Patterned Disks Around Young Stars Don't Always Harbor Planets

Young stars often have disks surrounding them, harboring the raw materials needed to create planets. Yet NASA scientists have now discovered that these debris disks don't always mean that planets surround these stars. Thanks to interactions between gas and dust, a debris disk may, under the right conditions, produce narrow rings on its own without any planets.

The disks that surround young stars are made up of dust and ice grains, particles produced by collisions among asteroids and comets also orbiting the star. Over time, the disks show sharply defined rings or spiral patterns. In order to better study these rings, researchers created models to see how the rings could possibly form.

"When the mass of gas is roughly equal to the mass of dust, the two interact in a way that leads to clumping in the dust and formation of patterns," said Wladimir Lyra, one of the researchers, in a news release. "In essence, the gas shepherds the dust into the kinds of structures we could expect to be seen if a planet were present."

When high-energy ultraviolet light from the central star strikes a clump of dust and ice grains, it drives electrons off the particles. These high-speed electrons then collide with heat and nearby gas. The rising gas pressure then changes the drag force on the orbiting dust, causing the clump to grow and better heat the gas. This interaction, known as photoelectric instability, continues to cascade. The clumps eventually grow into arcs, rings and oval features after tens of thousands of years.

"We were fascinated to watch this structure form in the simulations," said Lyra in a news release. "Some of the rings begin to oscillate, and at any moment they have the offset appearance of dust rings we see around many stars, such as Fomalhaut."

That's not the only thing that the researchers saw forming in these models. They also witnessed dense clumps with many times the dust density elsewhere in the disk. When a clump in a ring grows too dense, the rings break into arcs. These arcs gradually shrink until only a single compact clump remains.

The findings reveal that it may be a little harder than astronomers thought to find stars that harbor planets. At the same time, the new research also shows scientists what to watch out for as they comb the depths of space for planets.

The findings are published in the journal Nature.

Want to see the rings for yourself? Check out the video below, courtesy of NASA and YouTube.