<p>Vacuum does not mean a space that is completely devoid of matter, dark matter, or photons. While a perfect vacuum makes for a very nice spherical cow assumption, no such space exists. While physicists do like their spherical cow assumptions, they also self-effacingly deride themselves for making those spherical cow assumptions.</p>
<p>There is no such a thing anywhere in the universe as a perfect vacuum. However, the assumption of a perfect vacuum does yield useful results for many electromagnetic interactions. An imperfect vacuum -- now that is something that is encountered in space, and even in laboratories on the surface of the Earth. The best vacuum attainable in a lab is nowhere near what is attainable in space, even near-Earth space. For example, low Earth orbit space is considered to be both vacuum (it's a much himgher quality vacuum than any vacuum attainable in an Earth-bound vacuum chamber) and yet still in the atmosphere.</p>

CC BY-SA 4.0
2021-08-08T16:35:50.980Z
2021-08-08T16:35:50.980Z
David Hammen
<p>Space isn't an absolute vacuum. Even in intergalactic space there a few atoms in a sufficiently large volume (like a <span class="math-container">$m^3$</span>). In some respects you can approximate it with an absolute vacuum but in others not (for instance light travels through extremely large distances in intergalactic space, so it will still be affected by many atoms on its way).</p>

CC BY-SA 4.0
2021-08-08T08:46:00.280Z
2021-08-08T08:46:00.280Z
Thomas
<blockquote>
<p>How is space a vacuum when there are planets, gases, etc?</p>
</blockquote>
<p>Gravity and solar wind scrubs most of it away!</p>
<p>Lucky for us gravity pulls stuff together. This is what made our Sun collapse into a ball and the potential energy released by everything getting closer is what made it hot enough to start nuclear reactions and producing life-giving light.</p>
<p>Gravity also pulled matter together to produce the planets, and big balls of hydrogen (and other things) like Jupiter wouldn't exist either.</p>
<p>The reason our atmosphere's density drops a factor of two every 5-6 km is also due to gravity. Over time, gas seeks out sources of gravity and after a couple of collisions with other molecules forgets where it came from or how much kinetic energy it had. It <em>thermalizes</em>.</p>
<h3>But what about those atoms that escape planets?</h3>
<p>The tail end of thermal distributions means that some molecules will regularly escape, but the rate is quite slow. Once they do, the solar wind will eventually pick them up and take them out of (at least the inner) solar system.</p>
<p>The solar wind also picks off atoms from planet's atmospheres directly if it can. <s>This is why Mars' atmosphere is so small now. Without a magnetic field to deflect the charged particles from the Sun (like Earth does) the solar wind ran off with most of Mars' atmosphere.</s> Comments indicate there's recent reconsideration of this.</p>
<p>For more about that I've just asked</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/45205/7982">...but where did Mars&#39; atmosphere actually GO?</a></li>
</ul>
<p>So you can run and you can hide, but gravity and/or the solar wind will eventually scour the interplanetary spaces in our solar system and remove most of the gas that would otherwise orbit the Sun.</p>

