What Gets Kicked Out of Our Galaxy?

On occasion, objects will get kicked out of galaxies like the Milky Way. This includes planets, but also stars, entire star systems, and even black holes!
Matthew S. Williams

On a clear night, it's possible to gaze up in the night sky and see a hazy band of light stretching from one horizon to the other. What you are seeing is the central disk of the Milky Way, a massive formation made up of dust, gas, and billions of stars.

Based on modern observations, astronomers estimate that the Milky Way measures 150,000 to 200,000 light-years in diameter and contains between 100 and 400 billion stars. These stars, as well as nebular clouds of dust and gas, are tightly bound around the center of Milky Way.

In the past century, astronomers also realized that our galaxy is merely one of many in the observable Universe (current estimates say there could be as many as 1 or 2 trillion). However, astronomers have learned a great deal about what lies between galaxies during that time as well.

For the most part, intergalactic space is as close as the one can get to a total vacuum. While not completely empty, these regions are generally filled with only trace amounts of dust and debris that stretch like filaments from one galaxy to the next.

What Gets Kicked Out of Our Galaxy?
Panoramic view of the center of the Milky Way, Source: ESO

However, astronomers have also come to realize that in the space that lies between galaxies, there are also lots of objects that get kicked out of galaxies on a pretty regular basis.

These include rogue planets, rogue stars, and perhaps even a few supermassive things (more on that below). The existence of these extragalactic objects has led to some rather interesting realizations about our Universe.

A little cosmic history...

According to the most widely accepted cosmological theories, the Universe began with the Big Bang roughly 13.8 billion years ago. Roughly 100,000 years later, the first stars formed from primordial hydrogen and helium gas.

Over time, these stars began to group into large, spherical star clusters (aka. globular clusters). These then gravitated towards each other to form the first galaxies, which began to appear by about 1 billion years after the Big Bang (ca. 13 billion years ago).

By this point, the large-scale structure of the Universe had formed, which included galaxy clusters, superclusters, and the large filaments that connected them. After multiple generations of stars were born and died, heavier elements began to accumulate as well.

The first stars were formed from hydrogen and helium, but once they went supernova, metals that were formed within were blown out into space. Roughly six or seven billion years after the Big Bang, there were enough of these elements in the interstellar medium that planetary systems began to form.

All the while, new stars were born, planetary systems continued to form, and galactic mergers continued to take place. Given their importance to cosmic evolution, perhaps a little background on this is needed as well... 

Galactic mergers

For generations, astronomers have understood that over the course of cosmic history, galaxies have evolved through the merger process. This gradually caused dwarf galaxies to come together to form elliptical galaxies, which gradually grew to become spiral galaxies (like our own).

Whenever this occurred, the result would have been rather cataclysmic. It would begin in the outer regions of the merging galaxies, where their arms would make contact and stars and clouds of dust and gas would be exchanged.

Gradually, the galaxies would push into each other and denser concentrations of stars would come into contact. This would result in many star systems being destroyed through tidal disruptions, and perhaps even collisions between stars.

Someday, astronomers predict that the Milky Way Galaxy will collide with its nearest neighbor, the Andromeda Galaxy (aka. Messier 31). This massive spiral galaxy is located about 2.5 million light-years away and is comparable in size to our own.

Interestingly enough, this galaxy is approaching the Milky Way at a speed of roughly 482,800 km/h (300,000 mph). Based on the latest observations provided by the ESA's Gaia Observatory, from which astronomers have been able to anticipate the future movements of our two galaxies, this merger is estimated to take place about 4.5 billion years from now

For the record, that's 1 billion years longer than previously thought. What a relief, huh? And while civilization as we know it is likely to be long-dead at this point, any civilizations that are around at this point might need to get creative to ensure their survival.

Then again, they might not even notice. Basically, the process will take hundreds of millions (or even billions) of years to conclude. And according to astronomers, the process will likely involve five phases.

During Phase One, the Milky Way and Andromeda will continue to approach one another and Andromeda will grow ever larger and brighter in the night sky. In Phase Two, they will be close enough that giant molecular clouds in their outer reaches will become compressed and give birth to bright blue new stars, creating new constellations.

