A ring system that spans some 200 times the diameter of Saturn’s rings circles the exoplanet J1407 b. Astronomers suspect that undetected moons may create the observed gaps. This artist’s depiction shows the rings blocking light from the host star, J1407. RON MILLER
An exomoon alone could be a challenge to detect. While Beta Pic b’s entire Hill sphere will take from early April 2017 to late January 2018 to transit, a massive moon detectable from Earth would zip by in two days, says Wang. And it may make only a single transit. To confirm an exoplanet, scientists typically have to view at least three transits to rule out other possibilities. Since Beta Pic b takes more than 20 years to orbit its star — and the moon may not be visible during some transits, according to Heller — it could take a century to confirm a moon this way. Massive rings could be quite a bit easier to spot. Saturn boasts the solar system’s most massive ring system, but all the giant planets have rings. Saturn’s set remains an enigma, however, because its expected
lifetime is much shorter than the age of the solar system, says Heller. Fragments from colliding moons are one possible source, and exoplanets presumably could host similar systems. And a young planet like Beta Pic b may still possess a dazzling, pristine ring system that could put Saturn’s to shame. Matthew Kenworthy of Leiden Observatory in the Netherlands knows about rings. In 2007, he and his colleague, Eric Mamajek of the University of Rochester, spotted a massive ring system around a planet circling another star, J1407, only a few million years younger than Beta Pic. The enormous rings stretch nearly 200 times farther than Saturn’s, and they have gaps that the researchers tentatively identify with the gravitational pull
Rings around an exoplanet J1407 b Track of J1407
Astronomers deduced the presence of J1407 b’s ring system from the rapid brightness variations it caused as the rings passed in front of the Sun-like star J1407 in 2007. In this plot, the intensity of the red color corresponds to how much light each ring blocked. ASTRONOMY: ROEN KELLY, AFTER KENWORTHY AND MAMAJEK
of exomoons in much the same way that Saturn’s moons sculpt its rings. Ironically, although the rings are visible, the planet has yet to be discovered. The moons also have evaded direct detection; the evidence for them remains circumstantial. Nor have the strange rings eclipsed J1407 again, although Kenworthy and his colleagues are keeping their eyes on the star. But without seeing a planet, there’s no way to determine an orbit to know when the geometry might repeat. “We think that the same thing may happen with Beta Pic and its planet,” says Kenworthy. And this world might provide even better signs of its rings than J1407 did. “The difference is that we know when the planet is moving between us and its star.” There’s another reason Kenworthy is mildly optimistic about finding rings. Because Beta Pic is reasonably bright — at 4th magnitude, it’s easily visible to the naked eye from a dark site — astronomers have long used it as a standard star to help calibrate observations of other objects. In 1981, a strange fluctuation changed the star’s usually steady light over a two-week period, creating a pattern that puzzled astronomers. According to Wang’s calculations, there’s a good chance that Beta Pic b’s Hill sphere transited the star at that time, suggesting that something associated with the planet blocked the star’s light. So when Kenworthy heard that the planet itself wasn’t going to transit, he began wondering if perhaps a set of massive rings could produce a signal. W W W.ASTR ONOMY.COM
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