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A far-off solar system
diagram of Kepler-30c
In this artist interpretation, the planet Kepler-30c is transiting one of the large starspots that frequently appear on the surface of its host star. The authors used these spot-crossing events to show that the orbits of the three planets (color lines) are aligned with the rotation of the star (curly white arrow). See also graphic without orbit lines in illustration below. Credit: Cristina Sanchis Ojeda

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Excerpt: CAMBRIDGE, MA -- …Researchers at MIT, the University of California at Santa Cruz and other institutions have detected the first exoplanetary system, 10,000 light years away, with regularly aligned orbits similar to those in our solar system. At the center of this faraway system is Kepler-30, a star as bright and massive as the Sun. After analyzing data from NASA’s Kepler space telescope, the MIT scientists and their colleagues discovered that the star — much like the sun — rotates around a vertical axis and its three planets have orbits that are all in the same plane.

“In our solar system, the trajectory of the planets is parallel to the rotation of the sun, which shows they probably formed from a spinning disc,” says Roberto Sanchis-Ojeda, a physics graduate student at MIT who led the research effort. “In this system, we show that the same thing happens.”

Their findings, published today in the journal Nature, may help explain the origins of certain far-flung systems while shedding light on our own planetary neighborhood.

“It’s telling me that the solar system isn’t some fluke,” says Josh Winn, an associate professor of physics at MIT and a co-author on the paper. “The fact that the sun’s rotation is lined up with the planets’ orbits, that’s probably not some freak coincidence.”

…Hot Jupiters’ orbits are typically off-kilter, and scientists have thought that such misalignments might be a clue to their origins: Their orbits may have been knocked askew in the very early, volatile period of a planetary system’s formation, … But to really prove this “planetary scattering” theory, Winn says researchers have to identify a non-hot Jupiter system, one with planets circling farther from their star. If the system were aligned like our solar system, with no orbital tilt, it would provide evidence that only hot Jupiter systems are misaligned, formed as a result of planetary scattering.

diagram of Kepler-30c without orbit lines
Same as diagram above, but without orbit lines. Graphic: Cristina Sanchis Ojeda

…In order to resolve the puzzle, Sanchis-Ojeda looked through data from the Kepler space telescope, … on Kepler-30, a non-hot Jupiter system with three planets, all with much longer orbits than a typical hot Jupiter. To measure the alignment of the star, Sanchis-Ojeda tracked its sunspots, dark splotches on the surface of bright stars like the sun.

“These little black blotches march across the star as it rotates,” Winn says. … If a planet crosses a dark sunspot, the amount of light blocked decreases, creating a blip in the data dip. …From the data blips, Sanchis-Ojeda concluded that Kepler-30 rotates along an axis perpendicular to the orbital plane of its largest planet. The researchers then determined the alignment of the planets’ orbits by studying the gravitational effects of one planet on another. By measuring the timing variations of planets as they transit the star, the team derived their respective orbital configurations, and found that all three planets are aligned along the same plane. The overall planetary structure, Sanchis-Ojeda found, looks much like our solar system.

The findings from this first study of the alignment of a non-hot Jupiter system suggest that hot Jupiter systems may indeed form via planetary scattering.

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