CC BY-SA 4.0
2021-08-08T09:13:27.820Z
I fail to see why this was voted down as it directly addresses the question.
StephenG - Help Ukraine
CC BY-SA 4.0
2021-08-08T10:44:36.907Z
"This is why Mars' atmosphere is so small now." -- It's probably a lot less clear than the simplistic "magnetic field protects against solar wind" model you're invoking (which completely fails to explain why Venus has a significant atmosphere, or why the current atmospheric loss rate of Venus, Earth, and Mars are roughly the same). See e.g. https://ui.adsabs.harvard.edu/abs/2018A%26A...614L...3G/abstract
Peter Erwin
CC BY-SA 4.0
2021-08-08T12:57:15.833Z
@PeterErwin When a "contrary to what has previously been believed" simulation paper is published it doesn't necessary upend everything and become the new gospel truth instantly. Is the paper *right?* Have planetary scientists rallied around these simulation results as providing *actual loss rates?* They cite measured data from these three planets that are all almost 20 years old, but so far I can't even see how well the paper agrees with them. Anyway the abstract of the paper says that a magnetic field isn't "sufficient" but it does not say that Mars' magnetic field wasn't *necessary*.
uhoh
CC BY-SA 4.0
2021-08-08T12:59:24.990Z
@PeterErwin "Therefore, contrary to what has previously been believed, magnetisation is not a sufficient condition for protecting a planet from atmospheric loss." Was it in fact ever believed that a magnetic field was "sufficient"? Or could this be a bit of linguistic gamesmanship?
uhoh
CC BY-SA 4.0
2021-08-08T13:53:15.990Z
@uhoh The recent review article by [Gronoff et al. (2020)](https://ui.adsabs.harvard.edu/abs/2020JGRA..12527639G/abstract) says, "We show that the paradigm of the magnetic field as an atmospheric shield should be changed..."
Peter Erwin
CC BY-SA 4.0
2021-08-08T13:54:03.603Z
From Section 7.1.2 of Gronoff et al: "while the presence of a magnetosphere has a clear impact on ionospheric outflow, recent developments in the study of the coupling between stellar wind, magnetospheres and ionospheres challenge the idea of a protective effect of magnetospheres on atmospheric erosion.... "
Peter Erwin
CC BY-SA 4.0
2021-08-08T13:54:46.587Z
And from the same section: "recent studies such as Brain et al. (2013); Gunell et al. (2018); Airapetian et al. (2017); Garcia-Sage et al. (2017), as well as the case of Mercury, show that an intrinsic magnetic field does not totally protect an atmosphere. A contrario, the case of Venus shows that a magnetic field absence does not prevent sustaining a dense atmosphere."
Peter Erwin
CC BY-SA 4.0
2021-08-08T14:25:02.910Z
"Without a magnetic field to deflect the charged particles from the Sun (like Earth does) the solar wind ran off with most of Mars' atmosphere." This certainly sounds like a clear statement that it's the magnetic field that matters. (Or was that just "a bit of linguistic gamesmanship"?)
Peter Erwin
CC BY-SA 4.0
2021-08-08T14:26:02.890Z
This answer does not address the OP's misconception.
David Hammen
CC BY-SA 4.0
2021-08-08T14:49:10.303Z
See also [Tarduno et al. (2014)](https://ui.adsabs.harvard.edu/abs/2014PEPI..233...68T/abstract) -- "It is true that a magnetic field is not necessary to retain an atmosphere but, the relevant issue for life as we know it is water" --; [Egan et al. (2019)](https://ui.adsabs.harvard.edu/abs/2019MNRAS.488.2108E/abstract); and [Johnstone (2021)](https://ui.adsabs.harvard.edu/abs/2021arXiv210511243J/abstract).
Peter Erwin
CC BY-SA 4.0
2021-08-08T18:20:35.640Z
@PeterErwin okay that's quite a collection, I'll take a look. In the mean time I've struck-through that sentence.
uhoh
CC BY-SA 4.0
2021-08-08T18:21:51.367Z
@DavidHammen I won't practice telepathy without a license https://www.youtube.com/watch?v=6giYSgxiWEo From a single sentence we can't really divine what conceptions  or misconceptions an OP may or may not hold.
uhoh
CC BY-SA 4.0
2021-08-08T05:12:10.103Z
2021-08-08T18:19:36.030Z
2021-08-08T18:19:36.030Z
uhoh
<p>Space has a lot of stuff so how can it be a vacuum?</p>

