"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.


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.

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  • $\begingroup$ 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? $\endgroup$ – Jonathan Dunn Jul 20 '15 at 18:07
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    $\begingroup$ 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. $\endgroup$ – David Hammen Jul 20 '15 at 18:27
  • $\begingroup$ 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! $\endgroup$ – Rob Jeffries Jul 20 '15 at 20:44
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    $\begingroup$ 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? $\endgroup$ – Jonathan Dunn Jul 21 '15 at 0:36
  • $\begingroup$ @RobJeffries So 0.1 hectopascals; compared to mars' 6 hectopascal and earths' 1013 hectopacal. $\endgroup$ – Wayfaring Stranger Jul 21 '15 at 4:00

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