Photograph by NASA
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An x-ray image shows the turbulent region of space that surrounds the supermassive black hole at the middle of the Milky Way, known as Sagittarius A*.

Photograph by NASA

Black Hole at Galaxy's Heart Launches Planet-Size 'Spitballs'

The massive object at the Milky Way’s core is shredding stars and tossing out their remnants, new simulations suggest.

The monster black hole at the center of our galaxy is shooting out “spitballs” the size of planets—and one may be sprinting through space several hundred light-years away from Earth.

It seems a somewhat odd phenomenon, given the prevailing idea that black holes tend to slurp things up rather than fling stuff out. But new simulations from Harvard University undergraduate Eden Girma, presented January 4 at the American Astronomical Society’s annual meeting in Texas, suggest that our galaxy’s massive black hole could be responsible for sending an astronomical amount of these free-floating objects zipping through space.

“Our galaxy could be populated by hundreds of millions of these cold fragments that are the direct remnants of stars,” Girma says.

Those rogue, planet-like bodies form in a most unusual way.

Every 10,000 years or so, a star tiptoes too close to Sagittarius A*, the supermassive black hole parked in the center of the Milky Way. When that happens, the star’s life as a round, incandescent nuclear furnace is over. The black hole’s intense gravity disrupts and spaghettifies the star, leaving streamers of gas strewn near the Milky Way’s heart. (Also see “Black Hole in Milky Way Seen Snacking on Asteroids?”)

That part of the story is fairly well known, but what happens next is where Girma and her mentor, James Guillochon, come in.

The pair simulated 50 star-shredding encounters and watched as that noodly, disrupted starstuff began to reconnect, balling itself up and forming planet-mass clumps of gas and dust.

“While tidal disruptions in the galactic center have been a subject of research since the late ‘80s, the idea that actual compact objects could form from this process is very new,” Girma says.

Cool Crowd

In their simulations, some 11,473 bodies grew out of the starguts—each more massive than Neptune, and sometimes many times larger than Jupiter. The black hole then slings these gassy balls into space, sometimes at speeds exceeding 20 million miles an hour.

Of those newly born planetoids, about 95 percent were flung from the galaxy and into the cosmic hinterlands separating the Milky Way from the next galaxy over. A much smaller percentage remained bound to Sagittarius A*, destined to endlessly circle the exotic behemoth that had ripped their parent star apart.

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An illustration of the frenzied activity at the core of our Milky Way galaxy. The black hole's presence is revealed by the orbital motion of stars and blobs of gas in this region.

And the smallest set of spitballed planets, fewer than one percent of the total, are now wandering the outskirts of the Milky Way, perhaps within about six hundred light-years of Earth. (See “New Milky Way Map Is a Spectacular Billion-Star Atlas.”)

It's an interesting and plausible scenario, says UCLA's Andrea Ghez, an expert on the antics of Sagittarius A*.

If Girma and Guillochon are right about how frequently Sagittarius A* shreds passing stars, then there could be millions of these strange almost-planets in the Milky Way. Still more could be interlopers from our neighboring galaxies, punted into space by their central black holes in an epic game of intergalactic pinball.

“In general, these fragments travel at extremely high speeds and can escape the galaxy completely,” Girma says. This raises the question: How many of the stellar fragments wandering through the Milky Way were actually created in other galaxies?

Whether we’d know one of these planetary spitballs when we see one is still up for debate. Without a star to call their own, the planetoids cool to a temperature that makes finding them a task for infrared eyes. And even then, they wouldn’t necessarily be festooned with a fingerprint advertising a stellar or extragalactic origin.

Yet, Girma says, “in detecting them we could then delve more deeply into the chemical composition of these fragments, learning more about the star it originated from and perhaps determining their own habitability.”