<p>The original research paper is published is found on A&amp;A and <a href="https://arxiv.org/abs/2101.09260" rel="nofollow noreferrer">available via arxiv</a>, too.</p>
<p>One of the mysteries to understand in planet formation is the distinction between terrestrial planets and gasous planets - there is no continuous distribution curve. That means that different processes must be involved. Understanding how the involved processes interact, what conditions are required to trigger the individual processes, and how they shape a final system is important for the overall understanding. And this is easiest, the more complete information you have about a system and the better you can deduce its past evolution from the current state. The laplace chaining of the <a href="https://en.wikipedia.org/wiki/Orbital_resonance#Mean-motion_resonances_among_extrasolar_planets" rel="nofollow noreferrer">MMR</a> of the planets helps in this endevour.</p>
<p>Thus using this somewhat surprising stability of the system, we can tackle further issues with the formation of the individual planets itself and possibly also the planetesimal-disk interaction. In their summary they state:</p>
<blockquote>
<p>In this respect, the different planets of the TOI-178 system lie on
both sides of the radius valley (Fulton et al. 2017). Therefore,
reconstructing the past orbital and atmospheric history of this planet
may provide clues regarding the origin of the valley.</p>
</blockquote>
<p>The conjected 'decreasing density' of planets from center outward directly comes from the density of available solid material in the protoplanetary disk: further inward at higher temperatures only the heavier elements can condense. Barring any migration of planets and swapping radial order, this defines the radial density distribution. You see the very same density variation inside our own solar system. The equation of state of matter at high pressures in the planets centres somewhat slightly distorts this, but the general trend applies.</p>

CC BY-SA 4.0
2021-01-26T15:12:18.223Z
I get the part with the possible two formation mechanisms for terrestrial and gas planets, and want to dig deeper into that issue. The radially decreasing temperature of a protoplanetary disk also makes sense intuitively, but couldn't simply the density fluctuation of the protoplanetary disk be the cause of the given density order of the exoplanets?
B--rian
CC BY-SA 4.0
2021-01-26T10:20:06.263Z
2021-01-26T10:30:38.080Z
2021-01-26T10:30:38.080Z
planetmaker
<p>I am excited reading the <a href="https://www.esa.int/Science_Exploration/Space_Science/Cheops/ESA_s_exoplanet_watcher_Cheops_reveals_unique_planetary_system" rel="nofollow noreferrer">news about ESA's latest exoplanet discovery</a> using <a href="https://en.wikipedia.org/wiki/CHEOPS" rel="nofollow noreferrer">CHEOPS</a>. The system <a href="https://en.wikipedia.org/wiki/List_of_exoplanets_discovered_in_2021" rel="nofollow noreferrer">TOI-178</a> consists of (at least?) five planets, following a 18:9:6:4:3 pattern. In the press release, the authors write:</p>
<blockquote>
<p>But while the planets in the TOI-178 system orbit their star in a very orderly manner, their densities do not follow any particular pattern. One of the exoplanets, a dense, terrestrial planet like Earth is right next to a similar-sized but very fluffy planet ­­– like a mini-Jupiter, and next to that is one very similar to Neptune.</p>
</blockquote>
<blockquote>
<p>In the few systems we know where the planets orbit in this resonant rhythm, the densities of the planets gradually decrease as we move away from the star, and it is also what we expect from theory.”</p>
</blockquote>
<p>My question: What exactly does &quot;the theory&quot; state <em>exactly</em>? And why exoplanets in <a href="https://en.wikipedia.org/wiki/Orbital_resonance" rel="nofollow noreferrer">orbital resonance</a> necessarily imply decreasing <em>density</em> of the planets if the system did not collide with any other system after its formation?</p>

<exoplanet><orbital-resonance><cheops>
Why does the exoplanetary system TOI-178 challenges current theories of planet formation?
B--rian
# Why does the exoplanetary system TOI-178 challenges current theories of planet formation?

(Asked by: username_0 on 2021-01-25)

I am excited reading the [news about ESA's latest exoplanet discovery](https://www.esa.int/Science_Exploration/Space_Science/Cheops/ESA_s_exoplanet_watcher_Cheops_reveals_unique_planetary_system) using [CHEOPS](https://en.wikipedia.org/wiki/CHEOPS). The system [TOI-178](https://en.wikipedia.org/wiki/List_of_exoplanets_discovered_in_2021) consists of (at least?) five planets, following a 18:9:6:4:3 pattern. In the press release, the authors write:

> But while the planets in the TOI-178 system orbit their star in a very orderly manner, their densities do not follow any particular pattern. One of the exoplanets, a dense, terrestrial planet like Earth is right next to a similar-sized but very fluffy planet ­­– like a mini-Jupiter, and next to that is one very similar to Neptune.

> In the few systems we know where the planets orbit in this resonant rhythm, the densities of the planets gradually decrease as we move away from the star, and it is also what we expect from theory.”

My question: What exactly does "the theory" state *exactly*? And why exoplanets in [orbital resonance](https://en.wikipedia.org/wiki/Orbital_resonance) necessarily imply decreasing *density* of the planets if the system did not collide with any other system after its formation?

**Tags:** exoplanet, orbital-resonance, cheops

---

## 1 answers

### Answer by username_1 on 2021-01-26. Score: 5 | Accepted answer

The original research paper is published is found on A&A and [available via arxiv](https://arxiv.org/abs/2101.09260), too.

One of the mysteries to understand in planet formation is the distinction between terrestrial planets and gasous planets - there is no continuous distribution curve. That means that different processes must be involved. Understanding how the involved processes interact, what conditions are required to trigger the individual processes, and how they shape a final system is important for the overall understanding. And this is easiest, the more complete information you have about a system and the better you can deduce its past evolution from the current state. The laplace chaining of the [MMR](https://en.wikipedia.org/wiki/Orbital_resonance#Mean-motion_resonances_among_extrasolar_planets) of the planets helps in this endevour.

Thus using this somewhat surprising stability of the system, we can tackle further issues with the formation of the individual planets itself and possibly also the planetesimal-disk interaction. In their summary they state:

> In this respect, the different planets of the TOI-178 system lie on
> both sides of the radius valley (Fulton et al. 2017). Therefore,
> reconstructing the past orbital and atmospheric history of this planet
> may provide clues regarding the origin of the valley.

The conjected 'decreasing density' of planets from center outward directly comes from the density of available solid material in the protoplanetary disk: further inward at higher temperatures only the heavier elements can condense. Barring any migration of planets and swapping radial order, this defines the radial density distribution. You see the very same density variation inside our own solar system. The equation of state of matter at high pressures in the planets centres somewhat slightly distorts this, but the general trend applies.

> **Comments:**
> **username_0**: I get the part with the possible two formation mechanisms for terrestrial and gas planets, and want to dig deeper into that issue. The radially decreasing temperature of a protoplanetary disk also makes sense intuitively, but couldn't simply the density fluctuation of the protoplanetary disk be the cause of the given density order of the exoplanets?  

---

