Gas, not planets, may be source of rings around stars

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Rings around distant stars aren’t necessarily a sign of orbiting planets. That’s the conclusion of a simulation that challenges a tantalizing notion in planetary science: that elliptical voids in a star’s dusty debris disk betray a planet’s presence. Instead, the rings could result from interactions between the dust and gas.

“People claim too often that the rings we see are due to planets,” says planetary astrophysicist Wladimir Lyra of Caltech.

As young star systems evolve, the surrounding gas dissipates and dust particles collide and clump together. The hardiest of those blobs carve out ring-shaped pathways in the star litter and, astronomers think, form planets. While distant planets are difficult to image directly, those rings have been found circling several stars (SN Online: 6/19/13).

But around older stars, traces of gas within the debris disks may fall below the limits of detection, Lyra says. “People always thought that the effect of gas would be negligible,” he says.

Predicting the outcome of the dance between gas and dust requires powerful computer simulations. Lyra and colleague Marc Kuchner, of NASA Goddard Space Flight Center in Greenbelt, Md., created a simulation that accounted for that interaction. In their model, published in the July 11 Nature, dust warmed by a parent star transfers heat to the surrounding gas. This warmed area attracts the dust and gas into clusters that expand sideways and form a ring, Lyra says.

The work will affect astronomers’ interpretations of rings in star disks, says planetary astrophysicist Thayne Currie of the University of Toronto. “We have to be careful about taking every single one of these rings as signposts of planets.”

Dust and gas surrounding a star form into sharp rings in this simulation, even in the absence of a planet pushing debris out of the way.
Credit: W. Lyra/JPL-Caltech/NASA