Two planets, one with twice the period as the other

If the inner planet has a speed v, and its orbit has a radius r and period T, what is the speed and radius of the outer planet if its orbital period is 2T?











T is proportional to r3/2 so r is proportional to T2/3.
Doubling T means the second planet has a radius of r * 22/3

v = 2πr / T

Doubling T and increasing r by a factor of 22/3 means the second planet has a speed of v * 2-1/3