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. These are considered the first planets to be discovered with ALMA.

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 [1]. 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 [2].

"Measuring the flow of gas within a protoplanetary disc gives us much more certainty that planets are present around a young star," said Christophe Pinte of Monash University in Australia and Institut de Planetologie et d'Astrophysique de Grenoble (Universite de Grenoble-Alpes/CNRS) in France, and lead author on one of the two papers. "This technique offers a promising new direction to understand how planetary systems form."

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) [3]. 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.

"We looked at the localised, small-scale motion of gas in the star's protoplanetary disc. This entirely new approach could uncover some of the youngest planets in our galaxy, all thanks to the high-resolution images from ALMA," said Richard Teague, an astronomer at the University of Michigan and principal author on the other paper.

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 led by Teague identified two planets located approximately 12 billion and 21 billion kilometres from the star. The other team, led by Pinte, identified a planet at about 39 billion kilometres from the star [4].

The two teams used variations on the same technique, which looks for anomalies in the flow of gas - as evidenced by the shifting wavelengths of the CO emission - that indicate the gas is interacting with a massive object [5].

The technique used by Teague, 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 Pinte, 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.

This new technique allows astronomers to more precisely estimate protoplanetary masses and is less likely to produce false positives. "We are now bringing ALMA front and centre into the realm of planet detection," said coauthor Ted Bergin of the University of Michigan.

Both teams will continue refining this method and will apply it to other discs, where they hope to better understand how atmospheres are formed and which elements and molecules are delivered to a planet at its birth.

Notes
[1] Although thousands of exoplanets have been discovered in the last two decades, detecting protoplanets remains at the cutting edge of science and there have been no unambiguous detections before now. The techniques currently used for finding exoplanets in fully formed planetary systems - such as measuring the wobble of a star or the dimming of starlight due to a transiting planet - do not lend themselves to detecting protoplanets.

[2] The motion of gas around a star in the absence of planets has a very simple, predictable pattern (Keplerian rotation) that is nearly impossible to alter both coherently and locally, so that only the presence of a relatively massive object can create such disturbances.

[3] ALMA's stunning images of HD 163296 and other similar systems have revealed intriguing patterns of concentric rings and gaps within protoplanetary discs. These gaps may be evidence that protoplanets are ploughing the dust and gas away from their orbits, incorporating some of it into their own atmospheres. A previous study of this particular star's disc shows that the gaps in the dust and gas overlap, suggesting that at least two planets have formed there.

These initial observations, however, merely provided circumstantial evidence and could not be used to accurately estimate the masses of the planets.

[4] These correspond to 80, 140 and 260 times the distance from the Earth to the Sun.

[5] This technique is similar to the one that led to the discovery of the planet Neptune in the nineteenth century. In that case anomalies in the motion of the planet Uranus were traced to the gravitational effect of an unknown body, which was subsequently discovered visually in 1846 and found to be the eighth planet in the Solar System. This research was presented in two papers to appear in the same edition of the Astrophysical Journal Letters. The first is entitled "Kinematic evidence for an embedded protoplanet in a circumstellar disc", by C. Pinte et al. and the second "A Kinematic Detection of Two Unseen Jupiter Mass Embedded Protoplanets", by R. Teague et al.

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