[meteorite-list] OT: New Smallest, Possibly Earth-like, Extra-Solar Planet Announced

[Sterling K. Webb]

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
------------------------------------------
"Sterling K. Webb" wrote:

> Hi, All,
>
>     The "Marcy Team," with 106 detections of extra-solar planets to its
> credit, announced today number 107, the discovery of the smallest yet
> detected (by about half the previous detection),  It's only 7.5 (+/-
> 1.5) Earth masses.  It orbits Gliese 876, an M-class dwarf about
> one-third the mass of our Sun and only 15 light years away.  We're
> practically neighbors...
>
>     The planet is close enough to its star to be pretty warm (200 to 400
> degrees C).  In theory, it could be a hot mini-Uranus, but Marcy seems
> to think it's a rocky terrestrial world since it would be hard to hang
> onto all that gas at 200 degrees C or more.  If confirmed, it would be
> the first detection of an Earth-like world outside the Solar System.
>
>     Gliese 876 had already been discovered to have, not one, but two
> Super Jupiters orbiting further out than the new discovery.  If the new
> planet is a rocky terrestrial world, it seems to me to be more likely to
> be a "Super Venus" than a "Super Earth"!
>
>     The kind of star these planets orbit, M-class, are the most abundant
> class of stars, far more numerous than the larger K-class and the even
> larger G-class (that's us!).  M class stars are all over the place.  We
> have more M class neighboring stars (within 10-20 light years) than all
> the other kinds put together.  If M class stars can have terrestrial
> planets close enough to be warm, then we may be sailing along in a cloud
> of Earths!
>
>     Here's the National Science Foundation press release:
> <http://www.nsf.gov/news/news_summ.jsp?cntn_id=104243&org=NSF&from=news>
>
>     And a somewhat less technical one:
> <http://skyandtelescope.com/news/article_1530_1.asp>
>
> Sterling K. Webb