Backward planets turn theory upside down

Artist's impression of an exoplanet in a retrograde orbit around a star

Up to now it was expected that exoplanets 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—as they do in our Solar System. However, new results unexpectedly show that with some, the orbit is completely reversed.

The discovery of nine new exoplanets as announced today at the Royal Astronomical Society’s National Astronomy Meeting in Glasgow. Exoplanet are ones that orbit stars beyond our Solar System.

When these new results were combined with earlier observations, the 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.

“This is a real bomb we are dropping into the field of exoplanets,” says Amaury Triaud, a PhD student at the Geneva Observatory who, with Andrew Cameron and Didier Queloz, leads a major part of the observational campaign.

Planets are thought to form in the flattened, disc-like cloud of gas and dust encircling a young star. This proto-planetary disc rotates in the same direction as the star itself, and up to now 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. This is the case for the planets in the Solar System.

After the initial detection of the nine new exoplanets with, the WASP (Wide Angle Search for Planets) team of astronomers used the 3.6-metre European Southern Observatory (ESO) telescope at the La Silla observatory in Chile, along with data from the Swiss Euler telescope, also at La Silla, and data from other telescopes to confirm the discoveries and characterise the exoplanets found in both the new and older surveys.

Surprisingly, when the team combined the new data with older observations they found that more than half of all the hot Jupiters studied have orbits that are misaligned with the rotation axis of their parent stars.

They even found that six exoplanets in this extended study (of which two are new discoveries) have “retrograde motion”—they orbit their star in the “wrong” direction.

“The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin,” says Andrew Cameron of the University of St Andrews.

Artist's impressions of the exoplanets that have "backward" orbits.

Artist's impressions of the exoplanets that have "backward" orbits. The exoplanets are shown moving across the face of their parent stars, which is how they were found. The object at the lower right is for comparison and has a “normal” orbital direction.

How do “hot Jupiter’s” form?

In the 15 years since the first hot Jupiters were discovered, their origin has been a puzzle. These are planets with masses similar to or greater than that of Jupiter, but that orbit very close to their suns.

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 planets or stars 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.

Two of the newly discovered retrograde planets have already been found to have more distant, massive companions that could potentially be the cause of the upset. These new results will trigger an intensive search for additional bodies in other planetary systems.

The current count of known exoplanets is 452.

Adapted from information issued by ESO / A.C. Cameron / L. Calçada.

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