Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have uncovered evidence that three infant planets are forming in the protoplanetary disk around HD 163296, a young star located approximately 398 light-years away in the constellation Sagittarius.
An artist’s impression of protoplanets forming around a young star. Image credit: NRAO / AUI / NSF / S. Dagnello.
Protoplanetary disks are the gas- and dust-filled planet factories that encircle young stars.
The rings and gaps in these disks provide intriguing circumstantial evidence for the presence of protoplanets. Other phenomena, however, could also account for these tantalizing features.
But now, using a novel technique that identifies unusual patterns in the flow of gas within planet-forming disks around young stars, two independent teams of astronomers have confirmed distinct, telltale hallmarks of newly formed planets orbiting an infant star.
“Measuring the flow of gas within a protoplanetary disk gives us much more certainty that planets are present around a young star. This technique offers a promising new direction to understand how planetary systems form,” said Dr. Christophe Pinte, an astronomer at Monash University, Australia, and the Institut de Planétologie et d’Astrophysique de Grenoble, France, and leader of one of the two teams.
To make their respective discoveries, each team analyzed ALMA observations of HD 163296, a 4-million-year-old star with a mass of two solar masses.
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.
“We looked at the localized, small-scale motion of gas in the star’s protoplanetary disk,” explained University of Michigan astronomer Dr. Richard Teague, leader of the second team.
“This entirely new approach could uncover some of the youngest planets in our Milky Way Galaxy, all thanks to the high-resolution images from ALMA.”
Rather than focusing on the dust within the disk, which was clearly imaged in earlier ALMA observations, the astronomers instead studied carbon monoxide gas spread throughout the disk.
Molecules of carbon monoxide emit a very distinctive millimeter-wavelength light that ALMA can observe in great detail. Subtle changes in the wavelength of this light due to the Doppler effect reveal the motions of the gas in the disk.
This image shows part of the ALMA data set at one wavelength and reveals a clear ‘kink’ in the material, which indicates unambiguously the presence of one of the HD 163296 planets; the annotation shows the predicted position of the planet in question. Image credit: ESO / ALMA / NAOJ / NRAO / Pinte et al.
Dr. Teague and co-authors identified two Jupiter-mass planets located approximately 83 AU (astronomical units) and 137 AU from the star.
Dr. Pinte’s team identified a planet of two Jupiter masses at about 260 AU from the star.
The researchers used variations on the same technique, which looks for anomalies in the flow of gas — as evidenced by the shifting wavelengths of the carbon monoxide emission — that indicate the gas is interacting with a massive object.
The technique used by Dr. Teague’s team, which derived averaged variations in the flow of the gas as small as a few percent, revealed the impact of multiple planets on the gas motions nearer to the star.
The technique used by Dr. Pinte and colleagues, which more directly measured the flow of the gas, is better suited to studying the outer portion of the disk. It allowed the scientists to more accurately locate the third planet, but is restricted to larger deviations of the flow, greater than about 10%.
In both cases, the authors identified areas where the flow of the gas did not match its surroundings — a bit like eddies around a rock in a river. By carefully analyzing this motion, they could clearly see the influence of planetary bodies.
The astronomers reported their results and analysis in a pair of papers in the Astrophysical Journal Letters.
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Richard Teague et al. 2018. A Kinematical Detection of Two Embedded Jupiter Mass Planets in HD 163296. ApJL, in press; arXiv: 1805.10290
C. Pinte et al. 2018. Kinematic evidence for an embedded protoplanet in a circumstellar disc. ApJL, in press; arXiv: 1805.10293
