<p>The implication of the question is that this extra 1000 miles should be added to Pluto's radius. The answer is no. For all of the solid planets, it's that solid surface (or solid+liquid surface in the case of the Earth) that counts, not the outer reaches of the atmosphere. The surface is a clear-cut, non-arbitrary boundary. The atmosphere? They can extend a long way out. A non-arbitrary boundary is always going to be preferred over an arbitrary one.</p>

<p>That's not possible in the case of the Sun and the giant planets. A somewhat arbitrary boundary is needed for those bodies. That somewhat arbitrary boundary explicitly excludes the upper reaches of the atmosphere, which extends out for many thousands of kilometers. For the giant planets, some use a tenth of the Earth's atmosphere as defining the arbitrary "surface", others, one atmosphere, yet others, ten atmospheres. That factor of 100 variance in pressure amounts to about a hundred kilometers. That's not that much considering how big the gas giants are.</p>

CC BY-SA 3.0
2015-07-20T18:07:25.093Z
This seems like an additional layer of arbitrariness then on the definition of "planet" - it is looking more arbitrary all the time! I understand that the density and pressure of Pluto's atmosphere is much greater than Mercury's. Does anyone think that Eris and the other smaller bodies have this type of atmosphere?
Jonathan Dunn
CC BY-SA 3.0
2015-07-20T18:27:29.807Z
Where's the arbitrariness for a solid planet? Other than terrain variations, there is very little arbitrariness in using the surface as the measure of the size of a planet. Using the atmosphere would be a step backwards.
David Hammen
CC BY-SA 3.0
2015-07-20T20:44:33.717Z
Agreed. And I don't think Pluto would be much bigger if you used a 1 bar definition for the height of its atmosphere! I believe the surface pressure is of order $10^{-4}$ bar!
ProfRob
CC BY-SA 3.0
2015-07-21T00:36:37.987Z
We don't absolutely know whether Jupiter has a solid core. If we found out that it did, would we use the "solid planet" method to determine the radius?
Jonathan Dunn
CC BY-SA 3.0
2015-07-21T04:00:33.127Z
@RobJeffries So 0.1 hectopascals; compared to mars' 6 hectopascal and earths' 1013 hectopacal.
Wayfaring Stranger
CC BY-SA 3.0
2015-07-21T14:22:05.613Z
@JonathanDunn No, because it is compositionally different than terrestrial planets.
called2voyage
CC BY-SA 3.0
2015-07-21T18:18:04.970Z
I suppose size per se isn't a part of the current definition of "planet". If Pluto's core were larger than Mercury's it will still not be a planet, right?
Jonathan Dunn
CC BY-SA 3.0
2015-07-22T12:53:26.313Z
"Pluto's atmosphere is more than 13 times farther out into space than Earth's. " http://www.theverge.com/2015/7/17/8994661/plutos-escaping-atmosphere-extends-1000-miles-out-into-space
Jonathan Dunn
CC BY-SA 3.0
2015-07-22T18:14:31.193Z
@JonathanDunn A basic physics principle is that you can't just instantly transition from non-vacuum to vacuum.  There is necessarily a tapering off.  Things transition more or less continuously from "dense enough to be visible to us" to vacuum, and any value between those is arguably a perfectly fine pick for what constitutes the atmosphere.  So there is necessarily a bit of arbitrariness.  The short of the matter is, the universe doesn't really allow us perfect, universal, and unequivocal definitions of things.  We're the ones that draw the line in the sand, not reality itself.
zibadawa timmy
CC BY-SA 3.0
2015-07-20T17:46:05.703Z
2015-07-20T17:51:51.973Z
2015-07-20T17:51:51.973Z
David Hammen
<p><a href="http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150717-3" rel="nofollow">http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150717-3</a></p>

<p>"Scientists working with NASA’s New Horizons spacecraft have observed Pluto’s atmosphere as far as 1,000 miles (1,600 kilometers) above the surface of the planet, demonstrating that Pluto’s nitrogen-rich atmosphere is quite extended." </p>

<p>I note that conventional descriptions of the gas giants' sizes include mostly atmosphere. </p>

CC BY-SA 3.0
2015-07-20T18:43:40.920Z
https://en.wikipedia.org/wiki/Atmosphere_of_Pluto
Mithoron
<planetary-atmosphere><pluto><mercury>
Shall we say now that Pluto is "larger than Mercury" ?
Jonathan Dunn
# Shall we say now that Pluto is "larger than Mercury" ?

