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While detecting low mass exoplanets at tens of au is beyond currentinstrumentation, debris discs provide a unique opportunity to study the outerregions of planetary systems. Here we report new ALMA observations of the80-200 Myr old Solar analogue HD 107146 that reveal the radial structure of itsexo-Kuiper belt at wavelengths of 1.1 and 0.86 mm. We find that theplanetesimal disc is broad, extending from 40 to 140 au, and it ischaracterised by a circular gap extending from 60 to 100 au in which thecontinuum emission drops by about 50%. We also report the non-detection of theCO J=3-2 emission line, confirming that there is not enough gas to affect thedust distribution. To date, HD 107146 is the only gas-poor system showingmultiple rings in the distribution of millimeter sized particles. These ringssuggest a similar distribution of the planetesimals producing small dust grainsthat could be explained invoking the presence of one or more perturbingplanets. Because the disk appears axisymmetric, such planets should be oncircular orbits. By comparing N-body simulations with the observed visibilitieswe find that to explain the radial extent and depth of the gap, it would berequired the presence of multiple low mass planets or a single planet thatmigrated through the disc. Interior to HD 107146's exo-Kuiper belt we findextended emission with a peak at ~20 au and consistent with the inner warm beltthat was previously predicted based on 22$\mu$m excess as in many othersystems. This warm belt is the first to be imaged, although unexpectedlysuggesting that it is asymmetric. This could be due to a large belteccentricity or due to clumpy structure produced by resonant trapping with anadditional inner planet.

A gap in the planetesimal disc around HD 107146 and asymmetric warm dust emission revealed by ALMA