Characteristics of the first planets in the Universe? What would have the very first planets looked like, based on their most likely chemical compositions?

For example:


Were they mostly grey gas giants with atmospheres of hydrogen and helium, coalescing around the first Population II stars?
Were they blue gas giants, enriched with water vapour?
Were they brownish-red and grey, like our own Jupiter and Saturn?
Were they a heterogeneous mix of all sizes, colours, and chemical compositions that included rocky and gas giants?


What papers support research into this area?


https://iopscience.iop.org/article/10.1088/0004-637X/751/2/81/meta
https://www.mdpi.com/2076-3263/9/3/105
https://arxiv.org/pdf/1805.00023.pdf


Note: By first planets I mean the first planets that most likely existed in the Universe. In The habitable epoch of the early Universe, for example, Loeb suggests that a mere 13.5 ± 3.5 Myr after the Hot Big Bang could have witnessed the first viable supernova followed by planet formation. Another paper (can't find it) suggested that gas giants could have formed from the primordial hydrogen and helium soup before reionization, without needing any stars.

Related:


http://www.sci-news.com/astronomy/science-helium-rich-gas-giants-02905.html
https://en.wikipedia.org/wiki/Sudarsky%27s_gas_giant_classification
https://www.nasa.gov/centers/goddard/news/topstory/2003/0709hstssu.html
https://www.space.com/17441-universe-heavy-metals-planet-formation.html
https://earthsky.org/space/how-and-when-did-the-first-planets-form-in-our-universe
https://medium.com/starts-with-a-bang/ask-ethan-how-fast-could-life-have-arisen-in-the-universe-863bd17b92ca
https://www.nationalgeographic.com/news/2012/3/120326-oldest-planets-found-jupiter-big-bang-space-science/
According to Mashian &amp; Loeb (2016) (also in ArXiv), one possibility is that planet formation may have occurred around carbon-enhanced metal poor (CEMP) stars in the early universe. The paper focuses on CEMP-no stars as the most metal-poor CEMP stars mostly fall into this category.

CEMP-no stars are thought to form from material polluted by supernova ejecta from Population III stars. The main grain species that forms in these ejecta is amorphous carbon. From section 2 of the paper:


  Tailoring the
  SN explosion models to reproduce the observed elemental
  abundances of CEMP-no stars, they find that: (i) for all the
  progenitor models investigated, amorphous carbon (AC) is
  the only grain species that forms in significant amounts;
  this is a consequence of extensive fallback, which results in
  a distinct, carbon-dominated ejecta composition with negligible amounts of other metals, such as Mg, Si, and Al, that
  can enable the condensation of alternative grain types; (ii)
  the mass of carbon locked into AC grains increases when
  the ejecta composition is characterized by an initial mass
  of C greater than the O mass; this is particularly true in
  zero metallicity supernova progenitors, which undergo less
  mixing than their solar metallicity counterparts (Joggerst,
  Woosley, &amp; Heger 2009)


In this case, the earliest planets may have been carbon planets. Carbon planets with bare graphite/amorphous carbon surfaces (the carbon planet equivalent of the Moon or Mercury) would be very dark. Atmospheres would modify this appearance. As the paper notes, hot carbon planet atmospheres are expected to have spectra dominated by carbon monoxide. Cooler carbon planets would likely support various kinds of hydrocarbon chemistry that would modify their appearance, possibly producing reddish tholins as a result of photochemistry depending on what else is around.

Mashian &amp; Loeb predict that the maximum distance at which planets form would depend on the carbon abundance:


  We find that carbon planets may be orbiting iron-deficient stars with carbon abundances [C/H]&nbsp;∼&nbsp;-0.6, such as HE 2356-0410, as far out
  as ∼20&nbsp;AU from their host star in the case where fcond&nbsp;=&nbsp;1.
  Planets forming around stars with less carbon enhancement,
  i.e. HE&nbsp;0107-5240 with [C/H]&nbsp;∼&nbsp;-1.6, are expected to have
  more compact orbits, with semi-major axes r&nbsp;&lt;&nbsp;2&nbsp;AU. If the
  carbon condensation efficiency is only 10%, the expected orbits grow even more compact, with maximum semi-major
  axes of ∼5 and 0.5&nbsp;AU, respectively.


So far this is just hypothetical: we don't know of any planets around CEMP stars, nor of any carbon planets but this may suggest that it is not completely pointless to search for them.