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鈥檝e known that big planets close to other stars can have orbits that are tilted at all sorts of weird angles.
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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 鈥� such as interstellar gas, a bucking planet-forming disc or magnetic fields 鈥� 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.
鈥淎 story was starting to develop about why that is,鈥� says of Massachusetts Institute of Technology, who pioneered this theory.
The idea is that smaller, colder stars have thicker atmospheres. 鈥淭hat provides handles with which the star can grab onto the planet and vice versa,鈥� 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, , seems to threaten that idea. Because it tightly circles a small star, its orbit should have flattened out quickly 鈥� but the orbit is instead tilted a whopping 76 degrees from the plane in which its star spins.
鈥淚t should have aligned with the spin of the host star, and what we鈥檙e finding is that it has not,鈥� says study leader , who conducted the research at the Australian National University in Canberra. 鈥淚t was quite obvious from some of the very first measurements that it was an outlier.鈥�
鈥淭his new system doesn鈥檛 fit the pattern, it鈥檚 an anomaly,鈥� Winn says.
Understanding why, or finding other planets like HATS-14b, could knock down the tidal theory 鈥� 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鈥檚 gravity grabs hold of a hot Jupiter鈥檚 atmosphere, the same forces that pull a tilted planet into line should cause the planet鈥檚 orbit to decay, eventually leading to the star gobbling it up. This means planets aligned this way shouldn鈥檛 stick around for long, but that can鈥檛 be true because we see them out there.
Winn isn鈥檛 sure his theory can fully explain the discrepancy. 鈥淚 happen to be the one that proposed this whole tidal story,鈥� Winn says. 鈥淏ut I鈥檓 not especially wedded to it.鈥�
Journal references: The Astrophysical Journal Letters, accepted, ;听The Astrophysical Journal, accepted,
Image credit: NASA, ESA and A. Schaller (for STScI)
