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Turning Planetary Theory Upside Down

Scientists have announced the discovery of nine new transiting exoplanets. When these new results were combined with earlier observations of transiting exoplanets astronomers were surprised to find that six out of a larger sample of 27 were found to be orbiting in the opposite direction to the rotation of their host star — the exact reverse of what is seen in our own Solar System.

The new discoveries provide an unexpected and serious challenge to current theories of planet formation. They also suggest that systems with exoplanets of the type known as Hot Jupiters are unlikely to contain Earth-like planets.

Planets are thought to form in the disc of gas and dust (proto-planetary disc) rotating around a young star in the same direction as the star itself, in more or less the same plane. Since Hot Jupiters have cores made of rock and ice particles found in the outer reaches of planetary systems, they form far from their star, and then migrate inwards to orbit it much closer via gravitational interactions with the proto-planetary disc.

A theory suggests that the proximity of hot Jupiters to their stars is not due to interactions with the dust disc at all, but to a slower evolution process involving a gravitational war with more distant planetary or stellar companions over hundreds of millions of years. After these disturbances have bounced a giant exoplanet into a tilted and elongated orbit, it would suffer tidal friction, losing energy every time it swung close to the star. It would eventually become parked in a near circular, but randomly tilted, orbit close to the star. A dramatic side-effect of this process is that it would wipe out any other smaller Earth-like planet in these systems.

Image: Artist’s impression of the planet WASP 8b transiting its parent star. Credit: ESO

Source: ESO, Royal Astronomical Society

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