  Context. The protoplanetary disk around the star HD 100546 displays prominent
substructures in the form of two concentric rings. Recent observations with the
Atacama Large Millimeter/sub-millimeter Array (ALMA) have revealed these
features with high angular resolution and have resolved the faint outer ring
well. This allows us to study the nature of the system further.
  Aims. Our aim is to constrain some of the properties of potential planets
embedded in the disk, assuming that they induce the observed rings and gaps.
  Methods. We present the self-calibrated $0.9\,$mm ALMA observations of the
dust continuum emission from the circumstellar disk around HD 100546. These
observations reveal substructures in the disk that are consistent with two
rings, the outer ring being much fainter than the inner one. We reproduced this
appearance closely with a numerical model that assumes two embedded planets. We
varied planet and disk parameters in the framework of the planet-disk
interaction code FARGO3D and used the outputs for the gas and dust distribution
to generate synthetic observations with the code RADMC-3D.
  Results. From this comparison, we find that an inner planet located at $r_1 =
13\,$au with a mass $M_1 = 8 M_{\rm{Jup}}$ and an outer planet located at $r_2
= 143\,$au with a mass $M_2 = 3 M_{\rm{Jup}}$ leads to the best agreement
between synthetic and ALMA observations (deviation less than $3\sigma$ for the
normalized radial profiles). To match the very low brightness of the outer
structure relative to the inner ring, the initial disk gas surface density
profile needs to follow an exponentially tapered power law (self-similar
solution), rather than a simple power-law profile.
