When do gas giants reach the point of no return? (Forwarded)

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6 December 2007

The fine line between stability and instability -- when do gas giants reach
the point of no return?

Planetary scientists at UCL have identified the point at which a star causes
the atmosphere of an orbiting gas giant to become critically unstable, as
reported in this week's Nature (December 6). Depending upon their proximity
to a host star, giant Jupiter-like planets have atmospheres which are either
stable and thin, or unstable and rapidly expanding. This new research
enables us to work out whether planets in other systems are stable or
unstable by using a three dimensional model to characterise their upper
atmospheres.

Tommi Koskinen of UCL's Physics & Astronomy Department is lead author of the
paper and says: "We know that Jupiter has a thin, stable atmosphere and
orbits the Sun at five Astronomical Units (AU), or five times the distance
between the Sun and the Earth. 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.

"Our paper shows that if you brought Jupiter inside the Earth's orbit, to
0.16AU, it would remain Jupiter-like, with a stable atmosphere. 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. This dramatic
change takes place because the cooling mechanism that we identified breaks
down, leading to the atmosphere around the planet heating up
uncontrollably."

Professor Alan Aylward, co-author of the paper, explains some of the factors
which the team incorporated in order to make the breakthrough: "For the
first time we've used 3D-modelling to help us understand the whole heating
process which takes place as you move a gas giant closer to its sun. The
model incorporates the cooling effect of winds blowing around the planet,
not just those blowing off the surface and escaping.

"Crucially, the model also makes proper allowances for the effects of H3+ in
the atmosphere of a planet. This is an electrically-charged form of hydrogen
which strongly radiates sunlight back into space and which is created in
increasing quantities as you heat a planet by bringing it closer to its
star.

"We found that 0.15AU is the significant point of no return. 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."

Professor Steve Miller, the final contributing author to the paper, puts the
discovery into context: "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. Now we know that at some point they all probably cross this point of
no return and undergo a catastrophic breakdown.

"Just twelve years ago astronomers were searching for evidence of the first
extrasolar planet. It's amazing to think that since then we've not only
found more than 250 of them, but we're also in a much better position to
understand where they came from and what happens to them during their
lifetime."

-Ends-

Notes to editors:

For additional information or to arrange an interview with a member of the
research team please contact:

Dave Weston in the UCL Media Relations Office on tel: +44 (0)20 7679 7678

About UCL:

Founded in 1826, UCL was the first English university established after
Oxford and Cambridge, the first to admit students regardless of race, class,
religion or gender, and the first to provide systematic teaching of law,
architecture and medicine. In the government's most recent Research
Assessment Exercise, 59 UCL departments achieved top ratings of 5* and 5,
indicating research quality of international excellence.

UCL is in the top ten world universities in the 2007 THES-QS World
University Rankings, and the fourth-ranked UK university in the 2007 league
table of the top 500 world universities produced by the Shanghai Jiao Tong
University. UCL alumni include Marie Stopes, Jonathan Dimbleby, Lord Woolf,
Alexander Graham Bell, and members of the band Coldplay.