<p>This is not a characteristic of the solar system. It is a characteristic of the definitions of the names you used. Neptune and Uranus are the bodies you believe to be missing. In fact, with the mass of Earth at 6*10^24 kg, Uranus at 9*10^25 kg and Jupiter 2*10^27 kg, you'll notice that Uranus is only ~15 times the mass of Earth while Jupiter is ~20 times the mass of Uranus.</p>

<p>The initial growth rate of planets was limited by how much solid dust there was that could clump together to form them. Planets that formed in colder conditions outside the ice line were able to grow faster because of ices (especially water but also ammonia, etc. depending on how far out) that were prevalent in those parts of the nebula from which the solar system formed. Once they reached a critical mass and were able to hold on to hydrogen and helium gas gravitationally, they would grow much faster.</p>

<p>Jupiter grew fast due to its location just outside the ice line, where it had a comparatively dense supply of ice, and could then quickly gobble up a large supply of gas. Neptune and Uranus did not grow as quickly and had only collected a small amount of gas by the time the solar wind blew the nebula away. This is why they are composed of a much larger percentage ices than Jupiter and Saturn; in fact they are often called the "ice giants" which is the correct intermediate classification. (The "ice" in "ice giant" refers to the substance, which is of course no longer generally solid in the atmospheres of these two bodies.)</p>

CC BY-SA 3.0
2016-11-27T15:35:06.533Z
2016-11-27T15:35:06.533Z
user25972
<p>Since 2016 <a href="https://en.wikipedia.org/wiki/Planet_Nine" rel="nofollow noreferrer">a ninth planet is hypothesized beyond the Kuiper belt</a> which, if it exists and if it's a planet, is either a super-Earth or a mini-Neptune.</p>

<p>But you might want to consider Venus a planet in-between. While it isn't bigger than Earth, on the surface of Venus the air pressure is almost a hundred times that on Earth which makes the atmosphere a supercritical fluid there like those of the gas giants. For comparison, the air pressure within Uranus and Neptune is more than a thousand times that of Earth and within Jupiter and Saturn more than a million. So Venus is kind of inbetween. From space, Venus looks similar to the gas planets since you can't see its surface at all, but its cloud layers. </p>

CC BY-SA 4.0
2020-05-19T14:20:13.523Z
Can you please show some reference from this hyptothetised planet? It looks a little bit... mysterious.
peterh
CC BY-SA 4.0
2020-05-20T05:17:48.927Z
@antispinwards I've just written if the user "might want to consider" it an inbetween planet, but of course it's clearly a solid planet of rock. I referred to the fact that the air on the surface is a supercritical fluid (like in the gas giants) and that the surface is invisible from space. And if Earth would become like that too, well you might consider it such an "intermediate planet" too then.
The Architect
CC BY-SA 4.0
2020-05-19T13:37:17.987Z
2020-05-19T14:24:44.740Z
2020-05-19T14:24:44.740Z
The Architect
<p><a href="https://en.wikipedia.org/wiki/Super-Earth" rel="noreferrer"><strong>Super-Earths</strong></a> and <a href="https://en.wikipedia.org/wiki/Super-Earth" rel="noreferrer"><strong>Mini-Neptunes</strong></a> are the "in-between" types of exoplanets you're looking for. A sweeping generalization would put most in the range of $\sim1$-$10M_{\oplus}$ (Earth masses), with some outliers a bit above that. They may have significant quantities of hydrogen and helium in their atmospheres, as well as water, in liquid or vapor form. The latter should also have what are referred to as <a href="https://en.wikipedia.org/wiki/Volatiles" rel="noreferrer">volatiles</a> (colloquially called "ices") - compounds such as ammonia and water that are present in the atmospheres of proper ice giants like Uranus and Neptune.</p>

<p>One reason we don't have any of these in the Solar System is simply that no worlds with the proper mass formed. I explained <a href="https://astronomy.stackexchange.com/a/18564/2153">here</a> that there is a transition region of sorts, but most super-Earths require minimum masses of $1.5$-$2M_{\oplus}$ to keep their large hydrogen/helium envelopes (which terrestrial planets may accrete early in their lives but later lose). There would have to be a planet just in that mass range <em>and</em> at the right distance from the Sun (see <a href="https://arxiv.org/abs/1606.08088" rel="noreferrer">D'Angelo &amp; Bodenheimer (2016)</a> for general constraints on semi-major axes). The environment in the protoplanetary disk may simply not have been suitable to produce the right body at the right time.</p>

