<p>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 (<span class="math-container">$M \sin i$</span>, where <span class="math-container">$i$</span> is the orbital inclination).</p>
<p>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 <a href="https://en.wikipedia.org/wiki/Rossiter%E2%80%93McLaughlin_effect" rel="nofollow noreferrer">Rossiter-McLaughlin effect</a> 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.</p>
<p>Another possibility is to search in the spectra for the Doppler-shifted scattered starlight from the planetary atmosphere (<a href="https://arxiv.org/abs/1504.05962" rel="nofollow noreferrer">e.g. Martins et al. 2015</a>). However the amplitude of this is <em>tiny</em> depends on <em>both</em> the radius of the planet and its albedo/atmospheric composition.</p>

CC BY-SA 4.0
2020-07-28T13:38:53.200Z
2020-07-28T13:45:46.123Z
2020-07-28T13:45:46.123Z
ProfRob
<p>I have the stellar and planet mass, planet velocity, radial velocity of star, stellar semimajor axis.</p>
<p>How should I go about to find the radius of this exoplanet?</p>

CC BY-SA 4.0
2020-07-28T09:47:15.627Z
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?
ProfRob
CC BY-SA 4.0
2020-07-28T13:21:33.653Z
it's not, just most online results show how to find planet radius using light curves
Oop
<exoplanet><radius>
How can I find the radius of a exoplanet using doppler spectroscopy
Oop
# How can I find the radius of a exoplanet using doppler spectroscopy

(Asked by: username_0 on 2020-07-28)

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?

**Tags:** exoplanet, radius
> **Comments:**
> **username_1**: 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?  
> **username_0**: it's not, just most online results show how to find planet radius using light curves  

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## 1 answers

### Answer by username_1 on 2020-07-28. Score: 1

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](https://en.wikipedia.org/wiki/Rossiter%E2%80%93McLaughlin_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](https://arxiv.org/abs/1504.05962)). However the amplitude of this is *tiny* depends on *both* the radius of the planet and its albedo/atmospheric composition.


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