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There can be smoke without fire. Sharp rings of dust around stars aren’t always carved by planets but can form on their own – bad news for those who use the structures to guide them to stars that host planets. The finding also has implications for the existence of a controversial candidate exoplanet.

The discs of dust and gas debris surrounding stars sometimes produce sharply defined or elongated rings. These were assumed to be the calling cards of unseen planets, carved by the bodies as they travel through the disc.

“I call it the dark matter argument,” says at NASA’s Jet Propulsion Laboratory in Pasadena, California. “There is something you are seeing that you cannot explain, and you blame the gravity of something you cannot see.”

Now Lyra and at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, have shown that interactions between dust and gas alone can account for the rings.

Dust concentrates in regions of high-pressure gas. As the star heats the dust, it in turn causes the gas to heat up and expand, creating higher pressure which then concentrates more dust. Lyra and Kuchner simulated this feedback process and, with no planets in their model, .

Controversial planet

The work suggests a bright dot in the disc around the star Fomalhaut (above, right) may not be a planet. In 2004, the Hubble Space Telescope spotted the dot inside a gap in the dusty disc around Fomalhaut. Some astronomers thought the dot was a giant planet that had carved out the gap. That would have made the object, called Fomalhaut b, one of only a few exoplanets to be imaged directly.

But follow-up observations did not detect the object at infrared wavelengths, suggesting the dot was not a Jupiter-like planet, whose infrared glow should have been observed. Instead, some ventured that the dot was a dust cloud created by colliding asteroids and that one or more might have carved the dusty disc’s sharp edges.

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. But he is intrigued that these structures can form due to the hydrodynamics of dust and gas alone: “I’m now more open to the idea that maybe there are no planets [around Fomalhaut] at all.”

Watch a simulation of without the help of a planet.

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