Giant planets with wonky orbits mostly circle blistering-hot stars, two new studies find. This pattern could explain why some "hot Jupiters" – planets from a third to 12 times the mass of Jupiter that sit scorchingly close to their stars – orbit the way their star spins, while others tilt so far that they orbit backward.

"It's a possible resolution of what would otherwise be a weird fluke," said astronomer Joshua Winn of MIT, a co-author of one of the new studies.

Originally, astronomers thought planets formed from a swirling disk of gas and dust that revolved around a central star like a record. When the disk's material cooled and congealed, the resulting planets all marched in line with the star's equator. Hot Jupiters were supposed to have formed around where Jupiter sits in our solar system, then spiraled calmly inward by exchanging gravitational energy with the disk, a process called migration. The first batch of extrasolar planets discovered fit this picture, reassuring astronomers that their model was right.

But in 2008, astronomers started finding giant planets whose orbits lay at jaunty angles with respect to their stars. A recent study declared that so many hot Jupiters have cock-eyed orbits – about half of the 28 whose angles have been directly measured – that scientists should throw out the disk-migration theory altogether. Instead, most hot Jupiters probably got where they are through a violent encounter with a sibling planet.

Whether a single process could have formed both regular and wrong-way hot Jupiters – and why the first batch of planets looks so different from the second – remained a puzzle. In a paper posted on arXiv.org and submitted to Astrophysical Journal Letters, astronomers propose an answer to both questions: Wonky hot Jupiters orbit hot stars.

Winn and his colleagues took 19 of the planets whose angles have been measured, and plotted their angles against the temperature of their star. Only two of 11 planets orbiting cool stars were misaligned, while six out of eight planets orbiting stars with temperatures hotter than 6,250 Kelvin (10,790 degrees Fahrenheit) had tilted orbits.

The team pointed out that the first hot Jupiters were found by observing how the star moved in response to the planet's gravitational tug. This method, called Doppler spectroscopy, has an easier time finding planets around relatively cool stars.

The second group was found in transit surveys, where the planet announces its presence by passing in front of the star and blocking some of the star's light. This method works better with hotter, brighter stars, because the contrast is greater.

"That's why the first bunch of stars we looked at showed well-aligned orbits, and the second batch showed misaligned orbits: because the second batch were mainly hot stars," Winn said.

A second paper accepted to the Astrophysical Journal came to the same conclusion by a different route. Kevin Schlaufman, a graduate student at the University of California, Santa Cruz, noted that current techniques measure only the angle between the star and the planet's orbit, but the angle between the star and the Earth is also needed to draw a complete picture in three-dimensional space.

One way to estimate this angle is to check how fast the star seems to spin. Astronomers can tell how fast a star should spin based on its age and its mass. If the star apparently spins too slowly, that's a clue that it's not facing the Earth edge-on. Planets that cross in front of their stars must have edge-on orbits from the Earth's point of view, or we wouldn't see them. So if a star spins too slowly but has a transiting planet, that means the planet is at a wonky angle.

Schlaufman did a statistical study of 75 exoplanet systems, and found that 10 of them should have tilted orbits. Several of the planets his computations picked out were already known to have funny orbits. And all of them circled large, hot stars.

"I find that encouraging, and a signal that we're onto something good," Winn said. "We have these two pretty much totally independent ways of checking, and they give the same result."

Winn suggested that the transition temperature could explain why only hot stars have tilted planets. Stars that burn cool have thick outer layers called convective zones that respond strongly to the gravitational pull of the planet. The friction from the planet and the star yanking each other around robs energy from the planet's orbit. The orbit slowly becomes circular and settles into alignment with the star's equator, a position that takes less energy to maintain.

Stars hotter than 6,250 Kelvin have thin or even nonexistent convective zones, Winn said, so their hot Jupiters stay wherever their violent histories parked them.

"It struck us as interesting that this transition from well-aligned planets to misaligned planets happens to be at about the same temperature as convective zones," Winn said.

The theory still has some kinks to work out, like keeping the planet from getting swallowed up by the star. Astronomer Andrew Collier-Cameron of the University of St. Andrews in Scotland, who was not involved in the new study, calls for more observations.

"It's early days yet, and we're still working with a grand total of only about 28 planets," Cameron said. "Until we go do the legwork and measure more of them, there's still plenty of wriggle room for theorists."

Cameron also noted that, though "rampaging hot Jupiters" could knock any other planets out of their systems, most of the galaxy's Earth-like planets are probably safe. Hot Jupiters are "rare beasts," he said. "By and large, although they may mess up their own systems, they don't really harm the chances of us finding terrestrial type planets."

Image: ESO/L. Calçada

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