The star HR 8799 hosts one of the largest known debris discs and at least four giant planets .
Previous observations have found evidence for a warm belt within the orbits of the planets , a cold planetesimal belt beyond their orbits and a halo of small grains .
With the infrared data , it is hard to distinguish the planetesimal belt emission from that of the grains in the halo .
With this in mind , the system has been observed with ALMA in band 6 ( 1.34 mm ) using a compact array format .
These observations allow the inner edge of the planetesimal belt to be resolved for the first time .
A radial distribution of dust grains is fitted to the data using an MCMC method .
The disc is best fit by a broad ring between 145 ^ { +12 } _ { -12 } AU and 429 ^ { +37 } _ { -32 } AU at an inclination of 40 ^ { +5 } _ { -6 } { { \degr } } and a position angle of 51 ^ { +8 } _ { -8 } { { \degr } } .
A disc edge at \sim 145 AU is too far out to be explained simply by interactions with planet b , requiring either a more complicated dynamical history or an extra planet beyond the orbit of planet b .