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For a planet which has a temperature gradient, hot in the center and cooler on the surface, why do we see absorption lines?

Similarly, why do we see emission lines if the planet is hot on the surface and gets cooler as you move to the center?

Note, for this question I am only thinking of the planet as a black body, not as something, for example, transiting a star and observing the spectra (transit spectra) of the light that shines through the planet's atmosphere. The source of the spectra is the planet itself.

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For a planet which has a temperature gradient, hot in the center and cooler on the surface, why do we see absorption lines?

The hot center sends photons within the black body spectrum with appropriate energies to excite surface cold atoms ,so the black body curve will have holes, where energy of the photons has been absorbed in exciting surface molecules.

Similarly, why do we see emission lines if the planet is hot on the surface and gets cooler as you move to the center?

The black body spectrum is a continuous spectrum from thermal excitations. There exists though a probability that from the high energy tail of the black body energy spectrum , electrons from atoms on the surface are taken to a higher energy level and then relax back to the ground state emitting the specific line of that atom.

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  • $\begingroup$ There may be more than one explanation. The one you state seems the most obvious. The one I've heard was stated in terms of opacity of wavelengths. 1) Temperature is high at core, low on surface. The photon flux that comes from center is greater than the flux that comes from the surface. High opacity wavelengths only come from the surface and thus have lower fluxes. This is the absorption lines for the blackbody Low opacity wavelengths come from the center (from everywhere, actually, but with highest intensity from the center) and have higher fluxes $\endgroup$ – yalis Jul 19 '14 at 17:04
  • $\begingroup$ 2) Temperature is low at core, high on surface. The photon flux from the surface is greater. Low opacity wavelengths come from near the surface and have greater flux than than those that come from near the center. These are the emission lines. $\endgroup$ – yalis Jul 19 '14 at 17:06
  • $\begingroup$ Opacity indicates absorption, so I am thinking explanations could be equivalent. $\endgroup$ – yalis Jul 19 '14 at 17:08
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Well...there are several possible things which look a little strange with your question, but in general, emission occurs in hot regions and absorption occurs in cold. So, your "planet" could be emitting (presumably thermally, unless it's actually a star...) from the center and that blackbody spectrum is being absorbed by the cooler atmosphere. This is exactly what happens in stars.

The reverse is easier - if you have a planet which is somehow hotter on its surface (mercury might be an example, but again there are more significant physical processes going on), the surface would be emitting as a blackbody, or with spectral lines, or however you like.

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