SOMETHING is amiss with the otherwise well-behaved planets in the solar system. They all line up with each other obediently enough, but their orbital plane is slightly offset from the sun's equator.
Now there is a clue to what caused this rebellious streak: it's possible the baby planets strayed while trying to keep up with their jet-setting parent.
Computer simulations show that asymmetrical jets on the young sun may have pushed it around in such a way that its family of planets became tilted.
Stars are thought to form out of collapsing clouds of gas and dust. The leftover material from star formation flattens out as it spins and clumps together to make planets. We would expect this disc to settle around the star's middle, so planets in our solar system ought to orbit in line with the sun's equator.
Planets around other stars have been found with wildly tilted orbits, or "obliquities". A favourite explanation is planet-on-planet violence, with bigger worlds tossing the smaller ones away and altering their own orbits.
In our backyard, though, the planets are tilted by just 6 degrees. This suggests that a gentler process must have been at work, says Fathi Namouni of the University of Nice in France. The answer may lie not in scattering planets, but in moving the sun.
Hubble observations in the past 10 years have revealed that young stars with planet-forming discs can shoot jets of material from their poles. Often one jet can carry up to twice as much material as the other. Whichever jet is stronger pushes on the star.
"It's like you're mounting a huge rocket on the star and accelerating it away from the disc," Namouni says. In trying to keep up with the moving star, the disc and any young planets can end up tilted, he says.
That can't be the whole story, though, says exoplanet expert Daniel Fabrycky of the University of California in Santa Cruz. The acceleration caused by the jets is so small - about a millionth of a metre per second squared - that the star would have to move in the same direction for a million years to create obliquity.
To complicate matters, stars' magnetic poles switch direction regularly, which means the more powerful jet would sometimes point in the opposite direction.
However, Namouni suggests that his model can still work if the young sun flipped its poles at the same rate as the bulk of the mass in the disc orbited.
He ran 2000 supercomputer simulations of the orbits of Jupiter and Saturn, which hold most of the solar system's planetary mass, over the lifetime of the sun. In most cases, the simulations that included the asymmetric jets produced planets with a 6-degree tilt (arxiv.org/abs/1208.1432).
No one knows why star jets are asymmetrical, and several factors, such as the sun's polar flips, would have to be timed just right for Namouni's model to work. But Fabrycky says the idea has merit.
"I think it's a cool mechanism to generate obliquities," he says.
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