Disc gaps could be ‘cosmic illusions’

While gaps in the discs surrounding young stars are a means of discovering newly formed, unseen planets, a study has found that some gaps could be a sort of cosmic illusion. Rather than the sign of a massive planet, some apparent gaps in the discs might be a result of smaller planetary particles.

This artist's concept illustrates a solar system that is a much younger version of our own. Dusty disks, like the one shown here circling the star, are thought to be the breeding grounds of planets, including rocky ones like Earth. Astronomers using NASA's Spitzer Space Telescope spotted some of the raw ingredients for DNA and protein in one such disk belonging to a star called IRS 46. The ingredients, gaseous precursors to DNA and protein called acetylene and hydrogen cyanide, were detected in the star's inner disk, the region where scientists believe Earth-like planets would be most likely to form.

Artist’s conception of a young planetary system. NASA/JPL-Caltech/T. Pyle (SSC)

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When stars are forming, they are surrounded by a protoplanetary disc of gas and dust.

This disc contains materials for creating new planets and, potentially, eventually a new planetary system.

Often, astronomers can spot gaps in the light from the gas and dust in the disc and use this information to infer the existence of growing planetary bodies.

But a new study suggests this isn’t always the case.

“If we don’t see light scattered from the disc, it doesn’t necessarily mean that nothing is there,” says lead author Til Birnstiel of the Max Planck Institute for Astronomy.

As part of the study, the team looked at stellar discs that shine in visible or near-infrared wavelengths due to scattered or reflected light.

This scattered light is a result of starlight reflecting off tiny particles in the disc.

These particles are initially spread evenly throughout the disc, but eventually begin to change and clump together over time.

Some particles will clump together to form larger objects and eventually grow to become planets. But sometimes these particles break apart when they collide, rather than clumping together.

They can also move farther from or closer to the star in a process called ‘migration’.

By observing these processes in theoretical computer models, the team were able to learn more about how they effect what we observe in the young star’s disc.

“Growth, migration and destruction can have tangible, observable effects,” explains co-author Sean Andrews of the Harvard-Smithsonian Center for Astrophysics (CfA).

“Specifically, these processes can create an apparent gap in the disc when the small particles that scatter light are cleared away, even though larger particles still remain.”

Birnstiel says:

“Throwing a stone in the air doesn’t obstruct my view, but throwing a handful of dust in the air does.

Similarly, as small particles grow bigger in some areas of the disc, they don’t obstruct our view any more and those regions appear empty.”

In order to tell the difference between the different causes of gaps in a disc, the team are now conducting observations of TW Hydrae, a stellar disc known to have a gap, with the Atacama Large Millimeter/submillimeter Array (ALMA).

If the team find large grains within the gap, this will suggest the absence of a planet.

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But if the gap appears empty, the argument will be stronger for the existence of an unseen planet.