Could gravity waves be used to confirm the existence of a ninth planet?

The maths on Wikipedia give a way of calculating the the amplitude of graviational waves.

One detail is that for the waves to be detected by a ligo apparatus, one needs to be far enough from the source of the waves. The distance required depends on the frequency. For planet 9, if it has orbit of 10000 years, one would need to be more than $2500pi$ light years distant. That is nearly 10000 light years away. This value is "R"

Then we can use the formula:
$$h_{+} =-frac{1}{R}, frac{G^2}{c^4}, frac{4m_1 m_2}{r}$$

using the above value of R, the known values of the gravitation constant and speed of light, and the masses of the sun and a hypothesised value for the mass and semi-major axis of planet 9.

That gives a strain of $10^{-32}$. That is far far far below what is detectable. It also would have a frequency of 10000 years or so. So you would be trying to detect an oscillation comparable to the plank length, you would need at least 10000 years of observation, and your detector has to be built on the other side of the galaxy.

Not possible.