<p>Our knowledge of planet formation processes comes from theoretical work and is supported by observations. I'm going to give it my take based on that.</p>

<p>On the lower mass end of the planet distribution, starting from comets up to Super-Earths/Mini-Neptunes one needs a high number of rock-forming elements in order to build rocky worlds.<br>
Without high concentrations of dust in protoplanetary discs that we have nowadays, the initial phases of planet formation, namely streaming instability, planetesimal/pebble accretion cannot happen. There is a debate recently, on the fact that low-metallicity stars can form terrestrial planets as well. But there must be a lower limit to this, as completely without heavy elements there just can't be rocky worlds.<br>
Also we have not identified any population III stars yet, in order to study their planet populations. This would be hard as well, as pop III stars would float around in the galactic center, which is quite far for our planet finding methods.</p>

<p>Also the classical <a href="http://adsabs.harvard.edu/abs/1996Icar..124...62P" rel="nofollow noreferrer">core-accretion</a> which would form gas giants indirectly would have trouble acting without a rocky core to begin with. However, protoplanetary discs can destabilize and form giant planets directly. This <a href="http://adsabs.harvard.edu/abs/2013MNRAS.432.3168F" rel="nofollow noreferrer">disc fragmentation process</a> is also dependent on how fast the disc can cool, and is thus sensitive to the amount of dust in the disc. However this sensitivity is not as strong as in the core-accretion scenario.</p>

<p>We also have observational evidence for those two processes forming gas giants (see this plot below from <a href="http://adsabs.harvard.edu/abs/2017A%26A...608A.129T" rel="nofollow noreferrer">Triaud et al, 2017</a>) in our galactic neighbourhood.</p>

<p><a href="https://i.sstatic.net/up2ZE.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/up2ZE.png" alt="(c) Triaud+2017"></a></p>

<p>On the left we have probably core-accretion gas giants, so those which were formed via the rock-rich path. On the right are the brown-dwarf type planets, that can be considered as the lower mass-end of star formation. The orange contours compare to a previous survey by <a href="http://adsabs.harvard.edu/abs/2006ApJ...640.1051G" rel="nofollow noreferrer">Grether &amp; Lineweaver 2006</a>.</p>

<p>Now concluding, I would argue that the left, rock-rich path would be essentially excluded in the early universe. Too many steps would have to be taken first and they all depend critically on the number of rock-forming elements available. Therefore, disc-instability gas giants, without much water or ammonia, should be the first planets to form.</p>

<p>Note that rock-forming elements are not all just formed in Supernovae. While <a href="http://adsabs.harvard.edu/abs/2011LPICo1639.9051G" rel="nofollow noreferrer">some are</a>, most dusty, macro-silicate compounds require to be formed in moderately warm red giant winds to be formed over an elongated amount of time. Until those solids are available, the direct collapse planets will already have formed, in the simulations this happens usually fast, on a timescale of several 10.000 years.</p>

CC BY-SA 4.0
2019-10-10T15:51:00.727Z
So to the question "Characteristics of the first planets in the Universe?" would a tl;dr be "disc-instability gas giants, without much water or ammonia"?
uhoh
CC BY-SA 4.0
2019-10-10T16:12:58.843Z
Thank you! What would those early planets have looked like?
Dave Jarvis
CC BY-SA 4.0
2019-10-10T16:57:20.850Z
I don't understand how you are using the diagram. This shows a single (pseudo-Gaussian) distribution of planets and nothing at all in the section marked "brown dwarfs". There are planets that probably formed by gas instability alone (though that too is a metallicty-dependent process), but I can't identify them on this plot.
ProfRob
CC BY-SA 4.0
2019-10-10T18:12:57.330Z
@RobJeffries: I'm arguing that the left pseudo-gaussian would be nonexistent in a low-metallicity star forming environment. The right part (which has no brown dwarves in it, as the numbers of the EBLM survey are limited) is the low-mass branch of star formation, and would exist. There is a distinction between cloud fragmentation and disc instability, but the plot serves the point. Yes, disc fragmentation is metal-dependent but much weaker than the hierarchical building of cores. I'm just repeating my text here.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-10-10T18:14:04.233Z
@DaveJarvis: I don't know. If your other sources say that H/He giants are grey, then probably they would look grey. In the planet forming community colours are not very frequently predicted.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-10-10T18:15:01.647Z
@uhoh: Yes. Simply from the fact that dust grains that could trigger disc instability would take longer than that to grow in the ISM.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-10-10T19:36:33.010Z
The only proposed direct collapse "planets" have masses of 5-15 Jupiters. Nothing else on this diagram other than the objects in the left hand peak is a planet, as is clearly indicated in the labelling. The peak on the right is low-mass stars. Not planets.
ProfRob
CC BY-SA 4.0
2019-10-10T20:10:19.667Z
@RobJeffries: As I said, this is from real data, hence the numbers are limited. The labeling refers to a gap in numbers in the brown dwarf desert. If you look into the Forgan&Rice paper, where they perform a direct collapse population synthesis, they claim to be able to form planets way below Jupiter masses, thus extending the star-tail that is not limited by numbers in the sims. If you still want to label those gas blobs as "stars", then go ahead, that's a technicality. But usually any companion way below deuterium burning mass is usually considered a planet, no matter the provenance.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-10-10T22:36:58.840Z
"On the right are the brown-dwarf type planets, that can be considered as the lower mass-end of star formation". This just isn't correct and the reader would assume from your words that the bimodal mass distribution in the picture somehow represents "observational evidence" of the two *planet* formation modes.
ProfRob
CC BY-SA 4.0
2019-10-10T11:27:27.950Z
2019-10-10T11:27:27.950Z
AtmosphericPrisonEscape
<p>What would have the very first planets looked like, based on their most likely chemical compositions?</p>

