[meteorite-list] How to Destroy a Giant Planet

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Wed, 5 Dec 2007 13:39:33 -0600
Message-ID: <193001c83776$9041a930$4b29e146_at_ATARIENGINE>

http://www.space.com/scienceastronomy/071205-giant-planets.html

How to Destroy a Giant Planet
By Robert Roy Britt -- 05 December 2007

Theorists have what they think is a good handle
on how rocky planets like Earth form. Leftovers
of star formation collide, stick together and eventually
form a ball of rock.

However, the formation of gas giant planets is more
mysterious. For starters, so many gas giants beyond
our solar system have been found improbably close
to their host stars-in some cases with blistering
effects and an unsustainable outflow of material-
that researchers figure they probably formed farther
out and then migrated inward.

Such a scheme would have huge implications for
the development of any planetary system, as a
migrating giant (like Jupiter or even more massive)
would tend to gobble up aspiring Earths on the way
in. And what's to stop the migrating worlds from
getting too close and vaporizing altogether?

Among many questions about all this, one has just
been answered: How close can a giant planet get to
a star before its atmosphere becomes unstable and
the planet is doomed to catastrophe?

Researchers at University College London (UCL)
explain their work in the Dec. 6 issue of the journal Nature.
The study involved comparing Jupiter to other giant exoplanets.

"We know that Jupiter has a thin, stable atmosphere
and orbits the sun at 5 Astronomical Units (AU)-
or five times the distance between the sun and the
Earth," explained UCL's Tommi Koskinen. "In contrast,
we also know that closely orbiting exoplanets like
HD209458b-which orbits about 100 times closer
to its sun than Jupiter does-has a very expanded
atmosphere which is boiling off into space. Our team
wanted to find out at what point this change takes
place, and how it happens."

So Koskinen's team brought a virtual Jupiter closer
and closer to the sun.

"If you brought Jupiter inside the Earth's orbit, to 0.16AU,
it would remain Jupiter-like, with a stable atmosphere,"
Koskinen said. "But if you brought it just a little bit closer
to the sun, to 0.14AU, its atmosphere would suddenly
start to expand, become unstable and escape."

Equally important in the research is what causes the sudden
catastrophic loss of air. A giant planet is cooled by its own
winds blowing around the planet. This helps keep the
atmosphere stable. Another cool effect: An electrically-
charged form of hydrogen called H3+ reflects solar
radiation back to space. As the virtual Jupiter was brought
closer to the sun, more H3+ was produced, bolstering
this cooling mechanism.

"We found that 0.15AU is the significant point of no
return," said study co-author Alan Aylward. "If you
take a planet even slightly beyond this, molecular
hydrogen becomes unstable and no more H3+ is
produced. The self-regulating, 'thermostatic' effect
then disintegrates and the atmosphere begins to heat
up uncontrollably."

"This gives us an insight to the evolution of giant planets,
which typically form as an ice core out in the cold depths
of space before migrating in towards their host star over
a period of several million years," said Aylward and
Koskinen's colleague Steve Miller. "Now we know that
at some point they all probably cross this point of no
return and undergo a catastrophic breakdown.
Received on Wed 05 Dec 2007 02:39:33 PM PST