<p>The terrestrial planets formed in their final configurations over the course of a hundred million years or so. The collision that formed the Moon probably occurred about 30-50 million years after the birth of the protosun.</p>
<p>There was probably no significant gas in the Solar System after about 10 million years. A typical disc lifetime for a solar-type star is a few million years and discs appear to clear from the inside outwards.</p>

CC BY-SA 4.0
2024-05-21T05:34:12.913Z
Thank you! Is there any particular reason why terrestrial planets formed later? I'm assuming it's because of less solid material.

Also does that rule out the possibility of any such exoplanet?
Ale Kid
CC BY-SA 4.0
2024-05-21T05:13:23.593Z
2024-05-21T05:13:23.593Z
ProfRob
<p>Outside the frost line, there were more solids available, so accretion of solids into planetary embryos was faster there. Planetary embryos quickly grew large enough to directly capture gas in the outer solar system, beating the 10 million year deadline for gas dispersal in the disk mentioned by @ProfRob.</p>
<p>But in the inner solar system, the process was slower. Gas was cleared out of the disk before any large embryos could develop. Solid planetesimals remained, but as @ProfRob says, those merged and collided over about 100 million years.</p>
<p>OP mentioned that the terrestrial planets are large enough to retain gas atmospheres against escape, but these atmospheres are &quot;secondary atmospheres,&quot; containing matter outgassed from the planetary interiors. The giant planets have &quot;primary atmospheres&quot; that they directly accreted from the disk.</p>

CC BY-SA 4.0
2024-05-28T19:03:56.677Z
2024-05-28T19:03:56.677Z
giardia
<p>How come planets beyond the frost line form with ice cores but terrestrial planets didn't accrete anywhere near as much water vapour?</p>
<p>The way I understand accretion to work, planets form out of molecules that are solid at their equilibrium temperature. Then once they are sufficiently massive, they start pulling gasses like helium and hydrogen from the accretion disc (if their escape velocity is significant enough) via atmospheric escape.</p>
<p>Most of the planets beyond the frost line were massive enough to retain hydrogen by atmospheric escape and became gas giants. Earth and Venus are massive enough to hold onto water vapour - yet they have orders of magnitude less water/ice than any planet beyond the frost line.</p>
<p>What causes this huge disparity?</p>

CC BY-SA 4.0
2024-05-21T16:29:21.837Z
The abundance of water in our protoplanetary disk is expected to have been around $1 \times 10^{-4}$ of hydrogen, as it is in the atmosphere of Jupiter, and about $1 \times 10^{-2}$ of the dust.  So, there was probably less than a 0.1 Earth mass of $H_20$ within the gravitational influence of the forming Earth.
eshaya
<planetary-atmosphere><planetary-formation><water><accretion-discs><atmospheric-escape>
What stops terrestrial planets from becoming watery gas giants with water vapour atmospheres?
Ale Kid
# What stops terrestrial planets from becoming watery gas giants with water vapour atmospheres?

(Asked by: username_0 on 2024-05-21)

How come planets beyond the frost line form with ice cores but terrestrial planets didn't accrete anywhere near as much water vapour?

The way I understand accretion to work, planets form out of molecules that are solid at their equilibrium temperature. Then once they are sufficiently massive, they start pulling gasses like helium and hydrogen from the accretion disc (if their escape velocity is significant enough) via atmospheric escape.

Most of the planets beyond the frost line were massive enough to retain hydrogen by atmospheric escape and became gas giants. Earth and Venus are massive enough to hold onto water vapour - yet they have orders of magnitude less water/ice than any planet beyond the frost line.

What causes this huge disparity?

**Tags:** planetary-atmosphere, planetary-formation, water, accretion-discs, atmospheric-escape

> **Comments:**
> **username_1**: The abundance of water in our protoplanetary disk is expected to have been around $1 \times 10^{-4}$ of hydrogen, as it is in the atmosphere of Jupiter, and about $1 \times 10^{-2}$ of the dust.  So, there was probably less than a 0.1 Earth mass of $H_20$ within the gravitational influence of the forming Earth.  

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## 2 answers

### Answer by username_2 on 2024-05-21. Score: 2

The terrestrial planets formed in their final configurations over the course of a hundred million years or so. The collision that formed the Moon probably occurred about 30-50 million years after the birth of the protosun.

There was probably no significant gas in the Solar System after about 10 million years. A typical disc lifetime for a solar-type star is a few million years and discs appear to clear from the inside outwards.

> **Comments:**
> **username_0**: Thank you! Is there any particular reason why terrestrial planets formed later? I'm assuming it's because of less solid material.  Also does that rule out the possibility of any such exoplanet?  

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### Answer by username_3 on 2024-05-28. Score: 0

Outside the frost line, there were more solids available, so accretion of solids into planetary embryos was faster there. Planetary embryos quickly grew large enough to directly capture gas in the outer solar system, beating the 10 million year deadline for gas dispersal in the disk mentioned by @username_2.

But in the inner solar system, the process was slower. Gas was cleared out of the disk before any large embryos could develop. Solid planetesimals remained, but as @username_2 says, those merged and collided over about 100 million years.

OP mentioned that the terrestrial planets are large enough to retain gas atmospheres against escape, but these atmospheres are "secondary atmospheres," containing matter outgassed from the planetary interiors. The giant planets have "primary atmospheres" that they directly accreted from the disk.


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