<p>For example:</p>

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

<p>What papers support research into this area?</p>

<ul>
<li><a href="https://iopscience.iop.org/article/10.1088/0004-637X/751/2/81/meta" rel="nofollow noreferrer">https://iopscience.iop.org/article/10.1088/0004-637X/751/2/81/meta</a></li>
<li><a href="https://www.mdpi.com/2076-3263/9/3/105" rel="nofollow noreferrer">https://www.mdpi.com/2076-3263/9/3/105</a></li>
<li><a href="https://arxiv.org/pdf/1805.00023.pdf" rel="nofollow noreferrer">https://arxiv.org/pdf/1805.00023.pdf</a></li>
</ul>

<p>Note: By <em>first planets</em> I mean the first planets that most likely existed in the Universe. In <a href="https://www.cfa.harvard.edu/~loeb/habitable.pdf" rel="nofollow noreferrer"><em>The habitable epoch of the early Universe</em></a>, for example, Loeb suggests that a mere 13.5 ± 3.5 Myr after the <em>Hot Big Bang</em> 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.</p>

<p>Related:</p>

<ul>
<li><a href="http://www.sci-news.com/astronomy/science-helium-rich-gas-giants-02905.html" rel="nofollow noreferrer">http://www.sci-news.com/astronomy/science-helium-rich-gas-giants-02905.html</a></li>
<li><a href="https://en.wikipedia.org/wiki/Sudarsky%27s_gas_giant_classification" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Sudarsky%27s_gas_giant_classification</a></li>
<li><a href="https://www.nasa.gov/centers/goddard/news/topstory/2003/0709hstssu.html" rel="nofollow noreferrer">https://www.nasa.gov/centers/goddard/news/topstory/2003/0709hstssu.html</a></li>
<li><a href="https://www.space.com/17441-universe-heavy-metals-planet-formation.html" rel="nofollow noreferrer">https://www.space.com/17441-universe-heavy-metals-planet-formation.html</a></li>
<li><a href="https://earthsky.org/space/how-and-when-did-the-first-planets-form-in-our-universe" rel="nofollow noreferrer">https://earthsky.org/space/how-and-when-did-the-first-planets-form-in-our-universe</a></li>
<li><a href="https://medium.com/starts-with-a-bang/ask-ethan-how-fast-could-life-have-arisen-in-the-universe-863bd17b92ca" rel="nofollow noreferrer">https://medium.com/starts-with-a-bang/ask-ethan-how-fast-could-life-have-arisen-in-the-universe-863bd17b92ca</a></li>
<li><a href="https://www.nationalgeographic.com/news/2012/3/120326-oldest-planets-found-jupiter-big-bang-space-science/" rel="nofollow noreferrer">https://www.nationalgeographic.com/news/2012/3/120326-oldest-planets-found-jupiter-big-bang-space-science/</a></li>
</ul>

CC BY-SA 4.0
2019-10-04T15:07:19.263Z
A possibility could be giant planets of H/He, ie centesr of accretion that couldn't became stars. They could have born hot, probably even hot enough to be luminous but not enough to ignite.
Alchimista
CC BY-SA 4.0
2019-10-04T22:47:50.000Z
I would say that the H/HE theory seems most likely, and brown/red color of Jupiter is unlikely because it's carbon compounds that give Jupiter it's dark streaks of color (and frozen ices that gives Jupiter it's lighter streaks).   I've read that helium planets are grey.  Not sure about hydrogen/helium planets, as I've never seen one.  Jupiter and Saturn's visible light colors are from their trace gases and clouds not their hydrogen/helium.   Uranus and Neptune have relatively little free hydrogen/helium.
userLTK
<planet><star-formation><planetary-formation><gas-giants><early-universe>
Characteristics of the first planets in the Universe?
Dave Jarvis
# Characteristics of the first planets in the Universe?

(Asked by: username_0 on 2019-10-04)

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*](https://www.cfa.harvard.edu/~loeb/habitable.pdf), 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/>

**Tags:** planet, star-formation, planetary-formation, gas-giants, early-universe
> **Comments:**
> **username_1**: A possibility could be giant planets of H/He, ie centesr of accretion that couldn't became stars. They could have born hot, probably even hot enough to be luminous but not enough to ignite.  
> **username_2**: I would say that the H/HE theory seems most likely, and brown/red color of Jupiter is unlikely because it's carbon compounds that give Jupiter it's dark streaks of color (and frozen ices that gives Jupiter it's lighter streaks).   I've read that helium planets are grey.  Not sure about hydrogen/helium planets, as I've never seen one.  Jupiter and Saturn's visible light colors are from their trace gases and clouds not their hydrogen/helium.   Uranus and Neptune have relatively little free hydrogen/helium.  

