Planets sacrifice themselves to protect us from supermassive black holes

Many galaxies have gigantic black holes located at their center, but we're not usually able to actually see just how gigantic they are. Supermassive black holes are shrouded in huge dust clouds...which might be essential for the development of life.

According to a research team from the University of Leicester, these mysterious clouds might well be formed by the high speed collisions of planets and asteroids, which in turn is created by the massive gravitational pull of the black holes. A similar process exists in our solar system — the zodiacal dust found in the solar system is the ultra-fine result of countless collisions of asteroids and comets.

The researchers call these conglomerations of planets and asteroids around supermassive black holes "Super Oort Clouds", after the giant spherical cloud of rocks and debris found at the very edge of our solar system. Their hypothesis builds on recent research that shows star formation really is possible within even a few light-years of Sagittarius-A*, the supermassive black hole at the center of our own galaxy.

Their model simply assumes that the formation of planets, comets, and asteroids can also happen around these stars. Even after the stars are destroyed by the black hole, their rocky detritus can hang around to pulverize each other, reaching collision speeds of over two million miles per hour. This constant shattering and fragmentation creates the giant dust cloud that keeps at least 50% of most supermassive black holes shrouded from the universe at large.

We also know that such collisions happen in the galaxy at large. The Hubble image up top, for instance, shows a "light echo" from the nearby star V838 Monocerotis, which briefly became 600,000 times brighter than the Sun. That massive increase in brightness is thought to be the result of some gigantic collision, either that of a star and planet or even of two stars smashing together.

Team leader Sergei Nayakshin says any planets formed so close to supermassive black holes would be lifeless from the start, even before their stars are destroyed and they themselves are crushed into countless tiny dust particles. But that sacrifice likely has a huge benefit for life in the rest of the cosmos:

"Too bad for life on these planets, but on the other hand the dust created in this way blocks much of the harmful radiation from reaching the rest of the host galaxy. This in turn may make it easier for life to prosper elsewhere in the rest of the central region of the galaxy."

That means that life on Earth likely directly benefited from the destruction of all these planets around Sagittarius-A*. He adds that understanding the true nature of this dust cloud will help us better understand the black hole itself:

"We suspect that the supermassive black hole in our own Galaxy, the Milky Way, expelled most of the gas that would otherwise turn into more stars and planets. "Understanding the origin of the dust in the inner regions of galaxies would take us one step closer to solving the mystery of the supermassive black holes."

arXiv via the Royal Astronomical Society. Image by NASA/ESA.