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# Dust-catching vortices around stars may explain how planets form, say astronomers

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How do planets form? A new study is shedding light on the mystery.

Photo credit: NASA

Science Recorder | Delila James | Thursday, June 06, 2013

Astronomers have discovered a cashew-shaped object in the outer regions of a young solar system that could act as a “dust trap,” allowing minute dust particles to cling together and eventually coalesce into a planet-sized body. If the observations hold up to further scrutiny, the find could solve a long-standing mystery of planet formation.

Astronomers have long puzzled over the question of planet formation. Computer models of various accretion scenarios always showed that friction between gas and dust particles, thought to be the seeds for early planet formation, inevitably led to larger grains of a certain size moving inward and falling back onto their parent star, leaving no chance for larger objects to form. To explain how planets form if dust particles self-destruct once they reach a particular size, astronomers theorized that vortexes in the disk could act as dust traps, allowing particles to coalesce and eventually grow into larger and larger objects. However, there was no observational proof that such stellar dust collectors actually existed.

Now, an international team of scientists, using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, have created ground-breaking images of the outer regions of a young solar system, called Oph IRS 48, located in the constellation Ophiuchus some 390 light-years from Earth. The images reveal what may be the up-to-now hypothetical dust trap in a crescent-shaped object made up of a concentration of millimeter-scale dust. This concentration is in sharp contrast to the gas and micron-sized grains of dust distributed evenly throughout the disk, according to researchers.

“At first the shape of the dust in the image came as a complete surprise to us,” said lead author Nenke Van der Marel, Ph.D. student at Leiden Observatory in the Netherlands, in a statement released June 6 by the National Radio Astronomy Observatory (NRAO). “Instead of the ring we had expected to see, we found a very clear cashew-nut shape! We had to convince ourselves that this feature was real, but the strong signal and sharpness of the ALMA observations left no doubt about the structure.”

Although the ALMA observations only showed the outer structure of the disk, which is more than 50 times distant from its star than the Earth is to the Sun, researchers say the principle would apply at a closer distance to the star where rocky planets form. According to co-author Til Birnstiel, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass, the structure observed with ALMA could be scaled down to represent what may be occurring in the inner region of the solar system. In case of the ALMA observations, he said that astronomers may be seeing something analogous to the formation of the Kuiper Belt or Oort Cloud, the area of our solar system where comets are believed to form.

The ALMA telescope observations were achieved with less than its full array of 66 antennas, which will be functioning later this year. When fully operational, ALMA will have one of the sharpest visions of any astronomical observatory and will be an important tool in understanding the entire planet formation process, according to the NRAO, a facility of the National Science Foundation.

The team’s findings will be published Friday (June 7) in the journal Science.

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