Why do gas giant planets emit more heat than they receive?

From a post to sci.astro.research, April 12, 2001, by Marshall Perrin

Paul Stowe wrote:

> I have a few questions pertaining to the gas giant planets.  
> First, it is my understanding that Jupiter emits a total of 2 times the 
> thermal energy that it receives from the Sun [6.8 x 2 or ~13.6 watts/m^2], 
> Saturn 2.3 times as much [2 x 2.3 or ~ 4.6 watts/m^2], Uranus and Neptune 
> are less well defined.  

> Could someone confirm the Jupiter/Saturn values above, and give good 
> values for Uranus and Neptune?  References would be appreciated.

The Voyager 2 IRIS results for Uranus are 34+-38 erg/cm^2/s. The values for Neptune are 433+-46 erg/cm^2/s. See

> Second, Saturn's excess is said to be due to Helium rainout.  Has there 
> ever been a calculation as to how much (in-falling mass), and how long 
> such must and could be sustained, before He depletion occurred?

Yes, see Bergstrahl & Miner 1991 (_Uranus_, Univ. Ariz. Press) and citations therein. Current models of He rain agree well with the observed depletion of He in the atmosphere of Saturn, and the smaller but still noticeable depletion in Jupiter.

I wrote a literature review paper for my graduate planetary astronomy seminar last fall, focusing on the question of why the value for Uranus is so much less than the other gas giants. There are a couple strong theories floating around, mostly having to do with restriction of convection in the interior of Uranus relative to the other giant planets, but the available data isn't yet strong enough to be entirely conclusive. The paper is available at

Take a look - it summarizes the general physics involved in planetary internal heat sources and has a pretty extensive bibliography to get you started digging for more.

> Finally, a similar question for the proposed carbon rainout on Neptune?

Don't know anything about this one, sorry.