Hi,
My example of a SuperEarth was based on taking the same materials (bulk
composition) as the Earth is made from and just piling more of them
together. We have
no idea (and no way of knowing, for now) if the planetesimals of the
Gliese 876 system
were the same mix as the Sol planetesimals, but we think the raw materials
of solar
systems are generally similar.
When I started suggesting the loss of some water so we could have
continents and a
higher albedo so it would be cooler and so forth, you were being treated
to an ugly
display of a rational mind crumbling under the pressures of "interstellar
optimism,"
the desire to improve things just a touch.
After all, planetary systems have unique histories. The Earth picked
up this
whonking huge Moon to stabilize its axis and its climate though blind luck
at
incredible odds. Mars got all these volariles (we think) but then got its
atmosphere
stripped off and died. Venus really got a dirty deal; don't know what it
was, but it
was nasty. Them's the breaks.
It does make it look like there's more planetary bad luck than good
luck, doesn't
it? I'm sure we all wish Europa well! And I have a soft spot in my head
for Titan.
Always have had...
The interstellar "optimists" tend to think of extra-solar worlds as
similar to what
we know, but 50-60 years ago the interplanetary "optimists" tended to
think of solar
worlds as more Earth-like than they turned out to be.
As a teenager in the early Fifties, I devoured every scientific book on
other
planets that there was, and the picture they presented was rosy compared
to reality,
The very best book on Mars, "The Physics of the Planet Mars," by the great
Gerald de
Vaucouleurs, was translated in English in 1953, a substantial tome filled
with
equations, graphs, and tables. I special ordered it, and it was damned
expensive.
It presented a Mars with an atmospheric pressure of 100 to 200
millibars and 85
degree F. noonday temperatures. All the astronaut would have to do was
slip on flight
mask with a 10 pound oxygen tank on his back and go for a stroll. He
could leave his
leather jacket and white scarf back in the rocket because it's comfortable
weather out
there, at least in the daytime.
The notions of Venus were rosier still. It seems that the less you
know, the
happier the picture you get.
Here's the kind of paradox that arises from "happy" thinking. Venus
gets twice as
much solar energy as the Earth. But the albedo (reflectivity for those of
you
listening in) of Venus is more than twice that of the Earth (as we
estimated the
Earth's albedo in those days), so Venus shouldn't be any warmer than the
Earth (and a
few brave souls even suggested it was cooler, with big polar ice caps)
In the 1940's, Rupert Wildt measured huge fat CO2 absorption features
in the Venus
spectrum and concluded that Venus was a waterless inferno as hot as hell's
hinges.
What he got for his suggestion was a lot of scowls and being ignored for a
decade or
so. Other scientists (big names and I ain't saying who) measured H2O
bands, They were
dead wrong about the water, because the water they were measuring was in
the atmosphere
of Earth, not Venus! Who knew?
So, Venus was maybe a little warm, very wet, always cloudy but bright,
kind of like
the Permian had been on Earth. Venus was so remarkably like the Earth on
paper that
everyone figured it was the twin it appeared to be. Probably had oxygen
under those
clouds, Get out of the spaceship, wear good boots (it was bound to be
muddy), and keep
an eye out for Venusian dinosaurs.
I'm not talking about science fiction writers here; I'm talking about
real
honest-to-gosh scientists. We had gotten over Lowell and his canals on
Mars, but not
by much. Pickering was still talking about life on the Moon, for heaven's
sake.
Although he had a great explanation for Martian canals: they were the
migratory routes
of Martian herbivores, fertilized by their droppings.
Tommy Gold had his own special heresy for Venus (doesn't he always?),
an ocean of
hydrocarbons, an idea that would get picked up from Venus and moved to
Titan for 30 or
40 years. Ain't there.
You don't suppose wishful thinking has anything to do with the notion
of a
planetary body with oceans of gasoline, do you? Nah... Sorry, no oceans
of free
gasoline. You can leave the SUV at home, buddy.
In some ways, science fiction writers could be more realistic than
scientists in
those days. Hal Clement (Harry Stubbs) wrote two novels about life on a
SuperJupiter
around 61 Cyngi which are better pieces of rigorous thinking about these
really alien
worlds than any scientific work that had been done on the question. He
made me think
about eutectic melts of ammonia and water in all its complexity until my
head really
hurt. Of course, in those days, no one was doing scientific work on the
question.
