[meteorite-list] How to Destroy a Giant Planet

[Sterling K. Webb]

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.