Whether the force is Coriolis or centrifugal, or if the planet accelerates radially is frame dependent. For mass $m$ planet _revolving_ at speed $v$ in a radius of $R$:

In an inertial frame, there are no fictitious forces, and the centripetal force caused by gravity is:

$$ F_g = \frac{mv^2} R $$

which keeps the planet in orbit

In a rotating frame ($\omega=v/R$) in which the planet is fixed, there is a centrifugal force:

$$ F_{cent} = {m\omega^2}R = {m(v/R)^2}R = \frac{mv^2}R = F_g$$

Since the planet is stationary, there is no Coriolis force. The total fictitious force balances gravity, and the planet remains fixed.

In a frame moving at $\omega'=\omega/2$, there is a centrifugal force:

$$  F'_{cent} = \frac{m\omega'^2} R = \frac{mv^2}{4R}  $$

In this frame, the planet's velocity is:

$$ v' = v - R\omega' = v-v/2 = v/2 $$

The Coriolis force is:

$$ F'_{cor} = 2m\omega' v' = 2m(v'/R)v' = \frac{mv^2}{2R}$$

which is in the anti-radial direction (see Eötvös effect, in while the motion is manifest as a change local gravity).

The total fictitious force is:

$$ F'_{fact} = F'_{cor} + F'_{cent} = \frac 3 4 F_g $$

Hence,the planet experiences a radial force:

$$ \frac 1 4 F_g = \frac{mv'^2} {R} $$

which keeps it on its orbit.

The latter analysis holds for any $\omega'$.