Astronomers have discovered a number of planets outside our solar system that orbit the "wrong" way, challenging theories about how planets form. Scientists announced the discovery of nine new planets at a meeting of the Royal Astronomical Society in Glasgow on Tuesday.

The planets are all transiting exoplanets, planets that pass between their host star and the Earth, blocking some of the star's light. Because the planets pass in front of their stars, astronomers know which way they're orbiting. The discovery brought the total of known transiting exoplanets to 27. Six of these planets were found to be orbiting in the opposite direction of their host stars' spin, which flies in the face of the prevailing model of how stars and their planets form.

"The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their star's spin," said Andrew Cameron of the University of St. Andrews in Fife, Scotland.

Astronomers think that planets form from a disk of gas and dust surrounding newly formed stars. In this model, the disk rotates in the same direction as the star, and as the dust and gas coalesce into planets, the planets all orbit in the same direction, too.

The protoplanetary disk model also results in planets that orbit around the same axis as the star's spin, all on the same plane.

All the planets in our own solar system orbit in the same direction as the Sun's spin, and all orbit in roughly the same plane. (The one exception, Pluto, is no longer considered a planet.)

But when the astronomers looked at the known 27 transiting exoplanets, they found that more than half of them were misaligned with the rotation of their stars, and six were orbiting in the opposite direction.

"This is a real bomb we are dropping into the field of exoplanets," said Amaury Triaud, a PhD student at the Geneva Observatory, in a statement.

All of the 27 planets are in a class known as "hot Jupiters," giant gas planets larger than Jupiter that orbit very close to their star.

Because gas giants are thought to form in the outer part of a star's planetary disk, the origins of hot Jupiters already posed a challenge to astronomers.

Cameron said the misaligned and backward orbits of these planets could be explained by gravitational interactions with other stars.

The tug-of-war between two stars could cause the planet's orbit to stretch, tilt or even flip direction, he said.

Such violent gravitational forces among three large astronomical objects would probably rule out the possibility of smaller, Earth-like planets in systems with hot Jupiters.

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

Backward, or retrograde, orbits are known to exist in our solar system. Triton, the largest moon of Neptune, orbits in the opposite direction to the planet's rotation. Many of the small moons of Jupiter and Saturn, and even one of Saturn's rings, orbit the "wrong" way, too.

In all these cases, astronomers have explained the backward orbit by suggesting that the moons are captured objects, rather than ones that formed along with the planet.