# Estimating the radius of a non-transiting exoplanet

I need to know the timescale of tidal locking of a non-transiting exoplanet. For that, I need to know (or at least have some constraint of) its radius (Gladman et al. 1996):

$$T_{lock} = k \frac{a^6}{G M_{star} R_{planet}^2 Q},$$

where

$$T_{lock}$$ is the timescale for tidal locking, $$k$$ is a constant that depends on the properties of the planet and the star, $$a$$ is the semi-major axis of the planet's orbit, $$G$$ is the gravitational constant, $$M_{star}$$ is the mass of the host star, $$R_{planet}$$ is the radius of the planet and $$Q$$ is the tidal quality factor, which represents the efficiency of energy dissipation within the planet.

How can I estimate the radius of a given exoplanet?

If you have its mass (and if it's a Doppler-discovered planet then you only have a lower limit to the mass, $$M\sin i$$), then about the best you can do is plot a mass-radius relationship for all the transiting planets with a known mass and radius and use a best-fit to that in order to estimate the radius of your planet.