Our neighboring Andromeda Galaxy (Messier 31, or M31) appears to sport a lopsided arrangement of satellite galaxies that defy scientific models, stumping astronomers who are also trying to figure out why so many of this galaxy’s family members point in our direction. All but one of M31’s brightest 37 satellites are on the side of the Andromeda spiral that faces our Milky Way galaxy – the odd one out being Messier 110, which is easily visible in amateur images of the Andromeda Galaxy.
“M31 is the only system that we know of that demonstrates such an extreme degree of asymmetry,” Kosuke Jamie Kanehisa of the Institut für Physik und Astronomie at Universität Potsdam in Germany told Space.com.
According to cosmology‘s standard model, the universe is filled with vast clouds of dark matter. Within these clouds, large galaxies such as Andromeda and our Milky Way grow — and they grow by merging with myriad smaller dwarf galaxies that have been pulled in by the gravity of the dark matter. The dwarf galaxies that we see around large galaxies today are the leftovers of this formation process. According to this picture of “hierarchical growth,” these dwarf satellite galaxies should be spread randomly around larger galaxies. However, that’s not quite what astronomers are finding.
A large fraction of M31’s dwarf satellites are confined to a plane around the Andromeda Galaxy, as confirmed earlier in 2025 by the Hubble Space Telescope, and a similar satellite plane exists around the Milky Way. There’s also tentative evidence for a plane of satellites around another nearby galaxy, Centaurus A. For galaxies farther afield, it’s difficult to find faint dwarf galaxies and measure the distance to them, so astronomers cannot say for certain what those systems are doing. However, even the planes of satellites scientists have confirmed are difficult to explain in the standard model of cosmology.
And now, M31 and its satellites have become even more mysterious, with almost all those satellites positioned on the side of M31 that faces the Milky Way.
New simulations by Kanehisa, along with his Potsdam colleagues Marcel Pawlowski and Noam Libeskind, show that, according to cosmology’s standard model, the odds of having such a lopsided arrangement are less than 0.3%. And yet, the closest galaxy to us is beating those odds. So when is a coincidence just a coincidence, and when does it open a window into a deeper mystery?
The dwarf galaxies are all orbiting M31 at different velocities and distances; in another billion years, they will be more spread out. So, on the face of it, the current alignment could just be seen as a coincidence. However, the low probability of this coincidence occurring means that it is the kind of coincidence that scientists have a hard time believing in.
If something in the cosmos appears a certain way, it’s usually because something has caused it to be like that. Still, there’s no firm evidence pointing towards any particular explanation so far, other than that for this alignment to be present now, its cause must be recent.
“The following is speculation, but I expect the underlying culprit behind the M31 system’s discrepancy with cosmological expectations to be some unique accretion history,” said Kanehisa. “The fact that we see M31’s satellites in this unstable configuration today —– which is strange, to say the least — may point towards many having fallen in recently, possibly related to the major merger thought to have been experienced by Andromeda around two to three billion years ago.”
This major merger involved a mid-size galaxy being consumed by the larger Andromeda Galaxy. The Hubble Space Telescope’s recent deep-dive into Andromeda has been able to track the disorder in M31 that the merger left in its wake: streams of stars cannibalized from the galaxy eaten by M31 and producing a burst of star formation. However, even if the dwarf galaxies are recent arrivals that lagged behind that galaxy merger, it still doesn’t quite explain why they are all on the side of M31 that faces our Milky Way galaxy.
Another thought is, well, could the dwarf galaxies on our side of M31 be literally pointing at the culprit? Not so fast. If our Milky Way did play a role, then we might expect our system of dwarf satellites to display their own lopsidedness towards M31 — but they do not, and gravitational tidal forces between the two galaxies are not strong enough to pull M31’s dwarfs into this arrangement.
There are also some caveats to note about the improbable lopsidedness exhibited by M31’s satellites.
Accurate data regarding the motion of the dwarf galaxies is known for only four of M31’s satellites (five if IC 10 is counted, which is a contested member of the M31 system), which means that simulations can be a little rough. However, astronomers are aiming to determine the motions for at least a dozen of M31’s dwarfs to high precision within another year or two with the release of the next dataset from the Gaia astrometric mission.
“Once we have a more substantial sample of M31 dwarfs with proper motions, we should be better positioned to study the time evolution of the observed asymmetry,” said Kanehisa. In other words, we would be able to better understand for how long this lopsided alignment can stay in place.
Another caveat is that there likely remains many fainter, as yet undiscovered, satellite dwarf galaxies around M31. We know that this is probably the case because cosmology’s standard model predicts that there should be more dwarf satellites around both M31 and our Milky Way than we have currently found. These undiscovered dwarfs would be small and contain few stars, making them hard to identify. Indeed, the faintest dwarf galaxy currently known around M31, named Andromeda XXXV, was found on the far side of M31 and does not fall into the lopsided pattern of the others.
“We need to wait and see if the global asymmetry persists with the detection of fainter dwarfs, although if we found that only brighter dwarfs participated in the asymmetry, this would also raise some more questions as to their origin,” said Kanehisa.
The final caveat is that perhaps the alignment of M31’s system is not as improbable as we think. Very few galaxies beyond our Local Group have had their dwarf families mapped out in detail — the galaxies are just too far away and too faint, and those that we can detect do not have accurate distance measurements, so we can’t say for sure which side of their parent galaxy they are on.
“We can’t yet be sure that similar extreme systems don’t exist out there, or that such systems would be negligibly rare,” said Kanehisa.
The Andromeda Galaxy’s family of dwarfs are an enigma. First it was the unusual flat plane around which more than half of them orbit, and now this lopsidedness. It just goes to show that sometimes it is our closest neighbors who are the ones that turn out to be most alien to us.
The findings were published on April 11 in the journal Nature Astronomy.