Phase Three will involve the disk of dust and stars that characterize our galaxy Andromeda will begin to come apart. As Andromeda swings past our galaxy, the sky will become a jumbled mess of dust, gas, and bright young stars. In this phase, many of the newly-formed massive stars will go supernova, lighting up the night sky.

In Phase Four, 100 million years after Andromeda makes its first pass, it will swing back and the two galaxies will merge again. This will cause the molecular clouds to be compressed again, triggering another round of star formation and supernovae. The winds created by this will blow away much of the remaining gas and dust.

In Phase Five, the two galaxies will have finally be merged and form a single elliptical galaxy (often referred to as "Milkomeda"). Any evidence that that the Milky Way and Andromeda once existed as separate galaxies will be gone.


The distribution of stars in Andromeda and the Milky Way means that the chance of any direct collisions between star systems will be negligible. However, the merger process will still cause massive upheaval due to the sheer gravitational forces involved.

Basically, whenever star clusters or galaxies merge, the gravitational influence will generate tremendous tidal forces. It will also result in the creation of massive Gravitational Waves (GWs) that ripple through the cosmos and shake up spacetime.

What's more, when particularly massive galaxies merge (which will be the case when Andromeda and the Milky Way come together) a lot more than just gas, dust, and stars are exchanged. As astronomers have known since the 1970s, most massive galaxies have supermassive black holes (SMBHs) at their centers.

So when massive galaxies merge, so do the black holes at their cores. Here too, the two massive bodies will pass each other by, orbit each other for a period of time, and eventually come together to form a single SMBH.

Like the galaxies themselves, by the time they are finished, no evidence will remain that they were once separate.

So how do things "go rogue"?

In the case of planets, the process is relatively simple. Shortly after stars are born and have formed a system of planets, shake-ups can occur as a result of all the gravitational interactions. These shake-ups can even lead to one or more planets being kicked out of a star system.

Recent research shows that this may have happened in the Solar System about 4.5 billion years ago, causing some of our planets to go rogue. These planets would have become part of a population of billions that orbit the Milky Way directly and are not bound to any particular star.

But in some cases, planets could be hurled from a star system with enough force that they go extragalactic. Evidence of such planets was first revealed in 2018 by astrophysicists who indirectly observed a population of about 2000 planets between the Milky Way and a galaxy 3.8 billion light-years away.

As for stars, the process through which they go rogue is a little more dramatic. In some cases, the tidal forces caused by the merger of two galaxies (and the SMBHs at their cores) will be enough to overpower the gravitational attraction that keeps star systems bound their galaxy.

As a result, these stars will be flung from the merging galaxies and find themselves in intergalactic space. Astronomer Jack Hills was the first to theorize that such "rogue stars" could exist in 1988.

Since that time, astronomers have made numerous discoveries that indicate that rogue stars are actually quite common. In some cases, they were found to be traveling at speeds of one-tenth to one-third the speed of light (0.1 to 0.33 c).

For reference, light travels at a constant velocity of 299,792,458 m/s (1,079 million km/h; 670.6 million mph). Doing the math, that means that these stars were moving at speeds of about 100 million km/h (67 million mph) to 360 million km/h (223 million mph). 

These incredibly fast-moving stars to them receiving the designation hypervelocity stars (HVS), the first of which was observed in 2005 by astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA).

While galactic mergers are a compelling reason why stars would go rogue and achieve hypervelocity speeds, there are other possible mechanisms for hurling celestial objects from their respective galaxies.

For instance, astronomers have noted that Sagittarius A* (the SMBH at the center of our galaxy) has stars that orbit it regularly (such as S2). Because of the intense gravitational forces involved, S2 has a highly eccentric orbit and accelerates considerably when it is closest to Sag A*'s event horizon.

Based on the calculations performed by Jack Hills and more recent studies, astronomers have found that if a binary system were pulled in by the gravity of an SMBH, one companion might be captured while the other was ejected from the galaxy altogether.

In fact, it was the original calculations performed by Jack Hills that suggested that black holes that are 4 million times the mass of our Sun would be able to generate the necessary force to do it. Incidentally, Sag A* is estimated to be between about 4 and 4.5 billion Solar masses.

More recent observations have found that mid-mass black holes (MMBHs) - which have about a dozen times the mass of our Sun - could do the trick as well. In these cases, the stars could have been ejected as a result of one star in a binary pair going supernova and pushing the other star out of the galaxy.

