Astronomers in the US believe they have discovered why a quarter of known "hot Jupiters" orbit their stars in reverse. The finding challenges our understanding of how planets form and may give us a clue as to how common solar systems like ours are.

More than 550 exoplanets – planets orbiting stars other than the Sun – have been discovered to date. Many of them have been dubbed hot Jupiters because they are about the same mass as the giant planet and orbit very close to their host stars. However, astronomers have wondered why one in four of these alien worlds orbit in the opposite direction to the spin of their star. This is unlike our solar system, where the planets bend to the will of the Sun, with their orbits all following its anticlockwise rotation. Now, astrophysicists at Northwestern University think they have the answer.

The team modelled a simple solar system with a Sun-like star orbited by a Jupiter-sized planet far from the star and a second large body (a planet or brown dwarf) even further away. When the model was run the gravitational interactions between the bodies began to change the orbit of the inner planet. "These are extremely weak gravitational perturbations that over millions or billions of years cause small, gradual changes in the planet's orbit, which build up to become very large," team member Fred Rasio told physicsworld.com.

Flipped over

Eventually, this changes the inner planet's orbit from almost circular, like our Jupiter's, to highly elongated – a journey that at times takes the planet very close to the host star. The gravity of the star then squeezes and heats the planet, causing it to lose orbital energy and shrinking its orbit. This part of the theory, which explains the planet's proximity to its star, had been modelled before.

However, Rasio and his colleagues saw something new. "These perturbations also caused the inclination of the orbit to change," Rasio explained. Inclination is the angle between the angular momentum of the spinning star and the orbital angular momentum of the planet. "In some cases the hot Jupiter became so inclined to its star that it completely flipped over it and orbited it in the other direction," he adds.

The key player here is angular momentum – a quantity that must to be conserved. As the inner planet moves from a circular orbit to an elongated one, its angular momentum drastically decreases – in turn increasing the angular momentum of the perturbing outer body by the same amount. This loss of angular momentum makes the inner planet much easier to flip. "It only takes a relatively small force to flip a planet with a tiny angular momentum," says Smadar Noaz, Rasio's colleague.

'Promising mechanism'

"This looks like a very promising mechanism," Gordon Ogilvie at the University of Cambridge, UK, told physicsworld.com. "However, what is less clear is how often this mechanism occurs and whether this is sufficient to explain the majority of observed systems," he adds. Other processes have also been suggested and it could turn out that a single mechanism isn't causing all of the "flipped" hot Jupiters. "Further theoretical work is certainly needed to distinguish between these possibilities," says Ogilvie.

The case could be settled by finding the smoking gun: the perturbing, outer planet in these systems. "It [the perturbing planet] should still be there; there is no easy way to get rid of it," Rasio explains. "It could be very faint and hard to detect but it should be there," he adds. Direct imaging of exoplanets, such as that of Beta Pictoris b, could find them (see "Exoplanet caught on the move").

If confirmed, Noaz believes it tells us something important about our own solar system and our theories of planetary formation. "The picture of our solar system is very neat and beautiful. However, we see a whole zoo of different planetary systems out there, including planets that seem to flip over," she says. "This not only means our solar system might be unusual but it emphasizes the need for a better understanding of how planets are formed," she adds.

The findings are published in Nature 473 187.