New computer simulations show that gravitational interactions between two planets can explain why a handful of peculiar extrasolar planets are orbiting in the opposite direction to the spin of their host star.

“How can one be spinning one way and the other orbiting exactly the other way? It’s crazy,” says Frederic A. Rasio of Northwestern University. “It so obviously violates our most basic picture of planet and star formation.”

When a star sparks into existence it is surrounded by a swirling disc of dust and debris that moves in the same direction as the star itself is spinning. As a result, any planets condensing out of the debris disc will be orbiting in the same direction. Yet around one-quarter of all known exoplanets have ignored theory and orbit counter to the spin direction of the star; may also orbit incredibly close to their star, too.

In a new study, the Northwestern University team use computer simulations to solve how these planets got to be so close to their stars, along the way uncovering how they got their "backward" orbits. They suggest that gravitational perturbations by a much more distant planet or stellar companion in the same system are to blame.

“Once you get more than one planet, the planets perturb each other gravitationally,” says Rasio. “This becomes interesting because that means whatever orbit they were formed on isn’t necessarily the orbit they will stay on forever. These mutual perturbations can change the orbits, as we see in these extrasolar systems.”

In the simulations, the team consider two Jupiter-like planets orbiting a Sun-like star, with the inner planet located far from the star where large gas giants are typically thought to form, and the outer planet even further away, also on an eccentric orbit. The model also assumes that the two orbits start with a relatively high inclination to one another.

Initially small gravitational interactions between the two planets build up over a long period of time, such that the orbits start to exchange angular momentum, with the inner one losing energy to the outer via strong tides. Eventually this forces the inner planet to career into a brief, eccentric orbit, during which time the orbit of the planet may even change direction. As angular momentum is dumped onto the outer planet, the inner planet's energy is lessoned and so its orbit shrinks, bringing it close to the parent star. Tidal interactions with the star then rapidly circularizes its orbit.

"The flip happens after a very brief excursion of a very high eccentricity event," lead author of the study, Smadar Naoz, tells Astronomy Now. "The length of the entire process will depend on the configuration of the system [i.e. the masses of the two interacting objects and their orbital periods] but for systems where forced eccentricity plays an important role, this flipping will always happen."

Naoz adds that there are two main routes to forming a closely orbiting hot-jupiter through the dynamical evolution of the systems they consider. "In the first, tidal friction slowly dampens the growing eccentricity of the inner planet, resulting in a circularized, prograde hot jupiter. In the second, a sudden high-eccentricity spike in the orbital evolution of the inner planet is accompanied by a flip of its orbit. The planet is then quickly circularized into a retrograde short-period orbit."

The scientists suggest that observed systems with closely orbiting, "misaligned" hot jupiters could also contain a much more distance massive planet or brown dwarf on an inclined orbit. And the fact that a significant proportion of observed exoplanets are orbiting in the opposite direction to their star's spin suggests that planet-planet interactions play an important role in the dynamical history of these extrasolar systems.

“We had thought our Solar System was typical in the Universe, but from day one everything has looked weird in the extrasolar planetary systems,” says Rasio. “That makes us the odd ball really. Learning about these other systems provides a context for how special our system is. We certainly seem to live in a special place.”

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