(Asked by: username_0 on 2015-07-20)

<http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150717-3>

"Scientists working with NASA’s New Horizons spacecraft have observed Pluto’s atmosphere as far as 1,000 miles (1,600 kilometers) above the surface of the planet, demonstrating that Pluto’s nitrogen-rich atmosphere is quite extended."

I note that conventional descriptions of the gas giants' sizes include mostly atmosphere.

**Tags:** planetary-atmosphere, pluto, mercury

> **Comments:**
> **username_1**: https://en.wikipedia.org/wiki/Atmosphere_of_Pluto  

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## 1 answers

### Answer by username_2 on 2015-07-20. Score: 8

The implication of the question is that this extra 1000 miles should be added to Pluto's radius. The answer is no. For all of the solid planets, it's that solid surface (or solid+liquid surface in the case of the Earth) that counts, not the outer reaches of the atmosphere. The surface is a clear-cut, non-arbitrary boundary. The atmosphere? They can extend a long way out. A non-arbitrary boundary is always going to be preferred over an arbitrary one.

That's not possible in the case of the Sun and the giant planets. A somewhat arbitrary boundary is needed for those bodies. That somewhat arbitrary boundary explicitly excludes the upper reaches of the atmosphere, which extends out for many thousands of kilometers. For the giant planets, some use a tenth of the Earth's atmosphere as defining the arbitrary "surface", others, one atmosphere, yet others, ten atmospheres. That factor of 100 variance in pressure amounts to about a hundred kilometers. That's not that much considering how big the gas giants are.

> **Comments:**
> **username_0**: This seems like an additional layer of arbitrariness then on the definition of "planet" - it is looking more arbitrary all the time! I understand that the density and pressure of Pluto's atmosphere is much greater than Mercury's. Does anyone think that Eris and the other smaller bodies have this type of atmosphere?  
> **username_2**: Where's the arbitrariness for a solid planet? Other than terrain variations, there is very little arbitrariness in using the surface as the measure of the size of a planet. Using the atmosphere would be a step backwards.  
> **username_3**: Agreed. And I don't think Pluto would be much bigger if you used a 1 bar definition for the height of its atmosphere! I believe the surface pressure is of order $10^{-4}$ bar!  
> **username_0**: We don't absolutely know whether Jupiter has a solid core. If we found out that it did, would we use the "solid planet" method to determine the radius?  
> **username_4**: @RobJeffries So 0.1 hectopascals; compared to mars' 6 hectopascal and earths' 1013 hectopacal.  
> **username_5**: @JonathanDunn No, because it is compositionally different than terrestrial planets.  
> **username_0**: I suppose size per se isn't a part of the current definition of "planet". If Pluto's core were larger than Mercury's it will still not be a planet, right?  
> **username_0**: "Pluto's atmosphere is more than 13 times farther out into space than Earth's. " http://www.theverge.com/2015/7/17/8994661/plutos-escaping-atmosphere-extends-1000-miles-out-into-space  
> **username_6**: @JonathanDunn A basic physics principle is that you can't just instantly transition from non-vacuum to vacuum.  There is necessarily a tapering off.  Things transition more or less continuously from "dense enough to be visible to us" to vacuum, and any value between those is arguably a perfectly fine pick for what constitutes the atmosphere.  So there is necessarily a bit of arbitrariness.  The short of the matter is, the universe doesn't really allow us perfect, universal, and unequivocal definitions of things.  We're the ones that draw the line in the sand, not reality itself.  

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