A research team including university professor Douglas Hamilton made a surprising discovery in 2009 when it found Saturn’s nearly invisible Phoebe ring, the solar system’s largest.
This year, the team followed up its research with another unexpected development: The planet’s outermost ring is about 30 percent larger than originally expected.
Photos from the Wide-field Infrared Survey Explorer spacecraft helped Hamilton’s team get its first good look at the Phoebe ring. The ring is massive, Hamilton said, about 270 times larger in diameter than Saturn, which has a diameter of more than 72,000 miles.
“For the first time, we know exactly how big it is, which is one of the first questions people ask when we tell them we found the biggest ring in the solar system, so that’s a relief,” Hamilton said, laughing. The area of space that the Phoebe Ring covers is 7,000 times larger than Saturn.
The team’s 2009 discovery came from a small sample, just a “stripe across the ring,” he said. But this time, using data from the WISE spacecraft, which was launched in 2009, the researchers acquired a complete view of the ring.
Michael Skrutskie, a University of Virginia astronomy professor, was part of the WISE science team responsible for assembling the full image of the ring. WISE traveled around the solar system taking “shallow” pictures of the sky, and Saturn “happened to be in the right place at the right time,” he said .
“In the last study, there was a very slight germ of doubt, as with anything in astronomy,” he said. “So as WISE was painting the sky, we knew Saturn was coming. It was a relief as each stripe went nearer and nearer to Saturn and we started to see the ring.”
In addition to discovering the largest ring in the solar system, the team answered a lingering question — what size particles compose the ring. Through the WISE images, Hamilton said, the team now knows it is primarily made up of dust particles, unlike big blocks of rock and ice making up Saturn’s main rings.
The particles’ average width measures about a 100th of a millimeter. To help contextualize just how tiny that is, Hamilton offered a comparison: A human hair measures about a 20th of a millimeter — so five Phoebe dust particles equal one human hair.
These tiny particles don’t last very long, Hamilton said, meaning the ring is being produced quickly in real time.
The Phoebe ring is significant in that it is vast, faint and behaves differently than all other known rings, said Andrea Smith, a senior communication major and astronomy minor.
According to traditional understanding, if rings extend too far from the gravitational pull of the planet they orbit, they should collapse. Phoebe’s ring doesn’t do that; instead, it gradually fades out into the distance, Hamilton said, with its edges fading into invisibility.
“All of the textbooks will tell you that rings form near planets,” Hamilton said. “This thing flies in the face of that — it’s the furthest distance something can be from a planet and still have it be in orbit.”
Hamilton and his team’s 2009 research project stemmed from interest in one of Saturn’s moons, Iapetus, which has fascinated scientists for hundreds of years. Iapetus, unlike any other moon discovered in the solar system thus far, is black on one side and white on the other.
That, Hamilton’s team learned, is a result of the Phoebe ring’s black dust and debris striking the front face of Iapetus.
“It’s like a car driving through a rainstorm, where all the droplets hit the windshield,” Hamilton said. “The dust was only hitting the front side of the moon — it was only hitting the windshield.”
For Skrutskie, discovering Phoebe’s ring in 2009 came as a complete surprise, which he admits gratefully.
“Being surprised in astronomy is a great thing,” he said. “Because then you have to explain it, and that’s when the cool stuff happens.”
