Eat my dust (Image: NASA/JPL-Caltech)

PLANETS and asteroids may be smashing into each other by the thousand around monster black holes. Dust from the collisions could explain why many of these colossal objects, which would otherwise shine brightly as they swallow nearby matter, are hidden from view.

Most galaxies like the Milky Way harbour a central black hole with a mass upwards of a million suns. While the Milky Way’s black hole is currently inactive, many in other galaxies are swallowing up gas from dense discs that surround them, producing copious X-rays and other radiation in the process.

In addition to providing fodder for the black holes, these discs may give birth to millions of stars, explaining the dense star clusters seen in some galaxies’ hearts.


These stars may in turn host a lot of planets. But they might have trouble holding onto them, says a team led by Sergei Nayakshin at the University of Leicester, UK.

That is because the stars are crowded together so tightly that frequent gravitational encounters between them would strip planets away, sending them careening through space. En route, they would often crash into each other at speeds much higher than anything that happens in our solar system. Here, collisions typically occur at less than a few tens of kilometres per second. But around a supermassive black hole, objects zip around so fast that crashes would happen at up to 1000 kilometres per second, pulverising the colliding objects.

Objects would smash together at up to 1000 kilometres per second, pulverising them

That would be bad news for any life on the rocky bodies, but Nayakshin thinks the intense X-rays and ultraviolet radiation so close to a black hole would prevent life from forming on these planets in the first place. “It would be unimaginable that you could have life in these sorts of environments,” he says.

However, dust from the crashes could explain why many actively feeding black holes are hidden behind a fat doughnut-shaped dust shroud, the team says in an upcoming issue of Monthly Notices of the Royal Astronomical Society.

It is not clear how much dust each doughnut might contain, but assuming it is about 300 suns’ worth, that could be produced by grinding up planets from around 100,000 to a million stars, the team estimates. The idea is reasonable, says Richard Mushotzky at the University of Maryland in College Park, adding that the doughnuts have “so far been very difficult to explain”.

The dust may also help new stars to form, since it absorbs radiation from the feeding black hole that might otherwise push away star-forming gas.