Planets grow from very small but ‘fluffy’ dust particles 
New discoveries

Planets grow from very small but ‘fluffy’ dust particles 

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Just like a large tree grows from a tiny seed, planets grow from tiny dust particles. First, the dust particles stick together to larger and larger ‘dust bunnies’. Later on, they grow into pebbles and rocks. Those larger objects feel each other’s gravity and eventually clutter together into planets. 

For the first time ever, astronomers have now measured the sizes of dust particles that are the ‘seeds’ of future planets. The particles orbit a newborn star, at 500 light-years. That’s of course much too far to measure the sizes of the dust particles directly. Instead, the astronomers used ALMA, and a trick of light known as polarization. 

Waves of visible light (or millimeter and submillimeter waves) normally vibrate in every possible direction. But when the waves are reflected and bounced around by dust particles, they became polarized. As a result, they vibrate stronger in one direction than in other directions. 

The strength and the direction of this effect tells you something about the particles that are responsible for the polarization. From the ALMA observations, it follows that the dust 

particles around the newborn star are tiny: less than one-seventh of a millimeter across. It would be hard to see them without a spy-glass or a microscope. 

The result came as a surprise. Most astronomers thought that the dust particles would be much larger, at least as large as a millimeter across. But those earlier estimates assumed that the dust particles are perfectly round, like little spheres. Instead, the ALMA astronomers now believe that they are ‘fluffy’, and much more irregular in shape. 


HD142527 is a newborn star at a distance of 500 light-years in the constellation Lupus, the Wolf. It was born just five million years ago. The star is still surrounded by a disk of gas and dust. In the future, the dust particles in this disk will grow together, all the way into planets like Earth. HD142527 is about twice as heavy as our own Sun. 


The ALMA observations of HD142527 were carried out by Akimasa Kataoka. Akimasa is a Japanese astronomer who now works at Heidelberg University in Germany. He worked together with a large group of colleagues from Japan and Germany. The team published their results in a professional magazine, called The Astrophysical Journal.