Why planet formation by gravitational collapse depends on things that happen at small scales

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Research perspective written by Matthew D. Young
Institute of Astronomy
University of Cambridge, Cambridge, UK

How can I explain the importance of this research to the general public?

Some scientists argue that it is possible for giant planets like Jupiter and Saturn to form by gravity rapidly compressing the densest parts of discs, which are known to orbit young stars. This would only occur in discs where the gas had the right properties, specifically, the gas needs to be able to cool quickly. Scientists expected that planets would form in this way when a large patch of gas had the right average properties, but detailed computer simulations showed that the properties of the gas on small scales was important.

This work provides an explanation for why planet formation by rapid compression of gas depends on the properties of gas on smaller than expected scales. It also proposes a new mathematical model for predicting when gas has the right properties to form giant planets by gravitational collapse.

In so doing, this research has advanced our understanding of what physical processes play a role in forming giant planets. This improved understanding places further constraints on the properties that discs of gas need to have if they are to produce planets by rapid, gravity driven collapse. These constraints will allow us to better understand how common (or rare) our solar system is and to predict where and how often we will find gas giants around other stars.

Why is this important for researchers in fields other than astronomy?

This study provides further evidence that planet formation by direct gravitational collapse can only take place in the outer extremes of proto-planetary discs. Models of solar system and planetary system formation should take this constraint into account. Furthermore, the time scale criteria for fragmentation outlined in this study may be of interest to studies of fragmentation in galactic discs or in accretion discs around black holes.

Why is this important for researchers in the same field?

This paper explains why scales smaller than the disc scale height (H) must be taken into account in models of disc fragmentation. Using a series of 2D smoothed particle hydrodynamics and grid simulations, we demonstrate that discs only fragment when the cooling time exceeds ~3 times the dynamical time if scales smaller than H are allowed to play a role. To explain this, we propose that fragmentation needs to be understood as consisting of two modes; pressure supported collapse and dynamical collapse. Using a time scale argument based on the time between collisions with spiral arms, we provide constraints on these two modes of fragmentation.

Original article

Dependence of fragmentation in self-gravitating accretion discs on small-scale structure
M. D. Young, C. J. Clarke
Monthly Notices of the Royal Astronomical Society, published online 30 June 2015
Manuscript available on arXiv

Acknowledgements

Matthew D. Young gratefully acknowledges the support of a Poynton Cambridge Australia Scholarship. The simulations in this work were performed using HPC resources allocated through DiRAC project DP022. This work has been supported by the DISCSIM project, grant agreement 341137 funded by the European Research Council under ERC-2013-ADG. The original text was published by Oxford University Press on behalf of the Royal Astronomical Society.