CC BY-SA 4.0
2021-08-08T03:51:11.017Z
The amount of stuff per unit volume is mostly very small, apart from the occasional lumpy bits (dust grains, rocks, planets, etc). Even a whole galaxy doesn't contain a lot of stuff, relatively speaking, as I explain here: https://astronomy.stackexchange.com/a/41005/16685
PM 2Ring
CC BY-SA 4.0
2021-08-08T05:16:10.703Z
"Space" is the bits between the stars and planets. And there is very little gas there. The definition of "vacuum" doesn't mean no gas at all, just very little.    See https://astronomy.stackexchange.com/questions/33869/how-do-we-know-that-vacuum-is-devoid-of-matter
James K
CC BY-SA 4.0
2021-08-08T12:23:56.287Z
How is the inside of your vacuum cleaner a vacuum when there is dust and particles flying around inside it?
Daddy Kropotkin
CC BY-SA 4.0
2021-08-08T16:46:34.433Z
How is the sea called water, when there are boats, whales, fish, salt etc?
James K
<astrophysics>
How is space a vacuum when there are planets, gases, etc?
Omar Zayed
# How is space a vacuum when there are planets, gases, etc?

(Asked by: username_0 on 2021-08-08)

Space has a lot of stuff so how can it be a vacuum?

**Tags:** astrophysics
> **Comments:**
> **username_1**: The amount of stuff per unit volume is mostly very small, apart from the occasional lumpy bits (dust grains, rocks, planets, etc). Even a whole galaxy doesn't contain a lot of stuff, relatively speaking, as I explain here: https://astronomy.stackexchange.com/a/41005/16685  
> **username_2**: "Space" is the bits between the stars and planets. And there is very little gas there. The definition of "vacuum" doesn't mean no gas at all, just very little.    See https://astronomy.stackexchange.com/questions/33869/how-do-we-know-that-vacuum-is-devoid-of-matter  
> **username_3**: How is the inside of your vacuum cleaner a vacuum when there is dust and particles flying around inside it?  
> **username_2**: How is the sea called water, when there are boats, whales, fish, salt etc?  

---

## 3 answers

### Answer by username_4 on 2021-08-08. Score: 2

Vacuum does not mean a space that is completely devoid of matter, dark matter, or photons. While a perfect vacuum makes for a very nice spherical cow assumption, no such space exists. While physicists do like their spherical cow assumptions, they also self-effacingly deride themselves for making those spherical cow assumptions.

There is no such a thing anywhere in the universe as a perfect vacuum. However, the assumption of a perfect vacuum does yield useful results for many electromagnetic interactions. An imperfect vacuum -- now that is something that is encountered in space, and even in laboratories on the surface of the Earth. The best vacuum attainable in a lab is nowhere near what is attainable in space, even near-Earth space. For example, low Earth orbit space is considered to be both vacuum (it's a much himgher quality vacuum than any vacuum attainable in an Earth-bound vacuum chamber) and yet still in the atmosphere.


---

### Answer by username_5 on 2021-08-08. Score: 1

Space isn't an absolute vacuum. Even in intergalactic space there a few atoms in a sufficiently large volume (like a $m^3$). In some respects you can approximate it with an absolute vacuum but in others not (for instance light travels through extremely large distances in intergalactic space, so it will still be affected by many atoms on its way).


---

### Answer by username_6 on 2021-08-08. Score: 0

> How is space a vacuum when there are planets, gases, etc?

Gravity and solar wind scrubs most of it away!

Lucky for us gravity pulls stuff together. This is what made our Sun collapse into a ball and the potential energy released by everything getting closer is what made it hot enough to start nuclear reactions and producing life-giving light.

Gravity also pulled matter together to produce the planets, and big balls of hydrogen (and other things) like Jupiter wouldn't exist either.

The reason our atmosphere's density drops a factor of two every 5-6 km is also due to gravity. Over time, gas seeks out sources of gravity and after a couple of collisions with other molecules forgets where it came from or how much kinetic energy it had. It *thermalizes*.

### But what about those atoms that escape planets?

The tail end of thermal distributions means that some molecules will regularly escape, but the rate is quite slow. Once they do, the solar wind will eventually pick them up and take them out of (at least the inner) solar system.

