Image courtesy ESA/NASA and H. Bond, STScI
Published November 1, 2011
Almost all of the large galaxies observed so far have central black holes, each billions of times the mass of our sun.
But about half of these cosmic monsters are obscured by dense rings of dust, and astronomers have been uncertain where all this dust comes from and how it's remained intact over time.
According to the new theory, it's possible that newly formed planets and asteroids whirling close to these giant black holes are continually being smashed to smithereens, creating the thick, donut-shaped clouds of debris.
(Related: "Space Smash-Up Turned Planets to Dust.")
"Near a supermassive black hole, velocities are hundreds to a few thousand kilometers per second," said study leader Sergei Nayakshin of the University of Leicester in the U.K.
At such speeds, "hitting an Earth-size planet with a solid object a few kilometers across could fragment [the planet] into lots of smaller fragments that, over time, become nothing more than dust."
When Worlds Collide
In 2006 astronomers discovered a population of stars in two rotating disks around the Milky Way's central black hole.
Nayakshin's team believes these stars may have also formed their own planets and asteroids, similar to the objects in our solar system.
Such planets would have to be orbiting their host stars extremely closely to be able to form under the intense gravitational forces that exist near a black hole.
At the same time, stellar densities are very high near the giant black hole, so it's possible that the gravitational pull of closely passing stars frequently causes planets to unbind from their host stars and smash into each other, according to the new theory.
The dust created by these pulverized worlds might be similar to the zodiacal dust in our solar system—the result of ancient collisions between newborn planets, asteroids, and comets. (See "'False Dawn' This Week: Zodiacal Light Easier to See." [September 2010])
A similar mechanism could be at work across the universe, filling other galaxies with dust near their central black holes.
For the study, Nayakshin and colleagues looked at existing observations of dust clouds around supermassive black holes and compared them with computer models of dust-cloud creation in planetary systems.
"If you know this process to actually work on a smaller scale in a somewhat similar setting, then chances are that it may work in the bigger system you're studying," Nayakshin said.
The team found that the type of microscopic dust generated as planets and asteroids collided would in fact block light even from actively feeding black holes, which spew intense radiation as matter falls toward the black hole, compresses, and heats up.
Dust Rings Offer Galactic Protection?
Of course, the future for any newly formed planet orbiting a galactic black hole may seem bleak. But the violent demise of star systems could have a protective effect for other parts of some galaxies.
If the resulting clouds of dust are massive enough, Nayakshin believes, they can obscure much of the lethal x-rays and gamma rays that are constantly bellowing out from the edges of an actively feeding supermassive black hole.
While much of the area surrounding such a black hole would be sterile, Nayakshin said, "parts of the galaxy that are shielded by the ring of dust—about 50 percent of it—will have a safer and quieter environment for star and planet formation."
Overall, he added, the theory raises some interesting new possibilities for the exotic environments close to supermassive black holes.
People used to think all that existed around such black holes was gas and dust, Nayakshin said.
"But if we are right, then there are also planets, asteroids, and comets that exist there too—so it is a much more diverse environment than people ever thought."
The new black-hole dust theory will appear in an upcoming issue of the journal Monthly Notices of the Royal Astronomical Society.
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