Insight into the mysteries of planet formation

A combination of two exposures detail the complex material system around the star V960 Mon. © ESO/ALMA (ESO/NAOJ/NRAO)/Weber et al.

Are gas giants forming here in a special way? Researchers present astronomical evidence for the previously unconfirmed mechanism of massive planet formation through gravitational instability. Using images from the Very Large Telescope combined with data from the ALMA radio telescopes, the team has discovered large dust formations near a young star that appear to be collapsing and could give birth to planets.

Gas giants like our Jupiter or Saturn orbit many stars in the universe, as exoplanet research has shown in recent decades. As with us, they can also play a literally important role in the distant systems and thereby influence the emergence of habitable conditions on other planets. As for the formation of the giant planets, astronomers assume that they form mostly by the mechanism of so-called "nuclear accretion" in the protoplanetary disk around young stars. The agglomeration of dust grains initially forms a solid core, which then collects more and more gas from the nearby area of ​​the disk as it orbits the star.

So far theoretical alternative process

However, simulations suggest that there is also a second formation mechanism for giant planets: they could form in spiral arms of matter around young stars through “gravitational instability”. According to simulations, concentrations of matter develop there, which contract more and more until they finally collapse completely under their own gravity and form the protoplanets of the gas giants. While astronomers have already found clear evidence for the first of the two scenarios, there has been little astronomical evidence for this formation mechanism through gravitational instability.

But now the international team of astronomers led by Philipp Weber from the University of Santiago de Chile has discovered a possible example in a young star that is about 5000 light-years away from us in the constellation Unicorn. V960 Mon attracted attention for the first time because the star showed strong fluctuations in brightness. Observations with the SPHERE instrument of the European Southern Observatory's (ESO) Very Large Telescope (VLT) then provided evidence that these effects are associated with matter structures around V960 Mon.

Clumping in spiral arms

That's why Weber and his colleagues took a closer look at the star. To do this, they combined VLT images with data from the Atacama Large Millimeter/submillimeter Array (ALMA). In this way, the astronomers were finally able to develop a detailed picture of the system. Material around the young star was found to gather in a series of intertwined spiral arms spanning distances greater than the entire solar system. "The ALMA data revealed that the spiral arms are breaking apart, leading to the formation of clumps with planet-like masses," says co-author Alice Zurlo from the University of Diego Portales in Santiago de Chile.

The scientists say that this is the first solid evidence of the alternative formation process in the giant planets. "The observations fit well with the previous simulations," says co-author Joel Kastner from the Rochester Institute of Technology. "It's confirmation that one of the basic ideas of planet formation is working." Weber comments: "Nobody has ever seen a real observation of gravitational instability on a planetary scale - until now".

The astronomers now want to keep their sights on the exciting system in order to be able to uncover more details. They are pinning their hopes on the increased observation opportunities provided by ESO's Extremely Large Telescope (ELT), which is currently under construction in the Atacama Desert. "It will set the stage for exploring the chemical composition of the material surrounding these clumps, and therefore the substance from which potential planets form," says Weber.

Source: ESO, Rochester Institute of Technology, Article: The Astrophysical Journal Letters, doi: 10.3847/2041-8213/ace186

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