Detecting Planets via Circumstellar Spirals

Two NASA Hubble Fellows are proposing that spiral arm modifications in the circumstellar disks surrounding newborn stars may provide evidence of giant planets, offering a new method of planet detection.

Ruobing Dong, of the Lawrence Berkeley National Laboratory, and Zhaohuan Zhu, of Princeton Univ., came to the conclusion by computer modeling how gas-and-dust disks evolve around newborn stars.

According to NASA, gaps and rings present in circumstellar disks suggest the presence of planets. However, the gaps, which are believed to be swept clean by a planet’s gravity, don’t provide the location of the planet. Further, multiple planets may occupy a single gap, making it difficult to estimate the number of planetary bodies and their respective masses.

However, spiral disks formed in a planet’s wake may not just help astronomers detect planets, they may also provide information regarding the planet’s size.



“It’s difficult to see suspected planets inside a bright disk surrounding a young star,” said Dong. “Based on this study, we are convinced that planets can gravitationally excite structures in the disk. So if you can identify features in a disk and convince yourself those features are created by an underlying planet that you cannot see, this would be a smoking gun of forming planets.”  

Already, ground-based telescopes photographed two large-scale spiral arms in the young stars SAO 206462 and MWC 758. Until now, scientists had a hard time explaining the features, according to Dong. With the disks surrounding the two stars being just a few percent of the stars’ mass, they’re not gravitationally unstable, according to NASA.

When the team generated computer simulations with embedded planets in the circumstellar disks, they found the models created spiral structures which resemble previous observations made by ground-based telescopes.

“The mutual gravitational interaction between the disk and the planet creates regions where the density of gas and dust increases, like traffic backing up on a crowded expressway,” according to NASA. “The differential rotation of the disk around the star smears these over-dense regions into spiral waves.”

In order to produce such spiral arms as seen in SAO 206462 and MWC 758, the planets would have to be around 10 times the mass of Jupiter, the scientists determined.

This new method of planet detection may help scientists gain insights into planet formation.

“There are many theories about how planets form but very little work based on direct observational evidence confirming these theories,” said Dong. “If you see signs of a planet in a disk right now, it tells you when, where and how planets form.”