---

## 1 answers

### Answer by username_3 on 2019-10-10. Score: 3

Our knowledge of planet formation processes comes from theoretical work and is supported by observations. I'm going to give it my take based on that.

On the lower mass end of the planet distribution, starting from comets up to Super-Earths/Mini-Neptunes one needs a high number of rock-forming elements in order to build rocky worlds.  
Without high concentrations of dust in protoplanetary discs that we have nowadays, the initial phases of planet formation, namely streaming instability, planetesimal/pebble accretion cannot happen. There is a debate recently, on the fact that low-metallicity stars can form terrestrial planets as well. But there must be a lower limit to this, as completely without heavy elements there just can't be rocky worlds.  
Also we have not identified any population III stars yet, in order to study their planet populations. This would be hard as well, as pop III stars would float around in the galactic center, which is quite far for our planet finding methods.

Also the classical [core-accretion](http://adsabs.harvard.edu/abs/1996Icar..124...62P) which would form gas giants indirectly would have trouble acting without a rocky core to begin with. However, protoplanetary discs can destabilize and form giant planets directly. This [disc fragmentation process](http://adsabs.harvard.edu/abs/2013MNRAS.432.3168F) is also dependent on how fast the disc can cool, and is thus sensitive to the amount of dust in the disc. However this sensitivity is not as strong as in the core-accretion scenario.

We also have observational evidence for those two processes forming gas giants (see this plot below from [Triaud et al, 2017](http://adsabs.harvard.edu/abs/2017A%26A...608A.129T)) in our galactic neighbourhood.

[![(c) Triaud+2017](https://i.sstatic.net/up2ZE.png)](https://i.sstatic.net/up2ZE.png)

On the left we have probably core-accretion gas giants, so those which were formed via the rock-rich path. On the right are the brown-dwarf type planets, that can be considered as the lower mass-end of star formation. The orange contours compare to a previous survey by [Grether & Lineweaver 2006](http://adsabs.harvard.edu/abs/2006ApJ...640.1051G).

Now concluding, I would argue that the left, rock-rich path would be essentially excluded in the early universe. Too many steps would have to be taken first and they all depend critically on the number of rock-forming elements available. Therefore, disc-instability gas giants, without much water or ammonia, should be the first planets to form.

Note that rock-forming elements are not all just formed in Supernovae. While [some are](http://adsabs.harvard.edu/abs/2011LPICo1639.9051G), most dusty, macro-silicate compounds require to be formed in moderately warm red giant winds to be formed over an elongated amount of time. Until those solids are available, the direct collapse planets will already have formed, in the simulations this happens usually fast, on a timescale of several 10.000 years.

> **Comments:**
> **username_4**: So to the question "Characteristics of the first planets in the Universe?" would a tl;dr be "disc-instability gas giants, without much water or ammonia"?  
> **username_0**: Thank you! What would those early planets have looked like?  
> **username_5**: I don't understand how you are using the diagram. This shows a single (pseudo-Gaussian) distribution of planets and nothing at all in the section marked "brown dwarfs". There are planets that probably formed by gas instability alone (though that too is a metallicty-dependent process), but I can't identify them on this plot.  
> **username_3**: @RobJeffries: I'm arguing that the left pseudo-gaussian would be nonexistent in a low-metallicity star forming environment. The right part (which has no brown dwarves in it, as the numbers of the EBLM survey are limited) is the low-mass branch of star formation, and would exist. There is a distinction between cloud fragmentation and disc instability, but the plot serves the point. Yes, disc fragmentation is metal-dependent but much weaker than the hierarchical building of cores. I'm just repeating my text here.  
> **username_3**: @DaveJarvis: I don't know. If your other sources say that H/He giants are grey, then probably they would look grey. In the planet forming community colours are not very frequently predicted.  
> **username_3**: @uhoh: Yes. Simply from the fact that dust grains that could trigger disc instability would take longer than that to grow in the ISM.  
> **username_5**: The only proposed direct collapse "planets" have masses of 5-15 Jupiters. Nothing else on this diagram other than the objects in the left hand peak is a planet, as is clearly indicated in the labelling. The peak on the right is low-mass stars. Not planets.  
> **username_3**: @RobJeffries: As I said, this is from real data, hence the numbers are limited. The labeling refers to a gap in numbers in the brown dwarf desert. If you look into the Forgan&Rice paper, where they perform a direct collapse population synthesis, they claim to be able to form planets way below Jupiter masses, thus extending the star-tail that is not limited by numbers in the sims. If you still want to label those gas blobs as "stars", then go ahead, that's a technicality. But usually any companion way below deuterium burning mass is usually considered a planet, no matter the provenance.  
> **username_5**: "On the right are the brown-dwarf type planets, that can be considered as the lower mass-end of star formation". This just isn't correct and the reader would assume from your words that the bimodal mass distribution in the picture somehow represents "observational evidence" of the two *planet* formation modes.  

---

