Dust rings around distant stars, considered by many as "smoking gun" evidence of orbiting planets, can form on their own, U.S. research suggests.

The finding is possible bad news for those who use the structures to guide them to stars in their search for distant planets, and may raise questions about the existence of a controversial candidate exoplanet.

Sharply defined or elongated rings seen in the disk of dust and gas around stars had been assumed to be caused by unseen planets carving paths through the disk.

"I call it the dark matter argument," Wladimir Lyra at NASA's Jet Propulsion Laboratory in Pasadena, Calif., told NewScientist.com. "There is something you are seeing that you cannot explain, and you blame the gravity of something you cannot see."

Now Lyra, with NASA colleagues, has shown interactions between dust and gas alone can account for the rings.

Dust concentrates in regions of high-pressure gas that when heated by the star expands, creating higher pressure which then concentrates more dust, the researchers said.

Lyra and his colleagues simulated this feedback process and, with no planets in their computer model, created several types of structure, including elongated rings and clumps.

The work calls into question a bright dot in the disc around the star Fomalhaut thought to be a planet, but which could be due to the dynamics of dust and gas alone, the researchers said.

Markus Janson of Princeton University in New Jersey, who has studied the Fomalhaut system, says it's still too soon to tell what the bright dot is and why the nearby dust ring appears so sculpted.

"I'm now more open to the idea that maybe there are no planets [around Fomalhaut] at all," he said.