Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found compelling evidence that two infant planets — each about the mass of Saturn — are forming around HD 163296, a young star located approximately 400 light-years away in the constellation Sagittarius.
ALMA image of the protoplanetary disk surrounding the young star HD 163296 as seen in dust. New observations suggested that two planets are in orbit around the star. These planets, which are not yet fully formed, revealed themselves by the dual imprint they left in both the dust and the gas portions of the star’s protoplanetary disk. Image credit: ALMA / ESO / NAOJ / NRAO / AUI / NSF / A. Isella / B. Saxton.
“ALMA has shown us amazing images and never-before-seen views of the rings and gaps around young stars that could be the hallmarks of planet formation,” said Dr. Andrea Isella, an assistant professor of physics and astronomy at Rice University.
“However, since we were only looking at the dust in the disks with sufficient detail, we couldn’t be sure what created these features.”
In studying HD 163296, Dr. Isella and co-authors used ALMA to trace, for the first time, the distribution of both the dust and the carbon monoxide gas components of the disk at roughly the same level of detail.
These observations revealed three distinct gaps in the protoplanetary disk around the star.
The first gap is located approximately 60 AU (astronomical units) from the star, which is about twice the distance from our Sun to Neptune.
The other two gaps are 100 AU and 160 AU from the star, well beyond the extent of our Solar System’s Kuiper Belt, the region of icy bodies beyond the orbit of Neptune.
“HD 163296 is one of many known to have a large disk of dust and gas,” Dr. Isella said.
The star is roughly 5 million years old and about twice the mass of the Sun.
“Of the material that formed HD 163296’s disk, about 1% is dust particles and 99% is gas,” he noted.
All the rings around HD 163296 are devoid of dust, and the astronomers are sure that young planets are responsible for clearing the outermost ones.
“But the inner ring has far more carbon monoxide than the other two, leading them to believe no planet exists there. That remains unexplained,” Dr. Isella said.
“Theoreticians have proposed other phenomena that can form dark rings without planets,” he added.
The researchers suspect one in particular: a lack of turbulence among non-ionized gas molecules in a magnetorotational instability ‘dead zone’ that allows gas and dust to condense into a Saturn-like ring at the edge of the dark zone rather than a planet.
The ring may also appear at the carbon monoxide frost line where the gas becomes cold enough to condense.
Details of the research were recently published in the journal Physical Review Letters.
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Andrea Isella et al. 2016. Ringed Structures of the HD 163296 Protoplanetary Disk Revealed by ALMA. Phys. Rev. Lett. 117 (25): 251101; doi: 10.1103/PhysRevLett.117.251101
