ALMA finds planet-on-the-move in outer part of planet-forming disk 
New discoveries

ALMA finds planet-on-the-move in outer part of planet-forming disk 

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ALMA has discovered a new planet, orbiting a distant star. But not by actually seeing it – the planet is too small to detect from Earth. Instead, ALMA saw the planet’s tracks in the dusty disk that whirls around the star. It’s like finding the Invisible Man through his footprints in the snow. 

Many young stars are surrounded by flat, rotating disks of gas and dust. Eventually, the material in the disk will clump together into planets. That’s why these disks are known as proto-planetary disks. And when a giant planet is already forming, it creates a wide, circular gap in the disk. That’s because the baby planet is sweeping up the dust in its orbit, just like a vacuum cleaner. 

Indeed, the protoplanetary disk around the star HD169142 is showing just such a wide gap. The dust disk isn’t smooth anymore. Instead, there’s an inner disk and an outer disk, with an empty region in between. Astronomers are pretty sure there must be a giant planet orbiting in the wide gap. 

But the new ALMA observations reveal something remarkable. The outer disk turns out to consist of three narrow rings, separated by narrow gaps. Because they are so close together, these narrow gaps cannot be produced by two separate planets. So what causes the three rings? 

Astronomers think they know the answer. With powerful computers, they have simulated the evolution of the young planetary system. They found that the observations can be explained by a single planet, about ten times as massive as the Earth. 

Such a planet would open up one gap in the outer disk. But it would also concentrate dust in its own orbit, thanks to pressure waves in the dust. The result: three narrow dust rings, one along the planet’s orbit, and one on either side. 

The exciting thing is that the middle ring is not exactly in the middle. It is located about one billion kilometers from the inner ring, but about 1.8 billion kilometers from the outer one. This suggests that the planet is slowly moving inward. 

If this explanation is confirmed by future observations, it will shed new light on the birth of planetary systems. Apparently, relatively small planets can form in the outermost parts of protoplanetary disks, even when a large giant planet already exists nearer to the star. 


HD169142 is a star at a distance of 370 light-years in the constellation of Sagittarius (the Archer). It is some 6 million years old (quite young for a star), and it is some 70 percent more massive than the Sun. The star was already known to be surrounded by a protoplanetary disk. The disk shows a wide gap, probably due to the presence of a giant planet. This gap is located some 6.5 billion kilometers from the star, comparable to the distance from the Sun to Pluto. The new ALMA observations have revealed that the outer disk consists of three narrow dust rings. The three rings are a few hundred million kilometers in width, and are separated from each other by one to two billion kilometers. 


The ALMA observations of HD169142 were carried out between September and November 2017, by an international team of astronomers. The teal was led by Sebastián Pérez of the University of Santiago in Chile. Sebastián worked together with his Chilean colleagues Simon Casassus and Lucas Cieza, and with Clément Baruteau (France), Ruobing Dong (Canada) and Antonio Hales (USA). The results of their research were published in The Astronomical Journal in July, 2019.