CC BY-SA 3.0
2016-11-25T14:45:57.520Z
OP asked *why*? Most systems we detected appear to have super earths. Were their protoplanetary disks different?
kubanczyk
CC BY-SA 3.0
2016-11-27T18:35:44.437Z
@kubanczyk Can you back up the statement "Most systems we detected appear to have super earths"? As far as I know, those are the exception, not the rule.
HDE 226868
CC BY-SA 3.0
2016-11-27T19:46:25.273Z
https://arxiv.org/abs/1508.00931
kubanczyk
CC BY-SA 3.0
2016-11-28T16:59:59.287Z
The current methods favor the detection of larger planets. I'm not sure what the exact numbers are, but that detection bias might account for some of the reason why we think super earths are common.
ventsyv
CC BY-SA 3.0
2016-11-28T19:30:44.813Z
@ventsyv I'm well aware of that; I just didn't quite understand how a detection bias implies that there are lots of super-Earths.
HDE 226868
CC BY-SA 3.0
2016-12-03T12:27:02.297Z
One of the latest in a series of "Copernican discoveries" since Galileo is that our solar system is not the "blueprint" from which other solar systems are measured. There are still several plausible theories for how planetary systems form. Most fall under the general models of forming in their entirety from discontinuities in the proto-planetary disc, accretion onto rocky cores or some combination of the above. In other words, we don't "really know" why we are the way we are. It may be more due to random luck than anything.
Jack R. Woods
CC BY-SA 4.0
2022-03-19T15:00:46.850Z
I think the OP is asking _exclusively_ about the Solar system. The question is why we have gas giants immediately after terrestrial planets without those "in-between"s.
Snack Exchange
CC BY-SA 3.0
2016-11-24T02:47:35.177Z
2016-11-24T02:47:35.177Z
2017-04-13T12:59:06.910Z
HDE 226868
<p>The planets in our system are most often grouped into two categories:  </p>

<p><strong>Terrestrial:</strong></p>

<ul>
<li>Mercury</li>
<li>Venus</li>
<li>Earth</li>
<li>Mars</li>
</ul>

<p><strong>Gas Giants:</strong></p>

<ul>
<li><p>Jupiter</p></li>
<li><p>Saturn</p></li>
<li><p>Uranus</p></li>
<li><p>Neptune</p></li>
</ul>

<p>Why is it that we don't see "in-between" planets in our system - large, rocky worlds larger than Earth but with thick atmospheres somewhat similar to those of gas giants? Are they possible? Did they not occur by random chance?</p>

<planet><gas-giants><planetary-formation><terrestrial-planets>
Why don't we have in-between planets?
Zxyrra
# Why don't we have in-between planets?

(Asked by: username_0 on 2016-11-24)

The planets in our system are most often grouped into two categories:

**Terrestrial:**

* Mercury
* Venus
* Earth
* Mars

**Gas Giants:**

* Jupiter
* Saturn
* Uranus
* Neptune

Why is it that we don't see "in-between" planets in our system - large, rocky worlds larger than Earth but with thick atmospheres somewhat similar to those of gas giants? Are they possible? Did they not occur by random chance?

**Tags:** planet, gas-giants, planetary-formation, terrestrial-planets


---

## 3 answers

### Answer by username_4 on 2016-11-24. Score: 13 | Accepted answer

[**Super-Earths**](https://en.wikipedia.org/wiki/Super-Earth) and [**Mini-Neptunes**](https://en.wikipedia.org/wiki/Super-Earth) are the "in-between" types of exoplanets you're looking for. A sweeping generalization would put most in the range of $\sim1$-$10M\_{\oplus}$ (Earth masses), with some outliers a bit above that. They may have significant quantities of hydrogen and helium in their atmospheres, as well as water, in liquid or vapor form. The latter should also have what are referred to as [volatiles](https://en.wikipedia.org/wiki/Volatiles) (colloquially called "ices") - compounds such as ammonia and water that are present in the atmospheres of proper ice giants like Uranus and Neptune.

