A dominant theory about planets has been challenged by the discovery of nine worlds transiting distant stars, astronomers reported on Tuesday.

The belief that planets always orbit their sun in the same direction, imitating the rotation of the star itself, has been turned upside down, they said.

"This is a real bomb we are dropping into the field of exoplanets," said Geneva Observatory astronomer Amaury Triaud, referring to planets outside the Solar System.

Triaud's team are to report their findings at a meeting this week of the Royal Astronomical Society (RAS) in Glasgow, Scotland.

Their revolutionary notion is based on the discovery of nine exoplanets, which bring the tally of these phenomena to a grand 452 since they first came to light in 1995.

The latest planets are especially useful, as they were not discovered indirectly -- by calculating their gravitational pull on the star's light -- but because they passed directly in front of the sun.

These rarely-captured "transit" events are especially coveted, for they can yield much more information about the planet.

After combining the new results with previous observations of transiting exoplanets, Triaud and fellow astronomers Andrew Cameron and veteran exoplanet hunter Didier Queloz were stunned.

Six of 27 exoplanets they sampled were found to orbit in the opposite direction of their host star.

The big hypothesis about planets is that they coalesce from a disc of dust and gas orbiting a young star and move in the same direction of the star's own rotation.

"The new results really challenge the convention wisdom that planets should always orbit in the same direction as their stars spin," said Cameron, of the University of St. Andrews, Edinburgh.

The transiting planets are so-called "hot Jupiters," or planets with a mass similar to or greater than Jupiter.

Unlike our Jupiter, which encircles the Sun at a great distance, hot Jupiters are found very close to their star, sometimes roastingly so.

Until now, hot Jupiters were believed to form from material far from the host star and then gradually migrate to a closer orbit as a result of gravitational interaction between the star and the proto-disc of planetary dust.

How the "renegade" hot Jupiters came to exist dents this theory, too.

It could be that, in their infant stage, these planets became caught up in a "gravitational tug-of-war" with distant planets or even nearby stars, the astronomers suggest in a press release.

As a result, the exoplanet would have been hauled into a tilted or elongated orbit. Eventually, due to a phenomenon called tidal friction, it would have been snared by the star, parked in a weird, randomly tilted orbit close by.

Another question is what this means about hopes for finding another Earth -- a small, rocky planet that is in the "Goldilocks zone," where it is neither too hot nor too cold but just right so that water can exist in liquid form.

The Jupiter in our Solar System is believed to play a protective role, its huge mass taking the hit from rogue comets or asteroids that could smack into the small, vulnerably rocky planets nearer the Sun, including Earth.

But the new retrograde hot Jupiters would be killers, not guardians.

Essentially, they would be like a giant ball in a game of space billiards, wiping out any smaller planet in the vicinity as they lurched around.

"A dramatic side-effect of this process is that it would wipe out any other smaller Earth-like planet in these systems," said Queloz, also of Geneva Observatory.

The discoveries, made using the European Southern Observatory's giant 3.6-metre (11.7-feet telescope in La Silla, Chile, have been submitted to peer-reviewed journals for publication, ESO said.