Weirdly tilted planet knocks formation theory out of line

A freshly discovered off-kilter exoplanet is knocking our best theory on how such beasts form out of line.

In our solar system, the planets all orbit the sun in the same plane, perpendicular to the axis around which the sun spins. But for half a decade, we’ve known that big planets close to other stars can have orbits that are tilted at all sorts of weird angles.

We thought we had a grip on why. Stars and their planets all grow out of the same spinning disc, which means that a system needs something extra – such as interstellar gas, a bucking planet-forming disc or magnetic fields – to explain the mismatch.

One possible clue was that small, cold stars tend to have close-in gas giants called hot Jupiters that stay in line, whereas bigger, hotter stars are more likely to have hot Jupiters with tilted orbits.

“A story was starting to develop about why that is,” says Joshua Winn of Massachusetts Institute of Technology, who pioneered this theory.

The idea is that smaller, colder stars have thicker atmospheres. “That provides handles with which the star can grab onto the planet and vice versa,” Winn says. Over time, those gravitational handles exert a tidal force on the planet, pulling it and its star into alignment.

HATS off

But one Jupiter-mass planet discovered earlier this year, HATS-14b, seems to threaten that idea. Because it tightly circles a small star, its orbit should have flattened out quickly – but the orbit is instead tilted a whopping 76 degrees from the plane in which its star spins.

“It should have aligned with the spin of the host star, and what we’re finding is that it has not,” says study leader George Zhou, who conducted the research at the Australian National University in Canberra. “It was quite obvious from some of the very first measurements that it was an outlier.”

“This new system doesn’t fit the pattern, it’s an anomaly,” Winn says.

Understanding why, or finding other planets like HATS-14b, could knock down the tidal theory – which even Winn is starting to doubt.

In another new paper, Winn and Gongjie Li of Harvard University address another flaw in the traditional idea. Once the star’s gravity grabs hold of a hot Jupiter’s atmosphere, the same forces that pull a tilted planet into line should cause the planet’s orbit to decay, eventually leading to the star gobbling it up. This means planets aligned this way shouldn’t stick around for long, but that can’t be true because we see them out there.

Winn isn’t sure his theory can fully explain the discrepancy. “I happen to be the one that proposed this whole tidal story,” Winn says. “But I’m not especially wedded to it.”

Journal references: The Astrophysical Journal Letters, accepted, http://arxiv.org/abs/1510.08575; The Astrophysical Journal, accepted, http://arxiv.org/abs/1511.05570

Image credit: NASA, ESA and A. Schaller (for STScI)