The solar wind also picks off atoms from planet's atmospheres directly if it can. ~~This is why Mars' atmosphere is so small now. Without a magnetic field to deflect the charged particles from the Sun (like Earth does) the solar wind ran off with most of Mars' atmosphere.~~ Comments indicate there's recent reconsideration of this.

For more about that I've just asked

* [...but where did Mars' atmosphere actually GO?](https://astronomy.stackexchange.com/q/45205/7982)

So you can run and you can hide, but gravity and/or the solar wind will eventually scour the interplanetary spaces in our solar system and remove most of the gas that would otherwise orbit the Sun.

> **Comments:**
> **username_7**: I fail to see why this was voted down as it directly addresses the question.  
> **username_8**: "This is why Mars' atmosphere is so small now." -- It's probably a lot less clear than the simplistic "magnetic field protects against solar wind" model you're invoking (which completely fails to explain why Venus has a significant atmosphere, or why the current atmospheric loss rate of Venus, Earth, and Mars are roughly the same). See e.g. https://ui.adsabs.harvard.edu/abs/2018A%26A...614L...3G/abstract  
> **username_6**: @PeterErwin When a "contrary to what has previously been believed" simulation paper is published it doesn't necessary upend everything and become the new gospel truth instantly. Is the paper *right?* Have planetary scientists rallied around these simulation results as providing *actual loss rates?* They cite measured data from these three planets that are all almost 20 years old, but so far I can't even see how well the paper agrees with them. Anyway the abstract of the paper says that a magnetic field isn't "sufficient" but it does not say that Mars' magnetic field wasn't *necessary*.  
> **username_6**: @PeterErwin "Therefore, contrary to what has previously been believed, magnetisation is not a sufficient condition for protecting a planet from atmospheric loss." Was it in fact ever believed that a magnetic field was "sufficient"? Or could this be a bit of linguistic gamesmanship?  
> **username_8**: @uhoh The recent review article by [Gronoff et al. (2020)](https://ui.adsabs.harvard.edu/abs/2020JGRA..12527639G/abstract) says, "We show that the paradigm of the magnetic field as an atmospheric shield should be changed..."  
> **username_8**: From Section 7.1.2 of Gronoff et al: "while the presence of a magnetosphere has a clear impact on ionospheric outflow, recent developments in the study of the coupling between stellar wind, magnetospheres and ionospheres challenge the idea of a protective effect of magnetospheres on atmospheric erosion.... "  
> **username_8**: And from the same section: "recent studies such as Brain et al. (2013); Gunell et al. (2018); Airapetian et al. (2017); Garcia-Sage et al. (2017), as well as the case of Mercury, show that an intrinsic magnetic field does not totally protect an atmosphere. A contrario, the case of Venus shows that a magnetic field absence does not prevent sustaining a dense atmosphere."  
> **username_8**: "Without a magnetic field to deflect the charged particles from the Sun (like Earth does) the solar wind ran off with most of Mars' atmosphere." This certainly sounds like a clear statement that it's the magnetic field that matters. (Or was that just "a bit of linguistic gamesmanship"?)  
> **username_4**: This answer does not address the OP's misconception.  
> **username_8**: See also [Tarduno et al. (2014)](https://ui.adsabs.harvard.edu/abs/2014PEPI..233...68T/abstract) -- "It is true that a magnetic field is not necessary to retain an atmosphere but, the relevant issue for life as we know it is water" --; [Egan et al. (2019)](https://ui.adsabs.harvard.edu/abs/2019MNRAS.488.2108E/abstract); and [Johnstone (2021)](https://ui.adsabs.harvard.edu/abs/2021arXiv210511243J/abstract).  
> **username_6**: @PeterErwin okay that's quite a collection, I'll take a look. In the mean time I've struck-through that sentence.  
> **username_6**: @DavidHammen I won't practice telepathy without a license https://www.youtube.com/watch?v=6giYSgxiWEo From a single sentence we can't really divine what conceptions  or misconceptions an OP may or may not hold.  

---

