A young star observed by the Spitzer Space Telescope appears
to be home to a wild – and young – planetary system that shares some of the
frenetic dynamics thought to have shaped the early years of our own solar
system.
The Spitzer observations suggest young planets circling the
star are disturbing smaller comet-like bodies, causing them to collide and kick
up a huge halo of
dust.
The star, called HR 8799, became one of the first of two
stars with planets that were directly imaged from Earth in November 2008. Ground-based
telescopes at the W.M. Keck Observatory and the Gemini Observatory, both in
Hawaii, took images of three planets orbiting in the far reaches of the system.
Each of the three distant worlds is roughly 10 times the mass of Jupiter.
HR 8799 is younger and more massive than our sun, which is
more than 4.5 billion years old and more than 300,000 times the mass of Earth.
It is about 129 light-years from Earth, so scientists weren't sure if Spitzer
would be able to snap a picture of its debris disk. But to their amazement, it
succeeded.
The Spitzer team, led by Kate Su of the University of Arizona, Tucson, says the giant cloud of fine dust around the disk is very
unusual. The researchers say this dust must be coming from collisions among
small bodies similar to the comets or icy bodies that make up today's Kuiper
Belt objects in our solar system.
"The system is very chaotic and collisions are spraying
up a huge cloud of fine dust," Su said.
The gravity of the three large planets is throwing the
smaller bodies off course, causing them to migrate around and collide with each
other. Astronomers think the planets may have yet to reach their final stable
orbits, so more violence could be in store.
A similar setup has also been seen by Spitzer and the Hubble
Space Telescope around the star Fomalhaut, which sits about 25 light-years from
Earth.
"What's exciting is that we have a direct link between
a planetary disk and imaged planets," Su said. "We've been studying
disks for a long time, but this
star and Fomalhaut are the only two examples of systems where we can study
the relationships between the locations of planets and the disks."
When our solar system was young, astronomers think it went
through similar planet migrations. Jupiter and Saturn moved around quite a bit,
throwing comets around, sometimes into Earth.
Some say the most extreme part of this phase, called the
late heavy bombardment, explains how our planet got water. Wet, snowball-like
comets are thought to have crashed into Earth, delivering life's favorite
liquid.
The Spitzer results are detailed in the Nov. 1 issue of
Astrophysical Journal.
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