Estimating strangeness

But just how surprising is the new finding? A few other satellite galaxies moving in a single plane have been found before. Two such cases were discovered right in our cosmological backyard, one around our own Milky Way and one around the Andromeda galaxy. Three is not a big number, but we have not looked for these features much farther away yet. However there is tantalising evidence that about half of galaxies like the Milky Way may have satellites on ordered orbits.

So far, cosmologists have been writing off these planes as rare events, odd occurrences that don’t represent the wider universe. Using computer simulations of galaxy formation and sifting through all possible orientations of satellite galaxies in these models, scientists can estimate the number of such “outlier” galaxies we can expect to find in the universe. This shows that such planar distribution should not be freakishly rare – there is in fact a 10% chance of it happening.

However, the chance of seeing a large number of satellite galaxies rotating in the same direction, such as those around Centaurus A, is less than 0.5%. This means it is not impossible, but if we find too many such cases, the standard model would have to be rethought. In addition to the new study, we know of two other cases where that happens (also in Milky Way and Andromeda). So by now there are already three such examples in a sample of observations that is not yet very large.