When do gas giants reach the point of no return?

Dec 05, 2007

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. 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.”

Source: University College London

Explore further: Gravitational wave detection likely within five years, according to researcher

Related Stories

What are extrasolar planets?

12 hours ago

For countless generations, human beings have looked out at the night sky and wondered if they were alone in the universe. With the discovery of other planets in our solar system, the true extent of the Milky ...

What makes Mars sunsets different from Earth's?

May 19, 2015

Even robots can't tear their eyes from a beautiful sunset. NASA's Mars Curiosity rover pointed its high resolution mast camera at the setting sun to capture this 4-image sequence on April 15 at the conclusion ...

Using a sounding rocket to help calibrate NASA's SDO

May 19, 2015

Watching the sun is dangerous work for a telescope. Solar instruments in space naturally degrade over time, bombarded by a constant stream of solar particles that can cause a film of material to adhere to ...

A conversation with astronomer Dimitri Mawet

May 18, 2015

Associate Professor of Astronomy Dimitri Mawet has joined Caltech from the Paranal Observatory in Chile, where he was a staff astronomer for the Very Large Telescope. After earning his PhD at the University ...

Video: Kepler observes Neptune dance with its moons

May 15, 2015

NASA's Kepler spacecraft, known for its planet-hunting prowess of other stars, is also studying solar system objects. In its new K2 mission, Neptune and two of its moons, Triton and Nereid, have been imaged. ...

Does the red planet have green auroras?

May 14, 2015

Martian auroras will never best the visual splendor of those we see on Earth, but have no doubt. The red planet still has what it takes to throw an auroral bash. Witness the latest news from NASA's MAVEN ...

Recommended for you

Hubble video shows shock collision inside black hole jet

14 hours ago

When you're blasting though space at more than 98 percent of the speed of light, you may need driver's insurance. Astronomers have discovered for the first time a rear-end collision between two high-speed ...

A bubbly cosmic celebration

May 27, 2015

In the brightest region of the nebula RCW 34, gas is heated and expands through the surrounding cooler gas. Once the heated hydrogen reaches the borders of the gas cloud, it bursts outwards into the vacuum ...

Image: XMM-Newton self-portraits with planet Earth

May 27, 2015

This series of images was taken 15 years ago, a couple of months after the launch of ESA's XMM-Newton space observatory. These unique views, showing parts of the spacecraft main body and solar wings, feature ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.