Wednesday, 20 September 2017  


 

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Alan P. Boss

Alan Boss


Among his many affiliations, Alan Boss was selected to join the Science Working Group of NASA's Kepler Mission - a space-borne telescope designed to search for habitable planets around nearby stars, scheduled for launch in 2006. Boss is also a member of the NASA External Independent Readiness Board for the Navigator Program, which is charged with overseeing all NASA missions leading to the detection and characterization of habitable Earths. He is a member of the American Academy of Arts and Sciences, and a fellow of the American Geophysical Union, the American Association for the Advancement of Science, and the Meteoritical Society.

 

Equatorial density for a protoplanetary disk

This image shows the equatorial density - after 373 years - for a protoplanetary disk with a mass 10% that of the Sun. The formation of a multiple-Jupiter mass clump can be seen as a tiny white dot at 12 o'clock (not to scale). The region has a radius of 20 AU. The surface density in this relatively low-mass disk is comparable to that required for gas giant planet formation by the core accretion mechanism.

Within the last decade, about 150 extrasolar planets have been detected orbiting nearby stars. Other objects with planet-size masses also have been discovered hovering alone in space. Many of these new finds have defied what we thought we knew about planetary and stellar formation. All this has kept astrophysicist Alan Boss hard at work refining his theories about how these objects came to be.

There are two main models to explain how gas giant planets formed - core accretion and disk instability. The most widely accepted is the core accretion model; but it is a slow process requiring millions of years. In this theory, collisions between small bodies of ice and rock form a massive, solid planetary core. Later, the solid core gains a gaseous atmosphere from the nebular disk, and the planet grows to its final size. Under this scenario, however, the time needed for the core to accrete is longer than the lifetime of the nebular gas from which our solar system formed.

To deal with this problem and others, Boss developed the disk instability model. It is a much faster process, requiring only about a thousand years for a protoplanet to form. Boss has devised several three-dimensional models to study what happens to protoplanetary disks under this scenario. All of the models account for the effects of gravity, radiative transfer, and thermodynamics. He shows that gravitational instabilities in the nebular disk cause the gas and dust to suddenly break up into clumps. Some of the clumps contract into a core and quickly grow into giant gaseous planets. Interestingly, these instabilities can occur in a marginally unstable disk with a mass as low as 10% of the Sun's mass inside radius of 20 astronomical units (1 AU is the distance between Earth and the Sun) - a size similar to the mass of the disks thought to be needed to make planets by core accretion.

To address the debate about what the planetary-mass free-floating objects are, Boss recently considered the effects of magnetic fields in his formation calculations of low-mass stars called brown dwarfs. He found that magnetic-field tension stops molecular cloud from collapsing into a single prestellar object at the cloud's center. This outcome results in smaller-mass objects that can then accrete into several stars. He also found that a close multiple protostar system is unstable to orbital decay, which can cause single objects to be ejected and float freely in space. He suggests that these objects be called sub-brown dwarfs.

SELECTED PUBLICATIONS

  • A. P. Boss 2004, On the fragmentation of magnetized cloud cores, Monthly Notices Royal Astronomical Society, 350, L57-L60.

  • A. P. Boss 2004, Convective Cooling of Protoplanetary Disks and Rapid Giant Planet Formation, Astrophysical Journal, 610, 456-463.

  • A. P. Boss 2004, Evolution of the Solar Nebula. VI. Mixing and Transport of Isotopic Heterogeneity, Astrophysical Journal, 616, 1265-1277.

  • A. P. Boss 2004, Early Solar System: Shock Fronts in Hawaii, Nature, 432, 957-958.

  • A. P. Boss and R. H. Durisen 2005, Chondrule-Forming Shock Fronts in the Solar Nebula: A Possible Unified Scenario for Planet and Chondrite Formation, Astrophys. J. Letters, 621, L137-L140.

  • A. P. Boss 2005, Collapse and Fragmentation of Molecular Cloud Cores. VIII. Magnetically-Supported Infinite Sheets, Astrophysical Journal, 622, 393-403.

  • A. P. Boss 2005, Evolution of the Solar Nebula. VII. Formation and Survival of Protoplanets Formed by Disk Instability, Astrophysical Journal, 629, 535-548.

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