[meteorite-list] Extrasolar Planets -- SuperEarths

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Tue Aug 31 23:04:51 2004
Message-ID: <41353C2D.9D4A0FDB_at_bhil.com>

Hi, All,

    The extrasolar planet news was the discovery of smaller and
smaller planets than the Jupiter and Saturn sized worlds that
have been discovered up until now. Now both ESO and Marcy & Co.
have discovered planets that are Uranus/Neptune sized, 10 to 20
Earth masses, with ESO claiming the record at 14 Earths.
    So, what would a Super-Earth be like? If you start with the
same recipe mix of ingredients 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, 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. Too close and the oceans boil away, creating a
SuperVenus. Even that is problematical, since it's hard to
strip the atmosphere and oceans away from a planet that has an
escape velocity of 27,400 meters per second!
    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.
Glub, glub!

Sterling K. Webb

    Footnote: David Brin has suggested that most Earth-like
worlds are bigger than our Earth and are mostly Water Worlds,
and that is why we never get visited by intelligent aliens, who
are all really smart whales with no astronomy, no hands, and no

Paul H wrote:

> Extrasolar Planets Discovered
> Kathleen Burton Aug. 30, 2004
> NASA Ames Research Center, Moffett Field, Calif.
> Phone: 650/604-1731 or 604-9000
> E-mail: Kathleen.M.Burton_at_nasa.gov
> media representatives are invited to view
> live televised coverage and commentary of
> a major extra-solar discovery announcement
> on Tuesday, Aug. 31, at 10 a.m. PDT. The
> coverage will be broadcast live from NASA
> Headquarters in Washington on NASA
> Television and can be viewed in the lobby
> of the N-201 auditorium at NASA Ames
> Research Center, located in California's
> Silicon Valley. Following the briefing,
> Dr. Jack Lissauer, a NASA Ames planetary
> scientist, will be available for interviews.
> NASA Television can be seen on AMC- 6,
> Transponder 9 located at 72 degrees west
> longitude with 3880.0 MHz. vertical
> polarization and audio at 6.8 MHz.
> RELEASE: 04-79AR
> A team of planet hunters will announce its
> discovery of a new class of planets located
> beyond our solar system at a NASA Science
> Update at 10 a.m. PDT on Tuesday, Aug. 31.
> The discovery represents a significant and
> much-anticipated advance in the hunt for
> extra-solar planets.
> Panelists include:
> -- Dr. Geoffrey Marcy, University of California,
> Berkeley;
> -- Dr. R. Paul Butler, Department of Terrestrial
> Magnetism, Carnegie Institution of Washington;
> -- Dr. Barbara McArthur, University of Texas, Austin;
> -- Dr. Alan Boss, staff research astronomer,
> Department of Terrestrial Magnetism, Carnegie
> Institution of Washington;
> -- Dr. Anne Kinney, moderator, director, Universe
> Division, Science Mission Directorate, NASA,
> Washington.
> The news conference will be carried live on
> NASA Television, with two-way question-and-
> answer capability from participating NASA
> centers. The event also will be Webcast live
> at:
> http://planetquest.jpl.nasa.gov/webcasts/ssu_0804.html
> Also, last Wednesday the European Southern
> Observatory announced the discovery of a 14
> Earth-mass extrasolar planet ...
> http://www.eso.org/outreach/press-rel/pr-2004/pr-22-04.html
> The following paper appeared on the
> astrophysics preprint server ...
> http://cul.arxiv.org/abs/astro-ph/0408471
> Paul
> Baton Rouge, LA
Received on Tue 31 Aug 2004 11:04:14 PM PDT

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