In a new study published in this week's issue of Nature, astronomer Konstantin Batygin suggests that tilted planetary orbits may be due to the influence of nearby stars. As a matter of coincidence, the planets in our solar system orbit roughly along the same plane, called 'the ecliptic', which is tilted 7 degrees from the Sun's equator. Batygin's study speaks of companion stars within the same solar system causing this phenomena, so does that suggest that such a massive object caused the tilt of our system?
"Misaligned orbits are actually a natural outcome of disk migration—once you take into account the fact that planetary systems are usually born in multistellar environments," says Batygin, who works at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
'Disk migration' is a phenomena where, as a planet forms, it slows down in its orbit around its parent star, and it moves closer to its star as a result. Our star system started out as an expansive disk of rock, ice and gas, stretching out far beyond the orbit of Pluto. As the planets formed in this disk, they swept up more and more of the disk's material. As they gained mass and interacted with the matter in the disk, this gradually slowed their orbit and they 'migrated' towards the Sun. Their orbits slowly spiraled inwards until most of the material from the disk was swept up or ejected from the system, and they settled into their current orbits (or very close to them).
In star systems with only one star — like our own — the disk of the star should be lined up with the star's equator, but if there is more than one star in the system — the multistellar environment Batygin refers to — the forces between these stars (especially when one star orbits the other at a far distance) can put some extra 'torque' into the system and skew the planetary disk from the equator of the central star.
Josh Winn, an astronomer with the Massachusetts Institute of Technology in Cambridge, wants to put Batygin's hypothesis to the test. He has already studied several 'Hot Jupiter' planets — planets the size of Jupiter or larger that orbit very close to their stars — whose orbits are tilted in relation to their star's equator.
"I think it's an entirely plausible idea," he said. "The best thing about it is we can test it."
Astronomers have only examined the orbital tilt of one star system so far: Kepler-30. This system has three planets which show that their orbits line up perfectly with the equator of their single Sun-like star. Winn plans on examining more multi-planet systems to find if their orbits also line up like in Kepler-30.
As for multi-stellar systems, Batygin predicts that we will find many tilted orbits for planets around these stars. Just recently, astronomers discovered an Earth-sized planet orbiting the second star of the Alpha Centauri system — the closest star system to our own — and examining this close neighbor will be a very good way to test this idea.
"There's a good chance that astronomers will find misalignment in the Alpha Centauri system," Batygin said.
As for our star system, with its 7 degree tilt from the Sun's equator, despite us only having the single star, Batygin has an idea about that too: He believes that our Sun once had a distant companion star, that was around long enough to tilt the orbit of the planets, but was somehow kicked out of the system at some time in the distant past, turning it into a rogue star.
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"I think somewhere in the Milky Way, there's a star that's responsible for tilting us." he said.