Re: One Face Gas Giant confusions

On Aug 10, 1:37 pm, Erik Max Francis <m...@xxxxxxxxxxx> wrote:
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.

Yeah, I finally got that.

(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.)

(By "big", do you mean mass, or radius? One exoplanet, XO-3b, is
claimed to have ~13 Jupiter masses:

http://www.iop.org/EJ/abstract/0004-637X/677/1/657/

though it's radius is only ~ twice Jupiter's.)

Some Googling indicates that a Jovian that forms far from its star,
then reorbits close-in will be similar in density to Jupiter because
like Jupiter, it radiated much of its primordial heat and shrank, but
hasn't likely had enough time since reorbiting to heat up enough to
swell to the .7-Jupiter density range of many hot Jovians.

Those that form close-in in the first place are supposedly more
likely to be less dense than Jupiter because what heat they lose is
replaced by their star.

Bottom line, the radius range is restricted by the low-altitude
hydrogen going high-density (metallic) faster than the high-altitude
gaseous phase can add girth? Is that about right?

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.

More than once I've daydreamed some sort of ginormous mass-
spectrometry effect where the heavier elements from a supernova are
concentrated in a small region while the lighter elements are mostly
separated away, leaving a very high-metallicity cloud from which a
solar system might form.

  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.

That's enough to prevent thermal-driven circulation permanently?


Mark L. Fergerson
.

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