Evidence That Black Holes Eat Planets And Asteroids Too

At the centre of our galaxy sits a black hole about a million times more massive than our Sun. Called Sagittarius A* or Sgr A*, this supermassive black hole is remarkably quiet compared to the ones we can see in other galaxies.

That passivity is almost certainly temporary. The centre of the Milky Way is crowded with stars; so crowded, in fact, that astronomers think that one must plunge into this black hole every 100,000 years or so.

When that happens, Sgr A* belches x-rays and other frequencies in a dramatic display of galactic fireworks. Indeed, last month, we looked at evidence that the last flare up occurred only a few hundred years ago.

In the meantime, however, Sgr A* just rumbles away. About once a day it emits tiny flares of x-rays and infrared light that are many orders of magnitude smaller than those caused by the death of a star.

But although small, the flare are still up to 100 times brighter than Sgr A*’s background emissions. That raises an obvious question: what’s causing this daily grumbling?

Today, Kastytis Zubovas, at the University of Leicester in the UK, and a couple of pals say they know. These guys think the small flares are the death throes of planets and asteroids as they fall into Sgr A*.

Astronomers have long thought that a black hole ought to be surrounded by a torus of dust and gas. This stuff is probably a mixture of primordial grime and the remnants of long-eaten stars.

In many ways, these clouds are similar to those that form around emerging stars. Astronomers know these condense into planets and asteroids. So it’s not much a stretch to imagine that the same processes can occur around black holes too.

Consequently, many astrophysicists now think black holes must have their own systems of planets, asteroids and Oort-type clouds, just like stars.

Whenever these clouds are disrupted, perhaps by a star as it plunges to its death, asteroids, comets and even entire planets can be hurled inwards, towards the black hole and the deaths of these objects should generate flares, albeit small ones.

Zubovas and co have worked out the energetics of these processes. They say the daily flares that erupt from Sgr A* are exactly the size you’d expect from the death throes of asteroids upwards of 10 kilometres across.

Planets could also come to grief in a similar process, although Zubovas and co say these kinds of events are probably as rare as the death of stars. But they happen, we will be able to witness the death of an entire planet and everything on it.

If this kind of thinking is correct, the rumblings at the centre of our galaxy will give astronomers a new way to examine conditions there. The size and frequency of the flares will indicate the mass and distribution of asteroids around our supermassive black hole, perhaps revealing some of the true complexity and richness of the most exotic region of our galaxy.

Ref: arxiv.org/abs/1110.6872: Sgr A∗ Flares: Tidal Disruption Of Asteroids And Planets?