Milky Way’s Black Hole Is Throwing Cosmic Spitballs

Every now and then, the Milky Way’s central, supermassive black hole tears apart a star and flings away some of its innards. Now astronomers think they know how to spot these cosmic spitballs.

Black hole tears star apart

This computer-simulated image shows gas from a tidally shredded star (orange) that passed too close to the black hole (black dot in upper left corner). About half the gas gets ejected away from the black hole (stream stretching right). The other half falls inward, but not all of it makes it into the black hole. Some of this stream collapses into planet-sized objects that later get slingshotted away.
STScI / NASA Goddard / S. Gezari / J. Guillochon

The supermassive black hole lurking in the center of our galaxy is a docile beast. But it’s always the quiet ones: astronomers only recently realized that this black hole might be throwing cosmic spitballs our way in the form of gas balls the size of Jupiter. And a new simulation presented on January 6th at the winter meeting of American Astronomical Society shows how we might go about catching our neighborhood trickster in the act.

Every ten thousand years or so, an unlucky star passes too close to our galaxy’s central beast. Tidal gravitational forces rip the star apart, spaghettifying it in a messy process that flings half of the mass away into galactic space, while inexorably pulling the remainder into the black hole’s maw.

But that’s not the end of the story for the captured stellar material. During the star’s spaghettification, the stream of gas swirls as though down a drain and, as it’s still settling into its final orbits, some of the gas can get far enough away from the black hole that it collapses into planet-size fragments.

In every so-called tidal disruption event, perhaps 100 or so Neptune- to Jupiter-size fragments form — over the course of the Milky Way’s history, that could add up to 100 million objects. And once they form, they’re no longer consigned to their fate. Instead, the black hole’s gravity slingshots them away at incredible speeds of 1,000 to 10,000 kilometers per second (2 million to 20 million mph).

Tracking Spitballs

But if this particular tree were to fall in a forest, would we hear it? These cosmic spitballs are so small and faint, what’s the chance we could actually spot them — and prove the process was even happening? That was the question Eden Girma, an undergraduate at Harvard University and a member of the Banneker/Aztlan Institute, set out to answer.

Girma simulated the path these fragments would take as they slingshotted away from the black hole. She did this by first simulating the disruption of 50 stars. Then, for each of these tidal disruption events, she summed the forces on the fragments that formed, tracking their trajectories over 10 billion years, and watching to see whether the fragments remained bound to the galaxy or escaped it altogether. The video below shows Girma's simulation (the yellow dot shows the Sun's position in the galaxy relative to the black hole's position at the center of the simulation).

Only 5% would remain bound to the black hole, and they’d probably stay within several hundred light-years never coming close enough to Earth to be detectable. But the vast majority, Girma found, escape not only the black hole but the galaxy altogether. 95% of the fragments would hurtle out of the Milky Way in disparate directions at 10,000 km/s, eventually ending up more than 10 million light-years away.

Not Your Regular Rogue Planet

Rogue planet

Billions of rogue planets traverse the Milky Way — they might even outnumber the stars. The cosmic spitballs Eden Girma studied would look like these rogues, but they would be traveling at far higher speeds, most of them out of the galaxy altogether.
Caltech / NASA

Based on their escape velocities and trajectories, Girma estimates that the closest of these objects could come as close as 700 light-years away. As these stellar fragments pass through the galaxy, they would look just like rogue planets but for their incredible speed.

But even though they may be planet-mass, they’re not at all planet-like. Unlike rogue planets, these fragments don’t form over many millions of years, they form in just a year or two. And they’re literally made of star stuff — the hydrogen and helium that made up the original star.

So could we ever spot these elusive objects, or will they stay in theorists’ imagination? It turns out that the fragments would be faint infrared sources, thanks to the heat of their formation, though most of that heat would radiate away during the million years it takes to travel close enough for us to spot them. Though these faint sources would be out of reach of current telescopes, the James Webb Space Telescope might be able to spot them after it launches in October 2018.

Another option is that the Large Synoptic Survey Telescope, which will survey the full sky visible from the southern hemisphere every few days once it comes online in 2022, might spot the slight brightening of a background star as a planet-size fragment passes in front of it. This gravitational lensing signal is still beyond reach of current telescopes.

And the chance that JWST or LSST spots something might be even higher than we think — after all, there’s a supermassive black hole at the center of Andromeda Galaxy too, and it’s probably throwing cosmic spitballs our way too!

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