[meteorite-list] Pluto-sized planet embryos detected

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Mon, 01 Oct 2007 16:45:50 -0400
Message-ID: <s3n2g3t00kk1jc47kdl1c6vrum86ekpvh9_at_4ax.com>

Pluto-sized planet embryos detected
Objects would represent first evidence of stage of early planet formation
By Ker Than
Staff Writer
Space.com
Updated: 11:12 a.m. ET Oct 1, 2007
Puffy debris disks around three nearby stars could harbor Pluto-sized
planets-to-be, a new computer model suggests.

The "planet embryos" are predicted to orbit three young, nearby stars, located
within about 60 light years or less of our solar system. AU Microscopii and Beta
Pictoris are both estimated to be about 12 million years old, while a third
star, Fomalhaut, is aged at 200 million years old.

If confirmed, the objects would represent the first evidence of a
never-before-observed stage of early planet formation. Another team recently
spotted "space lint" around a nearby star that pointed to an even earlier phase
of planet building, when baseball-sized clumps of interstellar dust grains are
colliding together.

The new finding will be detailed in an upcoming issue of the Monthly Notices of
the Royal Astronomical Society.

Planet puffer-uppers
Using NASA's Hubble Space Telescope, the researchers measured the vertical
thickness of so-called circumstellar debris disks around the stars, and then
used a computer model to calculate the size of planets growing within them.

The thickness of a debris disk depends on the size of objects orbiting inside
it. The ring of dust thins as the star system ages, but if enough dust has
clumped together to form an embryonic planet, it knocks the other dust grains
into eccentric orbits. Over time, this can puff up what was a razor-thin disk.

The new model the researchers created predicts how large the bodies in a disk
must be to puff it up to a certain thickness. The results suggest that each of
the three stars studied is harboring a Pluto-sized embryonic planet.

"Even though [the disks] are pretty thin, they turn out to be thick enough that
we think there's something in there puffing them up," said study team member
Alice Quillen of the University of Rochester in New York.

At least one of the stars is thought to contain at least one other planet in
addition to the circling Pluto-sized planet. The circumstellar disk of Fomalhaut
contains a void that scientists think is being cleared out by a Neptune-sized
world. The researchers think the embryonic planets predicted by their model are
too small to clear gaps like this in the disk.

"If you think of water flowing over pebbles, if the pebbles are very small at
the bottom of the water, it doesn't make a good ripple," Quillen told Space.com.

Too far?
All of the embryonic planets predicted to exist in the three systems are located
far away from their parent stars. AU Microscopii's budding planet is estimated
to lie about 30 AU from its star, or about the same distance that Pluto is from
our sun. One AU is equal to the distance between Earth and the sun. The
embryonic planets of Beta Pictoris and Fomalhaut are thought to lie even
farther, at 100 and 133 AU, respectively.

It is the large distances separating the planet embryos and their stars that
have drawn the most criticism by colleagues, Quillen said. Many find it hard to
believe that any planet, even a diminutive Pluto-sized one, could form at such a
far distance.

According to the standard theory of how our solar system formed, Pluto formed
much closer to the sun but was then knocked out to its current orbit due to
instability in the inner solar system. However, there are objects in our solar
system that are located even further from our sun and are difficult to explain
by this theory. Sedna, for example, is about three-fourths the size of Pluto and
is located about three times farther from the sun.

Mordecai-Marc Mac Low, an astrophysicist at the American Museum of Natural
History in New York City who was not involved in the study, said the new model
should be viewed as a plausibility argument for the presence of Pluto-sized
objects rather than proof of their existence.

"The work presented here shows that Pluto-sized objects stirring disks are
consistent with the observed disk thicknesses and other properties," Mac Low
said.

James Graham, an astronomer at the University of California, Berkeley, who was
also not involved in the study, expressed a similar sentiment. "This calculation
is making a bold extrapolation," Graham said in an e-mail interview. "It's bit
like describing an elephant given a single cell from that animal. With enough
knowledge, this is possible ? if you know enough about microbiology and genetics
and could read the DNA in the cell and in principle envision the entire
creature."


Quillen is now looking for more young stars to investigate with her model, but
the criteria to be a candidate are strict. The systems have to be young enough
to still have their circumstellar disks, but old enough to be forming embryonic
planets. They must also appear edge-on as seen from Earth and be near enough
that Hubble can accurately discern the thickness of their disks.

At the moment, the three stars Quillen has already observed appear to be the
only candidates that meet all the standards.

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Received on Mon 01 Oct 2007 04:45:50 PM PDT