Planets form early and easily in baby solar systems

Planets form early and easily in baby solar systems

Read time: 4 minutes

Before cameras and photography existed, you would never know how you looked just after you were born. The only way would be to go to a nursery, take a close look at newborn babies, and assume that you started out more or less the same.

Astronomers face a similar problem when they try to figure out how our solar system formed, 4.6 billion years ago. Back then, there was no one around to take pictures, so there are no photos available. But of course, you can take a look at other baby solar systems, to learn more about the way planets form.

This is exactly what a large international team of astronomers has done. Using ALMA, they have studied twenty nearby baby solar systems up close and personal. And they have learned a lot of new things.

We already knew that planets form in flattened disks around newborn stars. In these ‘proto-planetary disks’, particles of dust or ice clump together, to form larger and larger objects. The end result is a small number of full-fledged planets, orbiting the central star.

ALMA picks up the millimeter waves from warm dust particles. Thus, astronomers can map the distribution of dust in the young proto-planetary disks. They found that the dust isn’t distributed evenly. Instead, each and every disk has bright rings, with more dust than average, and dark gaps, with less dust.

The rings and gaps are probably created by the gravity of large planets that are already forming. But some of the stars studied by ALMA are less than a million years old. No one had expected that massive planets would form so quickly after the birth of a star.

The new ALMA images also explain a mystery about the birth of small, rocky planets. In fact, no one understood how planets like our own Earth could grow from small dust particles. The reason is that a proto-planetary disk also contains a lot of gas. Friction with the gas would slow down the tiny dust particles. As a result, you would expect them to fall into the star before they can clump together into larger pebbles.

But in the rings that ALMA has discovered, the dust particles are more tightly packed. They can drift together in so-called ‘dust traps’, where they are less susceptible to gas drag. Thus, they can easily grow into larger and larger structures.

Some of the protoplanetary disks that were studied by ALMA also exhibit curved arcs or spiral-like structures. No one knows exactly how they are formed. But the twenty new images will certainly help astronomers to better understand the origin of our own solar system.


The proto-planetary disks that ALMA has observed, are all relatively large and nearby – at most a few hundred light-years away. They are all very young; some are no more than a million years old. The dust particles in the disks emit millimeter radiation that can be detected by ALMA. By placing the 66 ALMA dishes as far apart from each other as possible, astronomers were able to see a lot of detail in the disks, like bright rings and dark gaps. The observations were part of a large observing program, called the Disk Substructures at High Angular Resolution Project (DSHARP).


The ALMA project to observe a large number of protoplanetary disks was led by four astronomers: Sean Andrews of the Harvard-Smithsonian Center for Astrophysics (USA), Andrea Isealla of Rice University (USA), Laura Pérez of the University of Chile, and Cornelis Dullemond of Heidelberg University (Germany). Together with their colleagues, Sean, Andrea, Laura and Cornelis have published no less than ten articles about the project results in Astrophysical Journal Letters.