Re: One Face Gas Giant confusions

alien8er wrote:
On Aug 10, 4:17 am, Mike Williams <nos...@xxxxxxxxxxxxxxxxxx> wrote:
Wasn't it Mark L. Fergerson who wrote:



This article:
http://www.nasa.gov/mission_pages/kepler/news/kepler-discovery.html
presents the early results from the Kepler space telescope,
identifying some characteristics of an exoplanet orbiting HAT-P-7, a
star ~ 1000 ly away.
The planet in question is said to be a "gas giant", with a period of
2.2 days, and is a One Face. Its day side reads to be about 4310
Fahrenheit, while its night side is closer to 1000 Fahrenheit.
The article, like many from NASA about large planets with small-
radius orbits, also calls the planet a "hot Jupiter" because it's
close to its sun, and has a mass close to that of Jupiter.
Am I just being overly sensitive when that term grates on my
sensibilities? For me any planet that's any kind of "-Jupiter" is
largely hydrogen, not just a planet with a near-Jovian mass. I suppose
I am and will just have to get used to it, but I'd think the
astronomers who do this for a living would be less provincial than to
always describe exoplanets in Solar System terms.
Okay, I'll stop ranting about that. However, the Kepler observations
seem to indicate the existence of an atmosphere on the planet as well.
Why is it assumed to not transport more heat to the night side?
Basically, I'd like to know how they know the planet qualifies as a
gas giant.
HAT-P-7b has been observed by radial Doppler shift so we have an upper
limit on its mass (1.8 * Jupiter) and been observed by transit which
gives an estimate of its diameter (1.4 * Jupiter). From those two data
points, we can deduce an upper limit for its density at 0.7 * Jupiter.

To me, that clearly spells gas giant.

To me, it only spells "planet with mass <= 0.7 Jupiter".

Where does the "gas" part come in? Where is the physics that says
there can be no rocky planets in that mass range?

As had been pointed out, the "70% Jupiter" figure was for density, not mass. Its mass is considerable larger than Jupiter, and its density is at or lower than Jupiter, so you're clearly talking about a gas giant. (Interestingly, for reasons of equation of state, we have good reason to believe that Jupiter is just about as big as a gas giant/brown dwarf can get. I'm not sure whether the data on this planet -- 40% larger in size than Jupiter -- is sufficiently out of whack with this hypothesis to make anyone worry.)

But we _do_ have really good reason to believe that any "pretty big" planet, except perhaps in some very exceptional circumstances, should be a gas giant. The lowest molecular weight that a planet can retain its depends on its mass; larger mass planets can retain lighter elements for longer, and lots of this stuff was floating around in the protoplanetary disk from which the planets form. As we can see in our Solar System, planets only several times larger than Earth (e.g., Uranus and Neptune) are sufficiently large to retain lots of hydrogen and helium, so once a planet gets big enough, it turns into a gas giant and there's not much you can do about it.

One could imagine extreme conditions where the hydrogen-helium envelope is blown off of gas giants -- smaller ones, anyway -- but these would have to be sufficiently energetic events (say, recurring novae?) that would probably make the place rather inhospitable and thus probably not a very interesting place to visit.

Also, how the hell can a gas giant be said to be a One Face? Its
rocky core, sure, but the bulk of its atmosphere? Where'd its
fomational angular momentum go? Why does not such a ferocious
temperature difference drive equally ferocious circulation?

Tidal friction, the same as any other planet. The tidal braking effects are stronger the bigger the planet and the closer in you get, too.

--
Erik Max Francis && max@xxxxxxxxxxx && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM/Y!M/Skype erikmaxfrancis
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