[meteorite-list] OT: 3 NEW EXTRA-SOLAR PLANETS: NEPTUNES OR SUPER-EARTHS?

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu May 18 03:03:11 2006
Message-ID: <00e801c67a49$1a41f7d0$ab2ce146_at_ATARIENGINE>

Hi,

    With the extra-solar planet count now up over
170, a system with no Jupiter-sized (or larger) world
has been found, but with three moderate sized
worlds about the mass of Neptune or Uranus only
41 light years away.
    While the article simply calls them "Neptune-like,"
that only describes their mass. (Mass and distance
are all we can detect, now.) The authors seem to
assume that they are gas giants that have migrated
inward their sun (a bias from one theory of planet
formation). Marcy, record-holder for the previously
smallest planet at 14 E-masses, called his find a
"Super Earth."

Here's the news article:
http://www.newscientistspace.com/article/dn9181-extrasolarplanet-hunters-find-tripleneptune-system.html

    But if they are not migrated gas giants, but formed
in place, at these distances from their star, then what
they may really be are terrestrial in composition, or
Super-Earths, with 10, 12, and 18 Earth masses.
They are all closer to their star than the Earth, and
the inner two are likely quite hot. What would a Super
Earth 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 as the Earth, only more so? Nope,
more of the same is definitely not the same.
    If the Earth were bigger, 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 and
bigger Earth.
    Since the solid crust of a larger "Earth" would be
much thinner, heat transfer to the surface much faster,
vulcanism 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 area,
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 Super Earth,
standing on the top of Super Earth's highest mountain, you would
have 5000 meters of water above you!
    Whoops! No continents. The Super Earth 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 Super Earth 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 Super Earth'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
Super Earth 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. Too close and the oceans boil away, creating a
Super Venus. Perhaps the 18 Earth mass planet is a Super
Venus. Even that is problematical, since it's really hard to
strip the atmosphere and oceans away from a planet that has an
escape velocity of more than 32,000 meters per second!
(Or in the case of the middle one, 12 Earth mass world,
27,400 m/sec.)
    So a bigger Earth is not just a bigger Earth. Knowing that
somebody will ask how big a bigger Earth has to be before
there's no land at all, just oceans, the answer is: somewhere
between 2-1/2 and 3 Earth masses is the point where the median
ocean depths equal the height of the highest possible mountain.


Sterling K. Webb
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Received on Thu 18 May 2006 03:03:00 AM PDT