Turns out the SuperJupiter around 61 Cyngi is really there! Along with
scores of
other SuperJupiters around other stars.
Even more fascinating is the possibility of other kinds of stars. We
have always
assumed that the tiniest faint dink of an M9 star is as small as "stars"
get. Less
mass and you get essentially non-luminous bodies: brown dwarves, very
brown dwarves,
and black dwarves.
But 2MASS (the Two Micron All Sky Survey) turned up huge numbers of
very, very
faint stars never seen before, too faint to be seen in visible light.
About twenty of
them have been assigned on their spectral characteristics to a new class
of stars: the
Class L main sequence stars. About six are brown dwarves. Since 2MASS
only sampled a
very small patch of sky chosen at random and since their low emission sets
a limit to
how far away we could detect these faintest of all stellar objects, we can
calculate
their abundance.
The astounding answer is that they are more abundant than the M Class
stars that we
used to think were the most abundant, so much so that the likelihood is
that there are
about 2000 L Class main sequence stars within 50 light years of the Sun.
Yes, that's
2000 neighboring stars to our Sun that we can't even see in visible light!
<http://astron.berkeley.edu/~basri/bdwarfs/sec4.htm>
<http://astron.berkeley.edu/~basri/bdwarfs/sec6.htm>
That's an average of about one such star per 260 cubic light years.
Hmmm... What's
the radius of a sphere with a volume of 260 cubic lightyears centered on
our Sun? It's
a shade less than the distance to the "nearest star," good old alpha
Centauri. I put
quotes around that phrase "nearest star" because, if 2MASS is right (and
it seems to
be), there is a good chance there's an "invisible" star just as close or
closer to the
Sun than that star we can see!
Really big bright stars don't seem to have planets. They gobble up all
the planet
food and blow off the rest; it too energetic an environment for a solar
system to form
in. We used to think low-mass stars, like M class, wouldn't have planets
because there
wasn't enough mass around, but the detections of extra-solar planets seem
to indicate
that the formation of a less energetic star leaves plenty of material left
over for
planets. Little Gliese 876 has two SuperJupiters!
So, following that line of reasoning, why wouldn't the new L Class
stars have lots
of planets too? Despite the fact that "happy" thinking would like alpha
Centauri to
have planets because it's the closest, hence easiest, star to get to, no
detection
attempt has ever found any hint of planets. A closer L Class main
sequence star with
planets would genuinely be worth looking for!
But trying to find a nearby "invisible" star is a truly daunting
technical problem
concerning which no light bulbs have turned on in my brain... It's as dim
in there as
the L dwarves themselves!
Sterling K. Webb
------------------------------------------------------
[EMAIL PROTECTED] wrote:
Hola Sterling,
Your Super-Earth got me thinking about viable life forms though I'm not
yet
too adjusted. There certainly will be continents, though they will be
floating quagmires of life and useful excreta, and will probably get
quite thick.
Easily enough to walk on, despite those who worry about finding a
surface on
such gas-liquid giant planets. Not that walking will be too easy, so I
guess
you would need bigger muscles to deal with that. The interesting thing
with
such a world is there would be several interfaces - multilevel
continents -
especially suitable as anchors (surfaces) for life depending on the
vertical
profile of gases and liquids present. Gravity might be similar to that
on
Earth believe it or not, or even less in some of the upper level
continents
since the rarification will reduce the gravitational acceleration by the
height
squared (If you are on a planet 8 times the mass of earth but at 2.8X
the
radius, "surface gravity" is the same as Earth.)
But you're right it would get stuffy, so life would probably be pretty
acuatic-like and evolution driven by the rise to an upper or lower
continent in
addition to competition for low hanging fruit resources. There would
probably
be heavy development using bouyancy, and things would probably fly in
that
fashion. So the mosquitos you would swat would land on you by
regulating their
body densities with intestinal waste gas. Yuck.
Dense Ice would be at least down where pressures (and depths) were at
3000
atm, and very unstable given the dynamics of the situation, it would be
more
like a cloud formation, as probably not to present much of an issue.
But the
sort of magnetosphere this planet would have...could metallic hydrogen
make
it Earth-like? Probably too small. It would be a pretty boring place,
though
as meteorites would not be much less likely than on the surface of ...
