So, this will be weird, this was requested by Prom on Discord, somewhat, was told that it is a secondary cannon feat, but oh well, here is the quote:
"The planet was the worst of the worst—a dark conglomeration of an intelligent, or at least responsive, anti-matter. It seemed to literally eat light, leaving a null darkness from which the eye could not be averted."
Za Caruco[]
Apparently the planet is 1200 miles in diameter, which gives about 1931213 for it and 965606.5 meters for its radius, which gives us an volume of about 3.7712832393528E+18 meters3
Now, here is the thing, according to what i researched, Antimatter mass is the same as its "normal" matter mass counterpart, we don't know what this was made off, so I'll just consider rock cause hey, planetoid, which is 2700 Kg/m3, so
3.7712832393528E+18*2700 = 1.018246474625256e22 Kg
U = (3*G*(M)2)/(5*R)
Where U = GBE in joules, M = the mass of the body in question in kilograms, r = its radius in meters, and G = the gravitational constant of 6.67408x10^-11.
U = (3*6.67408e-11*(1.018246474625256e22)2)/(5*965606.5)
U = 4.2998005231686182334e27 Joules, or 1.027676989285 Exaton of TNT, Multi-Continent level by existing, but, apparently it was going into contact to Earth and would have caused a big boom, so let's find out how strong this should be
1 gram of antimatter is equivalent of ~9×1013 joules, so 1.018246474625256e22 Kg should be about
(0.001/1.018246474625256e22) = (9e13/x)
0.001x = 9.164218271627304e35
x = 9.164218271627304e38 joules or 219.03007341365449 Ronnatons of TNT, Dwarf Star level