Planets form in a protoplanetary disk of dense gas surrounding young stars. Some of these disks have been observed, and they show clear gaps that were believed to suggest the presence of an unseen planet. But a new study suggests that this assumption might not always be correct. 

The most detailed observations of protoplanetary disks have shown dark gaps within the rings, and scientists always assumed that these gaps were left behind by unseen planets trailing through the disk. In the new study, published in the Astrophysical Journal Letters, the researchers argue that sometimes it is possible to form less dusty rings, giving the illusion of a gap with a planet in it. The team modelled how disks would behave using the Smithsonian's Hydra supercomputer cluster.

To form planets, small particles referred to as "pebbles" are thought to clump into larger objects, forming a planet over million of years. A simulation in the study highlighted that the particles can break just as easily as they form, and they can move closer or further from the star in a process called migration. So a gap could be seen if there's a planet, if dust has migrated, or simply if the area is full of pebbles, which reflect less light than dust.

"Growth, migration and destruction can have tangible, observable effects," said co-author Sean Andrews of the Harvard-Smithsonian Center for Astrophysics, in a statement. "Specifically, these processes can create an apparent gap in the disk when the small particles that scatter light are cleared away, even though larger particles still remain.” 

The theory that planets form in a disk from the same substance that makes their star was first proposed in the 18th century, but it only become widely accepted in the 1980s when observations of young stars showed them surrounded by cool disks made of gas and dust. This has been paramount in shedding light on planetary formation. Astronomers use visible and infrared light reflected from the disk to study the disk size and matter distribution.

While not visible in the visible and infrared light, the pebbles should be visible in longer wavelengths. For this reason, the team will conduct microwave and radio observations of TW Hydrae, a dwarf star surrounded by a protoplanetary disk 176 light years from Earth. 

The gaps should not be seen in the new observation if there are pebbles in them. But if the gaps are still visible, an unseen planet would be the most likely cause for them.