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.
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 (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."
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 0.15AU 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.
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