A new method of planet formation identified

Scientists have suggested a new explanation for the abundance in intermediate-mass exoplanets – a long-standing puzzle in astronomy.

Most of the exoplanets fall in the intermediate-mass category. From super-Earth to Neptune-sized bodies, such types of planets are the most common types of planets in the Galaxy. Even in our solar system, the formation of Uranus and Neptune remains a mystery.

Scientists from the Universities of Cambridge and Zurich, associated with the Swiss NCCR PlanetS, have proposed an alternative explanation.

According to scientists, during planet formation from the protoplanetary disk of gas and dust, gravitational instabilities could be the driving mechanism.

During this process, dust and gas in the disk clump together because of gravity. It then transforms into dense spiral structures and develops into planetary building blocks and, eventually, planets.

Professor Lucio Mayer from the University of Zurich said, “The scale on which this process occurs is very large – spanning the scale of the protoplanetary disk. But over shorter distances – the scale of single planets – another force dominates: That of magnetic fields developing alongside the planets.”

“These magnetic fields stir up the gas and dust of the disk and influence the formation of the planets.”

Lead author Dr. Hongping Deng from Cambridge’s Department of Applied Mathematics and Theoretical Physics said, “To get a complete picture of the planetary formation process, it is important to not only simulate the large-scale spiral structure in the disk: the small-scale magnetic fields around the growing planetary building blocks also have to be included.”

In any case, the distinctions in scale and nature of gravity and magnetism make the two forces challenging to coordinate into the same planetary formation model. Up until this point, computer simulations that capture one of the troops’ impacts well typically do poorly with the other.

By developing a new modeling technique, scientists overcame the issue. At first, a deep theoretical understanding of both gravity and magnetism was essential. Later, they had to find a way to translate the understanding into a code that could efficiently compute these contrasting forces in unison. Finally, due to the immense number of necessary calculations, a powerful computer was required – like the Piz Daint at the Swiss National Supercomputing Centre (CSCS).

Mayer said, “Apart from the theoretical insights and the technical tools that we developed, we were therefore also dependent on the advancement of computing power.”

Deng said, “With our model, we were able to show for the first time that the magnetic fields make it difficult for the growing planets to continue accumulating mass beyond a certain point. As a result, giant planets become rarer and intermediate-mass planets much more frequent – similar to what we observe in reality.”

Co-author Ravit Helled from the University of Zurich said“These results are only a first step, but they clearly show the importance of accounting for more physical processes in planet formation simulations. Our study helps to understand potential pathways to forming intermediate-mass planets that are very common in our galaxy. It also helps us understand the protoplanetary disks in general.”

Journal Reference:
  1. Hongping Deng, Lucio Mayer, and Ravit Helled. ‘Formation of intermediate-mass planets via magnetically controlled disk fragmentation.’ Nature Astronomy (2021). DOI: 10.1038/s41550-020-01297-6

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