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Hot jupiter shocks astronomers
GEMMA LAVENDER
ASTRONOMY NOW
Posted: 24 April 2011


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A team of astronomers at the University of St Andrews believe that Jupiter-like worlds around other stars push shock waves ahead of them, it was heard last week at the National Astronomy Meeting in Llandudno, North Wales.


An artist’s impression of the hot Jupiter WASP-12b orbiting its host star. Image: NASA/JPL–Caltech/R Hurt (SSC).

Dr Aline Vidotto, of the University of St Andrews, presented a new model based on observations made with the SuperWASP project and the Hubble Space Telescope, where she likened the magnetic ‘bow-shock’ of our home planet – a magnetic shock wave created in front of our planet as it and its magnetosphere move through the solar wind environment – to the shock waves preceding exoplanets. The bow shock acts as a barrier to the solar wind, causing it to drop in in strength and thus allows planets to protect themselves from their host star’s damaging emissions. SuperWASP, which stands for Super Wide Angle Search for Planets, allows astronomers to obtain a wealth of information about exoplanetary systems, including their composition and size, by watching for planetary transits, where the star’s light is periodically dimmed slightly by one of its planets moving in front of it.

Back in 2008 a planet with a size 1.4 times that of Jupiter was found around the eleventh magnitude yellow dwarf star, WASP-12, which is located 800 light-years away in the constellation Auriga. The planet, designated WASP-12b, is one of the largest exoplanets found to date with a diameter of more than 250,000 kilometres. The alien world orbits its host star at a distance of 3.4 million kilometres – which is extremely close compared to the Earth-Sun distance of 150 million kilometres – meaning that it is able to complete an orbit in the short time of 26 hours. The short distance means that the atmosphere of this ‘hot jupiter’ planet becomes hot and swollen, and violent interactions can occur between star and planet.

This gigantic exoplanet has provided scientists with a unique opportunity to observe these interactions. The presence of a magnetic field relies on a conducting, rotating interior within a planet and, thanks to Hubble Space Telescope data, it seems that WASP-12b has been revealed to have just that.


An artist’s impression of the bow-shocks around our home planet. NASA/CXC/M Weiss.

Using observations of the planet in ultraviolet wavelengths, astronomers at the Open University, UK, have discovered that the start of the dip in light from the star during the transit of the planet is earlier in ultraviolet than visible light. This phenomenon was originally believed to be caused by material flowing from the planet onto the star. However, studies by experts at the University of St Andrews have determined that the planet, in fact, ploughs into a supersonic headwind and pushes a shock ahead of it – in the same fashion as a supersonic jet aircraft.

Using simulations of a planet and its bow shock transiting a star and investigating various shock geometries, orientations and densities, Vidotto and her team have reproduced the dip in ultraviolet light observed in WASP-12b. “The location of this bow shock provides us with an exciting new tool to measure the strength of planetary magnetic fields,” says Vidotto. “This is something that presently cannot be done in any other way.” Joe Llama, a St Andrews PhD student, carried out the simulations of the bow shock and commented; “Our models are able to reproduce the data from the Hubble Space Telescope for a range of wind speeds implying that bow shocks could be far more commonplace than had been thought.” The bow shocks that he speaks of are believed to protect the atmospheres of hot jupiters from their harsh environment where they are constantly bombarded with highly charged, energised particles from the stellar radiation winds of their parent stars and which would otherwise erode their atmospheres. The presence of a magnetic field could greatly reduce the amount of stellar wind the planet is exposed to, acting as a shield and helping the atmosphere survive.

“Although our model predicts a bow shock similar to that of the Earth, we are not expecting any messages from WASP-12b as it is too hot to support life,” concludes Llama. “But the first hints that extrasolar planets have magnetospheres is a big step forward in understanding and identifying the habitable zones where we ultimately hope to find signs of life.”

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