June 23, 2018
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Distant planets in the same plane

By Sid Perkins, ScienceNOW

Scientists have discovered a distant solar system very much like our own, in which the orbits of all known planets lie in nearly the same plane and are aligned with the star’s rotation.

In recent years, astronomers have discovered a flurry of solar systems filled with exotic planets such as massive “hot Jupiters” that orbit close to their parent star, and rocky “super Earths” between one and 10 times the size of our planet. Some of these exoplanet systems have been discovered through wobbles of the star due to gravitational interactions with the orbiting planets; others betrayed their presence when planets regularly passed in front of, or transited, their parent star, temporarily blocking some of the light streaming toward telescopes here.

Earlier this year, using data gathered by NASA’s Kepler spacecraft (which is designed to monitor thousands of stars for hints of transiting planets), scientists discovered three planets circling a star dubbed Kepler-30, which is about the size and mass of our sun. One planet, with a diameter about four times that of Earth, orbits the star every 29 days; the other two, each with a diameter at least 10 times that of Earth, orbit the star every 60 days and 143 days, respectively. Further analysis revealed a huge, dark starspot on Kepler-30, similar to the sunspots that blemish the face of our sun. By tracking the spot, researchers determined that the star rotates once every 16 days or so. That’s about half the time our sun needs to rotate, which suggests that Kepler-30 is a relatively young, very active star — and that, in turn, helps explain why its starspot is so large and so persistent, says team leader Roberto Sanchis-Ojeda, an astrophysicist at the Massachusetts Institute of Technology in Cambridge.

Now, detailed analyses of the variations in light reaching Kepler’s sensors over a 30-month period reveal that the three known planets not only pass in front of the star as seen from Earth, but they repeatedly pass in front of the dark spot on Kepler-30’s surface. This reveals critical information about the distant solar system, Sanchis-Ojeda and his colleagues suggest: The planets’ orbits are aligned within a few degrees of one another, and the planes of those orbits are closely aligned with the rotational plane of the parent star, a remarkable parallel to our solar system, the team reports Wednesday in Nature.

“For the first time, we can probe a system of planets that looks like our own,” says Sanchis-Ojeda.

Although the Kepler-30 system isn’t the only one known to have planets orbiting in closely aligned planes, it is the first for which scientists have also determined the plane in which the parent star rotates.

“There are relatively few solar systems like ours,” says Drake Deming, an astronomer at the University of Maryland, College Park. “Every time we find one more, that’s a big increase.”

The team’s analytical technique could be used to find more solar systems like ours, Deming says. Many of the stars in Earth’s neighborhood are small stars called red dwarfs. Standard planet-detection methods don’t work well with red dwarfs because they’re cool and emit relatively little light, mostly in red and infrared wavelengths at which Kepler and other planet-observing sensors now in use aren’t efficient. But these stars are also very active and often have large starspots, so if spaceborne instruments specifically designed to scan for transiting planets around such stars were launched, the results could shed new light on how common planetary systems similar to our own might be. “We don’t really understand how such systems form around cool stars,” Deming notes.

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