Newly-discovered exoplanets found going backwards

ESO astronomers have discovered nine new exoplanets – and found that several are going the wrong way.

When data on the new discoveries was combined with earlier observations of transiting exoplanets, the scientists were surprised to find that more than half of the hot Jupiters studied had orbits that were misaligned with the rotation axis of their parent stars.

They even found that six exoplanets in the study – of which two are new discoveries – orbit their star in the ‘wrong’ direction.

It’s a bit of a bombshell, given current theories of planet formation.


“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.

Planets are thought to form in the disc of gas and dust encircling a young star. This rotates in the same direction as the star itself, and it’s always been assumed that planets formed from the disc would all orbit in more or less the same plane, and in the same direction as the star’s rotation. It’s what happens in our own solar system.

So-called hot Jupiters are closer to their stars than they should be, given their composition. The standard explanation has been gravitational interactions with the dust disc.

ESO astronomers now theorize that the reason is actually 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.