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ALMA Discovers Trio of Infant Planets around Newborn Star
Two independent teams of astronomers have used ALMA to uncover convincing evidence that three young planets are in orbit around the infant star HD 163296. Using a novel planet-finding technique, the astronomers identified three disturbances in the gas-filled disc around the young star: the strongest evidence yet that newly formed planets are in orbit there.
ALMA Discovers Trio of Infant Planets. Image credit: ESO (CC BY 4.0)
The Atacama Large Millimeter/submillimeter Array (ALMA) has transformed our understanding of protoplanetary discs — the gas- and dust-filled planet factories that encircle young stars. The rings and gaps in these discs provide intriguing circumstantial evidence for the presence of protoplanets. Other phenomena, however, could also account for these tantalising features.
But now, using a novel planet-hunting technique that identifies unusual patterns in the flow of gas within a planet-forming disc around a young star, two teams of astronomers have each confirmed distinct, telltale hallmarks of newly formed planets orbiting an infant star.
To make their respective discoveries, each team analysed ALMA observations of HD 163296, a young star about 330 light-years from Earth in the constellation of Sagittarius (The Archer). This star is about twice the mass of the Sun but is just four million years old — just a thousandth of the age of the Sun.
Rather than focusing on the dust within the disc, which was clearly imaged in earlier ALMA observations, the astronomers instead studied carbon monoxide (CO) gas spread throughout the disc. Molecules of CO emit a very distinctive millimetre-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 disc.
The team from University of Michigan identified two planets located approximately 12 billion and 21 billion kilometres from the star. The other team from Monash University, Australia, identified a planet at about 39 billion kilometres from the star.
The technique used, 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.
Another technique used, which more directly measured the flow of the gas, is better suited to studying the outer portion of the disc. It allowed the authors to more accurately locate the third planet, but is restricted to larger deviations of the flow, greater than about 10%.
In both cases, the researchers 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 analysing this motion, they could clearly see the influence of planetary bodies similar in mass to Jupiter. (ESO)
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