Tracking Messengers from the Galaxy's Core

cosmic spitwads
This artist's conception portrays a collection of planet-mass objects that have been flung out of the galactic center at speeds of 20 million miles per hour (10,000 km/s). These cosmic "spitballs" formed from fragments of a star that was shredded by the galaxy's supermassive black hole. Mark Garlick/CFA

If you know anything about our galaxy, you've probably heard there's a supermassive black hole at its heart. The thing just sits there, mostly quietly sucking in whatever happens to wander too close. This includes clouds of gas and dust, as well as stars. Most of the time, the material is lost inside the black hole, never to be seen again. However, every once in a while, something quite interesting happens. A star wanders too close and the tremendous gravitational pull rips the star apart. That sends a long streamer of gas out to space. The gas gathers itself into planet-size objects that get flung throughout the galaxy. The whole thing is rather like a gigantic case of galactic spitball.

The numbers are rather incredible: a single star that gets torn apart by the close encounter with the black hole can generate hundreds of these spitwads. They travel out from the scene of the crime, leading astronomers to wonder next: where do they go?

Modeling Gaseous Spitballs

To figure that out, a group of researchers took currently known information about these shredded stars and devised a computer program that estimated their speed and directions of travel. The results show that these objects could actually be passing fairly close to our planet (in galactic terms). The closest might be only a few hundred light-years away. The program also suggests that the mass of such objects would be somewhere around the mass of Neptune or a super-Jupiter.

What would a galactic spitball look like? Turns out that they're relatively difficult to see with current instruments. However, when astronomers do get a chance to look at them using the infrared-sensitive James Webb Space Telescope (the successor to the Hubble Space Telescope) or the Large-Synoptic Survey Telescope (both set to come on line within the next few years), they would be searching for objects that are glowing in the infrared.

Leaving the Galaxy for Good

So, where do the spitballs go? Not all of these gassy objects will stick around in the Milky Way. Many of them — perhaps as many as 95 percent of them — will head out of the galaxy on one-way trips to intergalactic space. This makes sense — they're moving at speeds of about 10,000 kilometers per second (20 million miles per hour), so they can go a long ways pretty quickly. To give you an idea of how quickly they can travel the tremendous distances across the galaxy, astronomers estimated that it would take about a million years for a given spitwad to leave the center of the galaxy and travel across 26,000 light-years to our neck of the woods.

Spitballs from Afar

The Milky Way isn't the only galaxy coughing up these cosmic hairballs. Most other galaxies also have supermassive black holes at their cores, so they're probably undergoing the same stellar/black hole interactions too, resulting in the same gassy upheavals. Andromeda, our nearest spiral neighbor, is likely sending a lot of these spitballs toward us, and so that gives astronomers something else to look for when they search these objects out.

What Are These Spitballs Like?

These cosmic spitballs are made quite literally of "star stuff" and appear to be planet-like, but they are different from a typical planet. However, because different ones would develop from different pieces of the former star, their compositions could vary. You could have a more planet-like object, or it could be a very tightly bound ball of gas.

One of the amazing things about these spitballs (aside from the fact that they exist at all) is that they form very quickly. It takes less than a day for a black hole to shred something the size of a typical star through the process of tidal disruption. The fragments take about a year to pull themselves together into a coherent spitwad of star stuff, all the while rushing away from the scene of the crime. Actual planets form much more slowly; a Jupiter-type world, for example, might take millions of years to assemble during the normal process of planetary formation.

Among the biggest challenges for astronomers will be to tell these objects apart from free-floating planets between stars. Those would be worlds that formed the old-fashioned way in a cloud of gas and dust around a newborn star. The black hole-flung ones are rare — maybe only one in a thousand objects "out there" is a cosmic spitwad. But, they're out there and giving astronomers a look at the long-term consequences of actions in the core of our Milky Way Galaxy.