If you’re an early riser then you get a treat over the next week or two. Three planets rise shortly before the Sun: Mercury, Saturn, and Venus will be visible in the pre-dawn sky. Venus is up first and will be highest in the sky, with the other two much fainter and lower. Binoculars will help with those two, though Venus should be obvious enough.

I wrote all about this lovely apparition and how to view it for the online magazine Condé Nast Traveler, so please go check it out! I’ve written a few articles for them this year, including a brief guide to some of the best night sights in 2025.

And hey, don’t forget I write an astronomy article every week for Scientific American for my column, The Universe. I’d appreciate it if y’all read those too.

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The Universe is doing weird stuff again.

This time, it involves the Andromeda Galaxy. Like our own Milky Way, Andromeda is a big spiral galaxy, a flattened disk somewhat larger than our galaxy but with about the same mass. Together, we dominate the Local Group, a clutch of some 80 or so galaxies, most of them quite small. To give you an idea of just how big Andromeda is, despite its terribly remote distance of 2.5 million light-years, it’s visible to the unaided eye from observers in the northern hemisphere.

Surrounding both our galaxies are dozens of tiny dwarf galaxies, orbiting either the Milky Way or Andromeda like moons. It’s hard to know exactly how many there are, as these galaxies can be so faint they’re difficult to see, and in the case of the Milky Way, since they surround us, they can be anywhere in the sky. Andromeda’s satellite galaxies should be more or less in the direction toward that galaxy, making their citizenship a little easier to determine.

At the moment 37 such dwarf satellite galaxies are known to orbit Andromeda, and a priori you’d think they surround their host as well. But this is where things get really strange: in new research just published, astronomers found that nearly all of them are located between our two galaxies. In other words, they’re on the near side of Andromeda, with only a handful on the other side from us, and even those are close to the near side border [link to journal paper].

That’s bizarre.

Specifically, 29 of the 37 are on the near side, and 36 out of 37 are located within 107° of the Milky Way’s position as seen from Andromeda. So some are on the far side, but just barely. This problem has been known for a while, but in this new work the astronomers use a consistent set of measurements for those satellite galaxies’ distances, letting them see just how unlikely this all is. In other words, they show this problem is really real.

When I first read this, my immediate thought was “observational effect” — galaxies on the other side of Andromeda are much fainter, and therefore harder to see. Maybe they just haven’t been discovered yet. But the astronomers show this likely isn’t the case. Even if you look at brighter satellite galaxies the asymmetry is there; dwarf galaxies that bright should be easily seen on the other side, yet none is found.

The thing is, we expect them to be roughly randomly distributed through space, so there should be close to half on the near side and half on the far side. The fact that they aren’t means something very weird is going on.

How weird? The odds of this happening by chance are incredibly low. Assuming each one has a 50/50 shot at being on the near side of the galaxy to us, getting 29 out of 37 on the near side has a chance of just 1 in 2,600!

I know this is a bit hard to picture in your head, but happily the folks at the Leibniz Institute for Astrophysics Potsdam (the host institute for the research) put together a video that shows the problem: 

How can this happen?

The current idea we have is about how galaxies form is that they condense from material less than a billion years after the Big Bang. Matter (including our own “normal” matter and dark matter) was pretty evenly distributed throughout space, but some spots had a tiny amount more matter in them than others. As the Universe itself expanded, any of these clumps with enough mass could hold themselves together via gravity, and would in fact begin to collapse.

Dark matter, we think, doesn’t interact with normal matter except through gravity, whereas normal matter can bump into itself and form large coherent structures. The normal matter (mostly hydrogen and helium gas) collapsed down to form galaxies, which would’ve been surrounded by dark matter haloes. Most of these galaxies would have been small dwarfs that collided and merged and grew. Eventually, in our Local Group, we wound up with two dominant galaxies (us and Andromeda), one smaller one (Triangulum), and a ton of dwarf galaxies orbiting the big ones in a cloud around them.

In this simple model there’s no explanation for why all of Andromeda’s satellite galaxies are all packed on one side. And to have that be on the side facing our own galaxy cannot possibly be a coincidence. Clearly something else must be going on.

The obvious thought is that somehow our own big galaxy is interacting with these other galaxies in a way that pulls on them, and they all wind up on Andromeda’s near side. However, that’s unlikely: we have a lot of our own satellites and they are distributed all around us. The authors point out “no noteworthy degree of asymmetry has been reported in the Milky Way’s satellite distribution”. This, despite Andromeda having about the same mass as the Milky Way.

So whatever explanation you come up with has to also explain why we don’t see this problem with our own galaxy’s satellites. Like I said, it’s bizarre.

The authors ran simulations of galaxy formation using everything we understand about the process, and found that this kind of distribution happens in a Local Group-like system less than 0.3% of the time. So yeah, this asymmetry is really super unlikely.

What’s causing it?

¯\_(ツ)_/¯

Literally, they don’t know. Something we don’t yet understand.

Now, there is some structure to the satellite distribution. For reasons still unknown, many of Andromeda’s satellite galaxies are in a single plane instead of being in a spherical halo around it. We actually see this a lot with big galaxies and their satellite galaxies, including our own, and is no doubt tied to the way the galaxies formed. The specifics are still being argued over by astronomers. It’s possible this is tied to the asymmetry somehow, but it’s not at all clear how, and Andromeda is such a heavily distorted case that even then it would be hard to explain. 

It really is a mystery. Maybe it’s tied to how dark matter interacts with normal matter, or some other aspect of dark matter we don’t yet understand. I will point out that this doesn’t mean we have to throw all of Big Bang cosmology out the window! I think we have a pretty good grasp on how this process works overall, it’s just that, obviously, there are still some (important) details we have to work out.

So I don’t have a nice ribbon to wrap this all up in. It’s an ongoing investigation, with no culprit IDed just yet. I still have to wonder if it’s some sort of observational bias, something not obvious. It pays to really deeply investigate the simplest solution, even if it’s very unlikely. I still think of the experiment that showed neutrinos moved faster than light, but it turned out to be a faulty cable in the equipment.

Maybe with deeper survey observatories coming online soon we’ll find more satellites of Andromeda (and the Milky Way!) that will provide more clues. Who knows? 

For now, we have to be OK with not knowing. Science is like that a lot; the fun is in figuring out what we’re missing.

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