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I am trying to model some exoplanets and I don't understand how this model works. Below is a link of the model. What I don’t understand is how does graphing the transit depth vs wavelength provide information. I am trying to determine the atmospheric chemistry/atmosphere/properties of planets.

https://exoctk.stsci.edu/fortney

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  • $\begingroup$ It will be much easier to answer this to your satisfaction if you can edit your question and point to some specific aspects of the information there for which you would like clarification. Right now, as it reads, this could be interpreted as "Somebody please read this for me and then tell me what it says" which is not how SE works. $\endgroup$ – uhoh Sep 15 at 2:55
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    $\begingroup$ $uhoh I just updated it. I hope it’s better now $\endgroup$ – Physics Geek Sep 15 at 3:39
  • $\begingroup$ looks better, thanks! $\endgroup$ – uhoh Sep 15 at 3:49
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Planets with atmospheres are not discs with hard edges, when viewed in transit across the star. The atmosphere allows light to pass through the limb of the planetary disc.

The radius within which the planet is effectively opaque to light from the star will depend on the density and composition of the atmosphere as a function of height and also the wavelength of light that it is illuminated by. This is because the opacity of the atmosphere will be much greater at wavelengths where some molecule or atom that is present has an absorption feature.

Therefore if you observe a transit at different wavelengths, you will see different transit depths, because the planet has a different effective area at different wavelengths.

For example, if the atmosphere contains lots of atomic sodium, then because sodium strongly absorbs light at 589nm, the planet is opaque to a greater radius at 589nm, and the transit depth will be bigger at 589nm than at wavelengths either side of this.

Ps. (Jonathan) Fortney is the name of the scientist who developed the grid of models.

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