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How to destroy a giant planet

Astronomers calculate how close a gas giant can get to its parent star before it suffers a "catastrophic breakdown" - which has implications for solar systems beyond our own.
Image: Planet and star
If a planet creeps too close to its host star, its atmosphere is blown off into space by energetic solar output. Here, this artist's impression shows an extended envelope of atmosphere around the exoplanet HD 209458b being cast off into space. Alfred Vidal-Madjar / ESA
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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 explain their work in Thursday's issue of the journal Nature.

Closer, closer...
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  — 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."

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

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

Cool effects
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 0.15 AU 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."