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| Month
Day,2003 |
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Growing
Planets in a Bad Neighborhood
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David Tytell
Sky
& Telescope
It
seems that the three most important factors in forming planetary systems in nebulae
just may be: location, location, location. According to a study conducted by Henry
Throop (Southwest Research Institute) and his colleagues, the environment surrounding
a protoplanetary disk dramatically effects the type of system that will form.
Using
the Hubble Space Telescope, Throop looked at various million-year-old disks within
the Orion Nebula. In the disks, he found dust grains as large as 5 microns in
size, about the one-tenth the width of a human hair. For comparison, normal interstellar
dust grains are only 0.1 to 0.2 micron. The apparent grain growth implies that
they are in the early stages of planetary growth.
However, the Orion Nebula
is also home to more than two dozen O-type stars. These stellar giants cause an
extreme ruckus in any neighboring disk within 0.3 light-year or so. The star's
tremendous energy outflow blows away the gas in the system, and the giant star's
intense ultraviolet radiation bakes away any ice. According to models calculated
by Throop and others, the resulting system is quite strange: there is no gas to
form Jupiterlike gas giant planets, nor is there ice to form a Kuiper Belt or
any comets. Instead, all that remains is a group of atmosphereless rocky bodies,
similar in many respects to Mercury.
But, if the disk is fortunate to reside
in a "shady spot," says Throop, then gas giants and normal planetary formation
can proceed. Because O-type stars have such short life spans (up to 100 million
years), disks could easily form after the stars had wreaked their havoc. Details
of the study appear in this week's Science.
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