Clean gaps hint at new planets

If you don’t clean your room, dust will settle everywhere. The same is true for space. Young stars are surrounded by flat disks of gas and dust. Without any cleaning going on, the dust will spread all over the disk.

So, what about the empty regions in this ALMA image of a young star? ALMA detected the millimeter waves of small dust particles. The bright rings in the image are regions with lots of dust. The dark areas contain much less dust. It is as if someone has been going around with a large cosmic vacuum cleaner.

Of course, there are no vacuum cleaners in space. But newborn planets may have acted like cleaning ladies. Planets can clump together from smaller particles of gas and dust. When a planet has grown massive enough, its gravity will sweep its own orbit clean.

Astronomers believe there are at least two giant planets orbiting the star. One has created the empty hole in the center of the disk. The other has created the empty gap further out.

From the amount of dust and the sizes of the gaps, it follows that the planets must be about as large and massive as the giant planet Jupiter in our own solar system.

ALMA has also studied other stars that are surrounded by disks of gas and dust. In many cases, similar rings and gaps were found. So, it looks like planets are forming around most young stars.



The star studied by ALMA is known as HD169142. It is in the constellation Sagittarius the Archer, at 470 light-years from Earth. The star is 70 percent more massive than our own Sun, and shines much more brightly. It’s also much younger than the Sun: just some 6 million years. The two dust rings are far away from the star. The inner one is about as far as the planet Neptune is from the sun. The outer one is much farther out than the dwarf planet Pluto is from the Sun.


The ALMA image of the dust surrounding the star HD169142 was obtained by a team of nine astronomers from Italy, the Netherlands, the United Kingdom, and Germany. The team’s leader was Davide Fedele, an astronomer at the National Institute of Astrophysics in Florence, Italy. The team has published their discovery in a professional magazine called Astrophysical Journal Letters.