 107146's exo-Kuiper belt we find extended emission with a peak at ∼ 20 au and consistent with the inner warm belt that was previously predicted based on 22µm excess as in many other systems. This warm belt is the first to be imaged, although unexpectedly suggesting that it is asymmetric. This could be due to a large belt eccentricity or due to clumpy structure produced by resonant trapping with an additional inner planet.

The organic content of protoplanetary disks sets the initial compositions of planets and comets, thereby influencing subsequent chemistry that is possible in nascent planetary systems. We present observations of the complex nitrile-bearing species CH3CN and HC3N toward the disks around the T Tauri stars AS 209, IM Lup, LkCa 15, and V4046 Sgr as well as the Herbig Ae stars MWC 480 and HD 163296. HC3N is detected toward all disks except IM Lup, and CH3CN is detected toward V4046 Sgr, MWC 480, and HD 163296. Rotational temperatures derived for disks with multiple detected lines range from 29 to 73 K, indicating emission from the temperate molecular layer of the disk. V4046 Sgr and MWC 480 radial abundance profiles are constrained using a parametric model; the gas-phase CH3CN and HC3N abundances with respect to HCN are a few to tens of percent in the inner 100 au of the disk, signifying a rich nitrile chemistry at planet- and comet-forming disk radii. We find consistent relative abundances of CH3CN, HC3N, and HCN between our disk sample, protostellar envelopes, and solar system comets; this is suggestive of a robust nitrile chemistry with similar outcomes under a wide range of physical conditions.

Cometary studies suggest that the organic composition of the early Solar Nebula was rich in complex nitrile species such CH 3 CN. Recent ALMA detections in protoplanetary disks suggest that these species may be common during planet and comet formation, but connecting gas-phase measurements to cometary abundances first requires constraints on formation chemistry and distributions of these species. We present here the detection of seven spatially resolved transitions of CH 3 CN in the protoplanetary disk around the T-Tauri star TW Hya. Using a rotational diagram analysis, we find a disk-averaged column density of A radially resolved rotational diagram shows the rotational temperature to be constant across the disk, suggesting that the CH 3 CN emission originates from a layer at z/r∼0.3. Through comparison of the observations with predictions from a disk chemistry model, we find that grain-surface reactions likely dominate CH 3 CN formation and that in situ disk chemistry is sufficient to explain the observed CH 3 CN column density profile without invoking inheritance from the protostellar phase. However, the same model fails to reproduce a solar system cometary abundance of CH 3 CN relative to H 2 O in the midplane, suggesting that either vigorous vertical mixing or some degree of inheritance from interstellar ices occurred in the Solar Nebula.

Nitrogen fractionation is commonly used to assess the thermal history of Solar System volatiles. With ALMA it is for the first time possible to directly measure 14 N/ 15 N ratios in common molecules during assembly of planetary systems. We present ALMA observations of the H 13 CN and HC 15 N J = 3 − 2 lines at 0 .5 angular resolution, toward a sample of six protoplanetary disks, selected to span a range of stellar and disk structure properties. Adopting a typical 12 C/ 13 C ratio of 70, we find comet-like 14 N/ 15 N ratios of 80 − 160 in 5/6 of the disks (3 T Tauri and 2 Herbig Ae disks) and lack constraints for one of the T Tauri disks (IM Lup). There are no systematic differences between T Tauri and Herbig Ae disks, or between full and transition disks within the sample. In addition, no correlation is observed between disk-averaged D/H and 14 N/ 15 N ratios in the sample. One of the disks, V4046 Sgr, presents unusually bright HCN isotopologue emission, enabling us to model the radial profiles of H 13 CN and HC 15 N. We find tentative evidence of an increasing 14 N/ 15 N ratio with radius, indicating that selective photodissociation in the inner disk is important in setting the 14 N/ 15 N ratio during planet formation.

H 2 CO ice on dust grains is an important precursor of complex organic molecules (COMs). H 2 CO gas can be readily observed in protoplanetary disks and may be used to trace COM chemistry. However, its utility as a COM probe is currently limited by a lack of constraints on the relative contributions of two different formation pathways: on icy grain-surfaces and in the gas-phase. We use archival ALMA observations of the resolved distribution of H 2 CO emission in the disk around the young low-mass star DM Tau to assess the relative importance of these formation routes. The observed H 2 CO emission has a centrally peaked and radially broad brightness profile (extending out to 500 AU). We compare these observations with disk chemistry models with and without grain-surface formation reactions, and find that both gas and grain-surface chemistry are necessary to explain the spatial distribution of the emission. Gas-phase H 2 CO production is responsible for the observed central peak, while grainsurface chemistry is required to reproduce the emission exterior to the CO snowline (where H 2 CO mainly forms through the hydrogenation of CO ice before being non-thermally desorbed). These observations demonstrate that both gas and grain-surface pathways contribute to the observed H 2 CO in disks, and that their relative contributions depend strongly on distance from the host star.