I finally got around to looking for a solution to this. The formula for the initial velocity from wikipedia is:
where G is the gravitational constant, M is the mass of the object being orbited and r is the distance. Since we aren't dealing with real distances we can just use 1 instead of G.
In my tests I didn't use the gravitation plugin, instead I just applied a gravitational force to the planet every tick:
applied toward sun.x,sun.y
The biggest issue was that box2d units are odd so I had to find a conversion factor, which ended up being about 48.8.
So my final formula for the planet's impulse was:
Plugging that into your capx worked, but the orbits aren't perfectly round when using the gravitation plugin.