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A kids question indeed, but I do not have a conclusive answer. An usual artist's impression uses red and yellow as dominating colors, and a rather homogenous, dense disk, but how reasonable is that?

If you follow e.g. Sciencedirect's teaser page on Protoplanetary disks

Dust grains are a relatively minor constituent of protoplanetary disks, but they represent the starting point for the formation of rocky planets like Earth, and possibly also gas-rich planets like Jupiter. These grains are small, typically 1 m in diameter or less.

Later on the same page:

However, the fact that roughly half of young stars have debris disks of dust thought to come from asteroids and comets implies that growth of large solid bodies occurs in many protoplanetary disks, even if the mechanism remains obscure.

This two statements for me seeded some doubts that homogenous red is really the correct color to associate with the inside of a protoplanetary disk.

I also found a paper by Alan P. Boss stating

Theoretical models of disks undergoing the accretion of mass from an infalling cloud envelope predict disk temperatures in good agreement with these constraints: a moderately warm (500–1500 K) inner disk, surrounded by a cool (50–150 K) outer disk.

This would at least support the color scheme if you assume black body radiation.

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  • $\begingroup$ A disk is not uniform but varies in both density and temperature from radially inside to outside, and from the vertical structuring depends on its age - and the model assumptions you make. $\endgroup$
    – planetmaker
    Dec 17, 2020 at 23:53
  • $\begingroup$ @planetmaker I understand that temperature is usually radially decaying as well as over time. I am interested in a time when the first planet is about to form, so I assume that density fluctuations must be rather prominent then. $\endgroup$
    – B--rian
    Dec 17, 2020 at 23:56
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    $\begingroup$ Not a thing I know anything about but a quick search online for spectrum protoplanetary disc gave some interesting results and you may be able to use some of those links to get an understanding of likely color scheme. $\endgroup$
    – StephenG - Help Ukraine
    Dec 18, 2020 at 2:05

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The answer seems to be a clear "it depends":

  1. Homogenity of the disks depends on the age of the protoplanetary disk.
  2. Temperature is a function of the radius and the color-scale used in (old) artists' impressions is indeed correct.

On Wikipedia on the topic there is a gallery of different observations, also in the visual spectrum. An impressive picture of the density fluctuations within the protoplanetary disk is provideded by the Disk Substructures at High Angular Resolution Project (DSHARP) as explained e.g. by the ESO.

Edit Remarks following a suggestion by Rob Jeffries:

  • Please note that the DSHARP images are not from the visual part of the spectrum.
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    $\begingroup$ The DSHARP images are at sub-mm wavelengths so are irrelevant to your question. I think almost all the other images on the Wikipedia page are also false-colour sub-mm or infrared images. There may possibly be some optical images in scattered light of debris (rather than protoplanetary) disks. This doesn't answer your own question. $\endgroup$
    – ProfRob
    Dec 18, 2020 at 21:41
  • $\begingroup$ @RobJeffries Good point indeed, I once again managed to formulate in a misunderstanding way. I will edit asap that I meant the DSHARP picture as example for the density fluctuations. $\endgroup$
    – B--rian
    Dec 18, 2020 at 21:44
  • $\begingroup$ Maybe, but the sub-mm disk is not necessarily the disk you would see at optical wavelengths. $\endgroup$
    – ProfRob
    Dec 18, 2020 at 22:21

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