Venus... so I guess these water breathing nitrogen-fixing creatures
would do
something else for kicks (Starlight would not be very plentiful - and we
need a
renewable energy source or biosphere equilibrium with net energy going
into
support the net entropy production of the system).
What I wonder is how the higher forms would generate and harness
electricity
for progress, considering the whole planet is sounding rather grounded
in a
lightening sauna? It would make for a hell of a set of oceanmill farms
working off the sea currents for anyone who could come up with a good
insulator...
Saludos, Doug
Sterling W. wrote:
Hi,
Is this an all time high or an all time low? I'm replying to
my own post (see below)!
While I don't have the most recent edition, I dug out my copy of
"Planetary Engineers' Handbook" (Dresden, 15th Ed., 2314 AD) to
investiigate the characteristics of a "SuperEarth." Here's what
I found:
So, what would a SuperEarth be like? If you start with the
same recipe mix of ingredients as the Earth and just made a
bigger batch of planet, is it just the same, only more so? Nope,
more of the same is not the same.
If the Earth were bigger, it would retain more volatiles to
begin with. But in addition, the volume of water would
increase faster than the increase in surface area, so the
oceans would be deeper. Because of the deeper oceans and the
greater gravity, the pressures at the bottoms of those oceans
would be much higher.
Continents and their mountains would be much lower, because
the temperatures in the crust would increase faster with depth,
until the fluid point would be reached in the crust instead of
the mantle like it is on "our" Earth. Mountains can only pile
up until the pressures under them are about 3000 to 3500
atmospheres, and that zone would be reached at shallower and
shallower depths on a bigger Earth.
The solid crust of a larger "Earth" would be much thinner,
heat transfer to the surface much faster, volcanism much
livelier, plate tectonics much zippier.
Imagine an "Earth" exactly twice the diameter of our Earth:
16,000 miles across. It would have four times the surface,
eight times the volume, and 12 times the mass (compressibility
squishes). It's surface gravity would be 3 times greater. The
escape velocity from the surface would 2.45 times greater.
Because it would have 12 times the water but only four
times the surface, the average ocean depth would be about 9000
meters! The pressure at the depths of these oceans would be
about 3000 atmospheres. The highest mountains possible would be
about 4000 meters (calculating from the median diameter), so if
you were the greatest mountain climber on the SuperEarth,
standing on the top of SuperEarth's highest mountain, you would
have 5000 meters of water above you!
Whoops! No continents. The SuperEarth is a WaterWorld.
On our Earth, the crust is about 30 kilometers thick, but
the lithosphere (rocks that stay stiff and not slushy and
slippy) is about 75 kilometers, so the Earth's lithosphere
contains all the crust and the top part of the mantle.
The crust of the SuperEarth would be about 90 km thick, but
the lithosphere would only be about 30 kilometers thick. This
means that it would be very difficult to sink pieces of crust
(subduction) and equally difficult to bring deep basalt magmas
to the surface.
On the other hand, the SuperEarth's silicate crust would be
recylced very rapidly with lots of local vulcanism and
"hotspots" and have a very similar composition everywhere. The
only weathering that would be possible would be chemical,
because all the volitiles are released into the oceans rather
than the atmosphere.
The only question we can't answer is how hot or cold a
SuperEarth would be, since that depends on the distunce to its
Sun. Too far away and the oceans turn to ice, even Ice III,
which sinks instead of rising. Wow, did you know that?.
Too close and the oceans boil away, creating a
SuperVenus. But I discover that making a Super Venus is
not as easy as it sounds. It's very hard to strip all that
atmosphere and immense oceans of volatiles away from a
planet that has an escape velocity of 27,400 meters per
second!
And remember, a SuperEarth would have
proportionately more volatiles than a puny little Earth
like ours. It could even afford to lose some of those
9000 meters of ocean, don't you think? Maybe
enough to have continents?
Its immense atmosphere would have a very high
albedo from a water cloud deck 100's of kilometers
deep, and the surface temperatures could well be below
100 degrees C. Hmm, starting to sound interesting.
(Originally posted to the List 08-31-2004 in anticipation
of the discovery of a "SuperEarth," and Heck! I didn't
even have to wait a year... What next?)
Sterling K. Webb
------------------------------------------
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