How Do Planets Form? New Estimate Of Protoplanetary Disk Dust Particle Size Sheds Light On Scarcely Understood Process

Over the years, astronomers have discovered and confirmed the existence of nearly 3,000 exoplanets, but there is one question that they still haven’t been able to answer satisfactorily — how exactly do planets form? Specifically, how do dust particles just a few micrometers in size aggregate to form gigantic planets thousands of miles in diameter?

In an important step toward furthering our understanding of how planets form, a team of researchers using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile have, for the first time, measured the size of dust particles around a young star.

Their estimate? 150 micrometers — 10 times smaller than previously thought.

The researchers arrived at the estimate by observing the polarization of radio waves emitted by HD 142527 — a five-million-year old star located just 500 light years from Earth. They theorized that depending on the size and structure of the constituent dust particles in the protoplanetary disk, the radio waves emitted by the star should have “unique” polarization signatures.

“In the previous studies, astronomers have estimated the size based on radio emissions assuming hypothetical spherical dust particles,” lead researcher Akimasa Kataoka, an astronomer at the Heidelberg University in Germany, said in a statement. “In our study, we observed the scattered radio waves through polarization, which carries independent information from the thermal dust emission. Such a big difference in the estimated size of dust particles implies that the previous assumption might be wrong.”

This is the first time the size of dust particles has been calculated based on polarization.

“The polarization fraction of radio waves from the dust disk around HD 142527 is only a few percent. Thanks to ALMA's high sensitivity, we have detected such a tiny signal to derive information about the size and shape of the dust particles,” Kataoka said. “This is the very first step in the research on dust evolution with polarimetry, and I believe the future progress will be full of excitement.”