Several extrasolar planets have been discovered to be orbiting backward — that is, they revolve in the opposite direction that their host star rotates — challenging accepted ideas of how planets form, according to the astronomers who made the discovery.

“This is a real bomb we are dropping into the field of exoplanets,” said team member Amaury Triaud, a doctoral student at the Geneva Observatory in Switzerland.

The team announced the discovery of nine new transiting exoplanets April 13 at the annual meeting of the Royal Astronomical Society in Glasgow, Scotland. Transiting exoplanets are ones that were discovered as they passed in front of their host star from the perspective of Earth, causing a dip in the light coming from that star.

When the new results were combined with earlier observations of 18 other transiting planets, the astronomers were surprised to find that six out of that larger sample of 27 exoplanets were orbiting in the opposite direction of the rotation of their host star, called retrograde motion — the reverse of our solar system. Astronomers first discovered a backward-orbiting exoplanet in August 2009.

The new finding suggests that astronomers might have to revise some aspects of planet formation.

Planets are thought to form in the disc of gas and dust that surround a young star. This proto-planetary disc rotates in the same direction as the star itself, and it was thought that any planets that formed out of the disc would revolve in that same direction.

The planets found in the new study are so-called hot Jupiters, which are Jupiter-size planets that orbit very close to their parent stars, experiencing extreme temperatures. It was originally thought that hot Jupiters formed far from their star and migrated inward over a few million years as a result of gravitational interactions with the disc of dust from which they formed. But this theory doesn’t account for the new observations, the team said.

To explain the retrograde motion of the six exoplanets, the hot Jupiters may have migrated in due to the gravitation tug-of-war between them and more distant planetary or stellar companions over the course of 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.

“The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their star’s spin,” said team member Andrew Cameron of the University of St. Andrews in Scotland.