Spiral-arm-like features seen around two newborn stars, SAO 206462 and MWC 758, may be evidence for the presence of giant, unseen planets, says a team of astronomers led by Ruobing Dong of Lawrence Berkeley National Laboratory.
Right: observations taken by the ESO’s Very Large Telescope show a protoplanetary disk around the young star MWC 758; the disk has two spiral arms that extend over 10 billion miles from the star. Left: a computer model reproduces the two-spiral-arm structure; the X is the location of a putative planet. Image credit: NASA / ESA / ESO / M. Benisty et al / Ruobing Dong / Zhaohuan Zhu.
In 2013, astronomers spotted large-scale spiral arms in protoplanetary disks around the young stars SAO 206462 and MWC 758, located some 457 and 670 light-years from Earth, respectively. A few other nearby stars also show smaller spiral-like features.
“How they are created has been a big mystery until now. Scientists had a hard time explaining these features,” Dong said.
“If the disks were very massive, they would have enough self-gravity to become unstable and set up wave-like patterns.”
But the disks around the stars are probably just a few percent of the central star’s mass and therefore are not gravitationally unstable.
Dong and his colleagues from Princeton University and the University of California at Berkeley generated computer simulations of the dynamics of a disk and how a star’s radiation propagates through a disk with embedded planets.
This modeling created spiral structures that very closely resemble spiral arms around SAO 206462 and MWC 758.
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.
The differential rotation of the disk around the star smears these over-dense regions into spiral waves.
“Although it had been speculated that planets can produce spiral arms, we now think we know how,” said team member Zhaohuan Zhu, of Princeton University.
“Simulations also suggest that these spiral arms have rich information about the unseen planet, revealing not only its position but also its mass.”
The simulations show that if there were no planet present, the disk would look smooth.
“To make the grand-scale spiral arms seen in the SAO 206462 and MWC 758 systems, the unseen planet would have to be bulky, at least ten times the mass of Jupiter,” the astronomers said.
“It’s difficult to see suspected planets inside a bright disk surrounding a young star. Based on this study, we are convinced that planets can gravitationally excite structures in the disk,” Dong said.
“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.”
The results were published in the August 10 issue of the Astrophysical Journal Letters (arXiv.org preprint).
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Ruobing Dong et al. 2015. Observational Signatures of Planets in Protoplanetary Disks II: Spiral Arms Observed in Scattered Light Imaging Can be Induced by Planets. ApJ 809, L5; doi: 10.1088/2041-8205/809/1/L5
