More evidence our solar system may not be the 'model' for other systems.
http://www.timesonline.co.uk/tol/news/science/space/article7096677.eceDistant planets that orbit the wrong way around their parent stars have been observed by astronomers, in a discovery that challenges prevailing theories of planetary formation.
All the planets in our solar system orbit in the same direction as the rotation of the Sun — anticlockwise — and standard explanations for their formation suggest that this model should apply whenever planets form from a disc of dust around a star.
Research presented yesterday at the Royal Astronomical Society’s annual meeting in Glasgow, however, has now indicated that this is not so.
A team led by Andrew Collier Cameron, of the University of St Andrews, has found six distant “exoplanets” that circle in the opposite direction to their host stars. “The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their star’s spin,” Professor Cameron said.
“Our picture of planetary formation and migration may have been coloured by the simplicity of our own solar system.”
Amaury Triaud, a doctoral student at the Geneva Observatory who was a member of the research team, said: “This is a real bomb we are throwing into the field of exoplanets.”
Standard theories of solar system formation hold that planets coalesce out of protoplanetary discs of dust and gas that spin around a star’s equator. As these discs rotate in the same direction as the star spins on its axis, planetary orbits ought also to follow this pattern. This is what happens in the solar system, where all eight of the planets orbit in the same direction and in roughly the same plane.
The new observations, however, have shown that this model does not always apply, at least to solar systems that contain “hot Jupiters”, or gas giants that orbit very close to their central star.
Of 27 hot Jupiters that were examined in close detail, more than half were found to have orbits that were misaligned with the rotation of their parent stars. Of these, six planets, of which two are newly discovered, orbit in the opposite direction to the star’s spin.
Hot Jupiters are known to form in the outer reaches of solar systems and then to migrate inwards. It had been thought that this was caused by the gravity of other parts of the planetary disc.Professor Cameron said that the planets with a retrograde orbit must be explained by a different phenomenon: the influence of gravity from more distant objects, typically nearby companion stars.
These would throw gas giants into highly eccentric orbits, causing a gravitational tug of war between two stars that could flip their orbital alignment.
This model also suggests that no Earth-like planets could survive in solar systems of this type.Didier Queloz, of the Geneva Observatory, another leader of the research team, said:
“A dramatic sideeffect of this process is that it would wipe out any other smaller Earth-like planet in these systems.”