One reason we don't have any of these in the Solar System is simply that no worlds with the proper mass formed. I explained [here](https://astronomy.stackexchange.com/a/18564/2153) that there is a transition region of sorts, but most super-Earths require minimum masses of $1.5$-$2M\_{\oplus}$ to keep their large hydrogen/helium envelopes (which terrestrial planets may accrete early in their lives but later lose). There would have to be a planet just in that mass range *and* at the right distance from the Sun (see [D'Angelo & Bodenheimer (2016)](https://arxiv.org/abs/1606.08088) for general constraints on semi-major axes). The environment in the protoplanetary disk may simply not have been suitable to produce the right body at the right time.

> **Comments:**
> **username_5**: OP asked *why*? Most systems we detected appear to have super earths. Were their protoplanetary disks different?  
> **username_4**: @username_5 Can you back up the statement "Most systems we detected appear to have super earths"? As far as I know, those are the exception, not the rule.  
> **username_5**: https://arxiv.org/abs/1508.00931  
> **username_6**: The current methods favor the detection of larger planets. I'm not sure what the exact numbers are, but that detection bias might account for some of the reason why we think super earths are common.  
> **username_4**: @username_6 I'm well aware of that; I just didn't quite understand how a detection bias implies that there are lots of super-Earths.  
> **username_7**: One of the latest in a series of "Copernican discoveries" since Galileo is that our solar system is not the "blueprint" from which other solar systems are measured. There are still several plausible theories for how planetary systems form. Most fall under the general models of forming in their entirety from discontinuities in the proto-planetary disc, accretion onto rocky cores or some combination of the above. In other words, we don't "really know" why we are the way we are. It may be more due to random luck than anything.  
> **username_8**: I think the OP is asking _exclusively_ about the Solar system. The question is why we have gas giants immediately after terrestrial planets without those "in-between"s.  

---

### Answer by username_1 on 2016-11-27. Score: 6

This is not a characteristic of the solar system. It is a characteristic of the definitions of the names you used. Neptune and Uranus are the bodies you believe to be missing. In fact, with the mass of Earth at 6\*10^24 kg, Uranus at 9\*10^25 kg and Jupiter 2\*10^27 kg, you'll notice that Uranus is only ~15 times the mass of Earth while Jupiter is ~20 times the mass of Uranus.

The initial growth rate of planets was limited by how much solid dust there was that could clump together to form them. Planets that formed in colder conditions outside the ice line were able to grow faster because of ices (especially water but also ammonia, etc. depending on how far out) that were prevalent in those parts of the nebula from which the solar system formed. Once they reached a critical mass and were able to hold on to hydrogen and helium gas gravitationally, they would grow much faster.

Jupiter grew fast due to its location just outside the ice line, where it had a comparatively dense supply of ice, and could then quickly gobble up a large supply of gas. Neptune and Uranus did not grow as quickly and had only collected a small amount of gas by the time the solar wind blew the nebula away. This is why they are composed of a much larger percentage ices than Jupiter and Saturn; in fact they are often called the "ice giants" which is the correct intermediate classification. (The "ice" in "ice giant" refers to the substance, which is of course no longer generally solid in the atmospheres of these two bodies.)


---

### Answer by username_2 on 2020-05-19. Score: -1

Since 2016 [a ninth planet is hypothesized beyond the Kuiper belt](https://en.wikipedia.org/wiki/Planet_Nine) which, if it exists and if it's a planet, is either a super-Earth or a mini-Neptune.

But you might want to consider Venus a planet in-between. While it isn't bigger than Earth, on the surface of Venus the air pressure is almost a hundred times that on Earth which makes the atmosphere a supercritical fluid there like those of the gas giants. For comparison, the air pressure within Uranus and Neptune is more than a thousand times that of Earth and within Jupiter and Saturn more than a million. So Venus is kind of inbetween. From space, Venus looks similar to the gas planets since you can't see its surface at all, but its cloud layers.

> **Comments:**
> **username_3**: Can you please show some reference from this hyptothetised planet? It looks a little bit... mysterious.  
> **username_2**: @antispinwards I've just written if the user "might want to consider" it an inbetween planet, but of course it's clearly a solid planet of rock. I referred to the fact that the air on the surface is a supercritical fluid (like in the gas giants) and that the surface is invisible from space. And if Earth would become like that too, well you might consider it such an "intermediate planet" too then.  

---

