Astrophysics

Producing Planetary-Mass Objects

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Science  04 May 2001:
Vol. 292, Issue 5518, pp. 811
DOI: 10.1126/science.292.5518.811d

Models suggest that stars form by the gravitational collapse of a molecular cloud, whereas planets form in circumstellar disks around young stars by accretion. Recent observations have uncovered isolated planetary-mass objects, and neither model is adequate to explain these lonely objects unless some additional assumptions are made. Boss has developed a refined model to create planetary-mass objects through the collapse of a molecular cloud. He includes magnetic pressure effects (which approximate a magnetic field) in a slowly rotating cloud. The cloud starts to collapse due to gravitational forces and heats up at the center. The combination of thermal and magnetic pressures then allows the central region to rebound radially outward, and clumps begin to form where the rebounding material meets the collapsing material. Under these circumstances, clumps as massive as Jupiter can form, and these objects eventually become unstable within the system. These instabilities can lead to the ejection of an object into the emptiness of space. The isolated objects produced do not have enough mass to burn hydrogen like stars or deuterium like brown dwarfs, and they do not form like planets. Thus, the author suggests that they should be called sub-brown dwarfs. — LR

Astrophys. J.55, L167 (2001).

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