A European team of astronomers has discovered that dust particles around a star already coagulate before the star is fully grown. Dust particle growth is the first step in the formation of planets. The researchers from the Netherlands, Sweden and Denmark publish their findings in Nature Astronomy.

Published by the editorial team, 25 June 2018

In recent years, astronomers have discovered numerous planetary systems around other stars. Almost every star is likely to have at least one planet orbiting it. Some of the major questions are centered around how planetary systems form and how this process leads to the observed diversity of planets in numbers and masses. The results of a European research project suggest that planet formation starts very early in the star formation process.

The researchers used the Atacama Large Millimeter Array for their discovery. ALMA is a collection of 66 linked radio telescopes spread over 16 kilometer in the Atacama desert in Chile. The researchers pointed the telescope toward TMC1A, a still developing star in the constellation Taurus (the Bull).

The astronomers saw a striking lack of carbon monoxide radiation in a disc-shaped area near the star. They suspected that the radiation was blocked by big dust particles. Using numerical models, they could demonstrate that indeed the dust particles in the young protoplanetary disk have probably grown from a thousandth of a millimeter to a millimeter.

Lead researcher Daniel Harsono (Leiden University, the Netherlands) explains why this is so surprising: "The results indicate that planets already start forming while the star is still developing. The star is only half to three-quarters of its final mass. This is new."

Per Bjerkeli (Chalmers University, Sweden) highlights the implication of early grain growth: "It can be an explanation for the formation of giant planets that are comparable to Jupiter and Saturn. Only early protoplanetary discs contain sufficient mass to form giant planets."

Co-researcher Matthijs van der Wiel (ASTRON, Netherlands Institute for Radio Astronomy) is pleased with the clear and unambiguous observations. "This early particle growth could be an exception, of course. Maybe this young disk is very special."

In the future, the researchers want to look for tell-tale signs of planet formation around other protostars in similar manner. "Currently, ALMA is the only observatory capable of resolving dust and gas emission at scales where new planets are forming, matching the scales in our Solar system. In the future, similarly high resolution observations will be attained with the dishes of the Square Kilometre Array (SKA) to be built in South Africa. Compared with ALMA’s millimeter wave detectors, the SKA will be sensitive to wavelengths of 2 cm and above, and will therefore help to localize centimeter-sized grains, the next step up in the journey from tiny dust particles to planets," says Van der Wiel.

(c) Daria Dall'Olio [CC-BY-SA 3.0]. Artistic impression of a star with a protoplanetary disk and growing grains.

(c) Jørgensen/Harsono/ESASky/ESAC [CC-BY-SA 3.0]. TMC1A is a still developing star in the constellation Taurus. Red are areas with many dust particles. Green and blue are two types of carbon monoxide. The absence of green / blue carbon monoxide in the inner part indicates that dust particles in the young protoplanetary disk have grown from less than a thousandth of a millimeter to a millimeter.


"Evidence for the start of planet formation in a young circumstellar disk." By: Daniel Harsono (1), Per Bjerkeli (2), Matthijs H.D. van der Wiel (4), Jon P. Ramsey (3), Luke T. Maud (1), Lars E. Kristensen (3) & Jes K. Jørgensen (3). 1. Leiden University, the Netherlands. 2. Chalmers University of Technology, Sweden. 3. University of Copenhagen, Denmark. 4. ASTRON, Dwingeloo, the Netherlands. In: Nature Astronomy, 25 June 2018.



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