But here's where things get really interesting. According to some observations and theoretical studies, some truly interesting things are being hurled from our galaxy (and others) as well.

Planets, and stars, and black holes!

To recap, planets are kicked out of galaxies relatively often and hypervelocity stars are also common. But what about whole systems, where stars and the planets that orbit them are kicked out of galaxies?

According to researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) and the Institute for Theory and Computation (ITC), it is entirely possible that stars ejected from our galaxy could carry their planetary systems along for the ride.

This means that entire star systems could be traveling from one galaxy to another at a portion of the speed of light. Even more intriguing is the possibility that some of these planets could be inhabited, and that the stars they orbit would eventually reach another galaxy.

In this respect, hypervelocity stars could be one way in which life is spread throughout the Universe. What's more, the same researchers indicated that there could trillions of these stars in the Universe out there, just waiting to be studied.

As Professor Abraham Loeb, one of the authors of the research indicated:

“Tightly bound planets can join the stars for the ride. The fastest stars traverse billions of light-years through the universe, offering a thrilling cosmic journey for extra-terrestrial civilizations. In the past, astronomers considered the possibility of transferring life between planets within the solar system and maybe through our Milky Way galaxy. But this newly predicted population of stars can transport life between galaxies across the entire universe.”

Sounds pretty freaky, huh? Well, it gets even freakier than that! The next possibility is so profound that it deserves to its own line:

Supermassive Black Holes!

What Gets Kicked Out of Our Galaxy?
Artist's impression of an SMBH being ejected from a galaxy, Source: NASA/CXC/M.Weiss

You read that right. According to recent research, the tidal interactions caused by galaxies colliding can be so intense that even Supermassive Black Holes (SMBHs) could find themselves being ejected from galaxies and going rogue - thereby being rogue Supermassive Black Holes (rSMBHs).

In 2018, astronomers from the National Radio Astronomy Observatory (NRAO) detected what they believed to be an rSMBH traveling away from its galaxy. Using data from NASA’s Chandra X-ray Observatory and other telescopes, the team spotted it in intergalactic space about 3.9 billion light-years from Earth.

Given the mass of the object (160 million times the mass of our Sun), as well as its bright X-ray signature, the team determined that it must either be an SMBH or dual SMBHs. They also theorized that it was likely to have been part of an elliptical galaxy at one time.

Since this object was over 80 times as massive as Sag A*, the galaxy that contained it would have had to have been very massive. What's more, the gravitational force responsible for kicking it out must have been truly enormous!

All of this adds weight to the theory that the object was ejected as a result of two particularly massive galaxies merging. One can only imagine the astronomical (no pun) forces involved. And the thought of something that massive and powerful flying through space... let us just be thankful we're not in its way!


What does all this mean for space exploration? Well, someday we may be able to study extragalactic stars and planets in detail, much in the same way that we expect to study extrasolar planets in detail. Who knows what we might find?

In addition, we might learn someday that life as we know it (or the necessary ingredients) came from another galaxy altogether. In fact, we might have distant relatives living in a galaxy billions of light-years away that are looking up at the stars and wondering if there's intelligent life beyond their world.

As for hypervelocity stars that are out there right now (and have inhabited planets orbiting them), one can only imagine what it must be like for intelligent creatures staring up at the night sky. Assuming they kept detailed records, they would realize that the sky was changing over long periods of time.

In one hemisphere, the stars would appear reddish since they would be getting farther and farther away. In the other, they would appear blue (blueshift) since they were getting closer. Eventually, the people in one hemisphere would have a clear view of the galaxy they left, while people in the other would realize that the galaxy in their sky was slowly getting bigger.

And as Professor Loeb explained, if some offshoot of humanity is still around  4.5 billion years now, they might end hitching a ride on a planet that orbits a hypervelocity star:

"In the past, astronomers considered the possibility of transferring life between planets within the solar system and maybe through our Milky Way galaxy. But this newly predicted population of stars can transport life between galaxies across the entire universe. Our descendants might contemplate boarding a related planetary system once the Milky Way will merge with its sister galaxy, Andromeda, in a few billion years."

If there's one thing that the study of the Universe has taught us, it is powered by some truly titanic forces. So it should come as no surprise that on occasion, planets, stars, and even black holes can get tossed around like billiard balls!

And yet, one can't help but feel amazed!

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