Not all exoplanets are created equal, and new discoveries about the orbits of newly found extra solar planets could challenge the current theories of planet formation. The discoveries also suggest that systems with exoplanets of the type known as hot Jupiters are unlikely to contain Earth-like planets. “This is a real bomb we are dropping into the field of exoplanets,” said Amaury Triaud, a PhD student at the Geneva Observatory who led an observational campaign from several observatories.
Six exoplanets out of twenty-seven 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 team announced the discovery of nine new planets orbiting other stars, and combined their results with earlier observations. Besides the surprising abundance of retrograde orbits, the astronomers also found that more than half of all the so-called “hot Jupiters” in their survey have orbits that are misaligned with the rotation axis of their parent stars.
Hot Jupiters are planets orbiting other stars that have masses similar to or greater than Jupiter, but which orbit their parent stars much more closely.
Planets are thought to form in the disc of gas and dust encircling a young star, and since this proto-planetary disc rotates in the same direction as the star itself, it was expected that planets that form from the disc would all orbit in more or less the same plane, and that they would move along their orbits in the same direction as the star’s rotation.
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“The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin,” said Andrew Cameron of the University of St Andrews, who presented the new results at the RAS National Astronomy Meeting (NAM2010) in Glasgow, Scotland this week.
At this writing, 454 planets have been found orbiting other stars, and in the 15 years since the first hot Jupiters were discovered, astronomers have been puzzled by their origin. The cores of giant planets are thought to form from a mix of rock and ice particles found only in the cold outer reaches of planetary systems. Hot Jupiters must therefore form far from their star and subsequently migrate inwards to orbits much closer to the parent star. Many astronomers believed this was due to gravitational interactions with the disc of dust from which they formed. This scenario takes place over a few million years and results in an orbit aligned with the rotation axis of the parent star. It would also allow Earth-like rocky planets to form subsequently, but unfortunately it cannot account for the new observations.
To account for the new retrograde exoplanets an alternative migration 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 tug-of-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,” says Didier Queloz of Geneva Observatory.
The observatories for this survey included the Wide Angle Search for Planets (WASP), the HARPS spectrograph on the 3.6-metre ESO telescope at the La Silla observatory in Chile, and the Swiss Euler telescope, also at La Silla. Data from other telescopes to confirm the discoveries.