I have the stellar and planet mass, planet velocity, radial velocity of star, stellar semimajor axis.

How should I go about to find the radius of this exoplanet?

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    $\begingroup$ You cannot find the radius of an exoplanet using Doppler spectroscopy. Why is the spectroscopy relevant if you already have all the parameters that you would derive from the spectroscopy? $\endgroup$ – Rob Jeffries Jul 28 at 9:47
  • $\begingroup$ it's not, just most online results show how to find planet radius using light curves $\endgroup$ – Oop Jul 28 at 13:21

In general, you can't. The spectroscopy tells you about the properties of the star and the properties of the orbit and gives some information about the mass of the planet ($M \sin i$, where $i$ is the orbital inclination).

If the spectroscopy were super-precise and with extremely high signal-to-noise and high time resolution, then you might be able to map the Rossiter-McLaughlin effect with such precision that you can estimate the planet radius. However, this would only work if the planet is transiting in front of the parent star - in which case you might as well have used transit photometry in the first place.

Another possibility is to search in the spectra for the Doppler-shifted scattered starlight from the planetary atmosphere (e.g. Martins et al. 2015). However the amplitude of this is tiny depends on both the radius of the planet and its albedo/atmospheric composition.

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