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Astronomers Are Closing in on Exoplanetary Rings

Monday, April 17, 2017 3:26
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Planetary rings abound in our solar system — every outer planet possesses rings, from Jupiter through Neptune. But only one extrasolar planet, J1407b, has a confirmed system of rings (largely because the rings are so large, extending out to the distance that Earth sits from the Sun). Now, astronomers are beginning to close in on an answer to the important question the lack of additional ring discoveries raises: Are exoplanet rings not readily found because they’re too hard to see, or are they simply extremely rare? 

Recent work led by Masataka Aizawa of the University of Tokyo and published in The Astronomical Journal on March 31 and highlighted by the American Astronomical Society examined the possibility of spotting rings around several long-period transiting planet candidates. Long-period transiting planets are farther out in their solar system, which promotes ring longevity. Planets that are close to their star experience conditions too hot for icy rings to last very long. Because planetary rings generally have negligible mass compared to the planet and its sun, detecting them via their gravitational influence is nearly impossible. But other methods, such as looking for ring signatures during planetary transits or via spectroscopy, which reveals the chemical makeup of the planetary system, hold greater promise. 

The rings of Uranus were definitively discovered via the first method in 1977, when James Elliot, Edward Dunham, and Jessica Mink watched the ice giant occult (pass in front of) a distant background star. Occultations, like transits, give astronomers information about a planet’s size, atmosphere, and, if present, ring system. Elliot and his team observed five smaller dips in the starlight before and after the sphere of Uranus blocked the star from view. These additional dips were smaller occultations caused by the planet’s rings.

Aizawa’s team examined a sample of 89 transiting planet candidates to look for rings. Using the transit light curves, which show the changes in starlight over time as the planet passes in front of its sun, they tried to fit the data to a model of a single planet without rings crossing the face of its star. A poor match between the predicted and actual changes in starlight could signal the presence of rings or other features. 


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