# Why don't galaxies orbit each other?

Planets orbit around stars, satellites orbit around planets, even stars orbit each other. So the question is: Why don't galaxies orbit each other in general, as it's rarely observed? Is it considered that 'dark energy' is responsible for this phenomenon?

• Related: physics.stackexchange.com/q/129429. Commented Nov 14, 2015 at 23:07
• A number of comments have been deleted. A nuanced discussion of the details of orbital motion and what is meant by a barycenter could have added to this question, but that wasn't what was here. Commented Nov 18, 2015 at 1:27
• Interesting news : mnras.oxfordjournals.org/content/early/2016/04/13/…
– user98038
Commented Apr 15, 2016 at 11:25

There are plenty of satellite galaxies orbiting larger galaxies. The question is how long are you willing to wait for an orbit?

The Milky Way has a mass $M$ of something like $6\times10^{11}$ solar masses, or $10^{42}\ \mathrm{kg}$. The small Magellanic Cloud is at a distance $R$ of $2\times10^5$ light years, or $2\times10^{21}\ \mathrm{m}$. A test mass orbiting a mass $M$ at a separation $R$ will have a period of $$P = 2\pi \sqrt{\frac{R^3}{GM}} = \text{2 billion years}.$$ Such a system could undergo at most $7$ orbits in the entire history of the universe. The universe isn't old enough for the nearest major galaxy to have completed a single orbit around us at its current separation.

Even if you did wait long enough, galaxies aren't particularly good at holding their shape. If you put them in a situation where gravity is strong enough to bend their path into a closed orbit, odds are they will also be tidally torn apart by that same gravity. And we see this all the time, as for example with the Mice Galaxies:

• Beat me... and you even have a picture AND an equation. Commented Nov 14, 2015 at 23:34
• That's a 10^9 "billion" or a 10^12 "billion"? (great picture!) Commented Nov 15, 2015 at 1:06
• @RolazaroAzeveires: in English, always the smaller one. Some older texts use the "long billion", especially British English, but for all official purposes even the UK switched some time in the 1970s. There are still living humans speaking English and using the long billion, but they are sufficiently rare and they diverge from all official definitions/styles, that for any practical purposes they are just wrong :-) Of course, the fact that the French (and other European language) equivalent of the English "trillion" is "billion" can lead to translation errors! Commented Nov 15, 2015 at 11:22
• @SteveJessop It leads to translation errors all the time - especially when the difference isn't "important" (especially in movies - I lost count of all the movies that try to convince us that the Earth is a thousand times older than the universe). Still, the "European" definition of billion makes a lot more sense, and it's commonly used in scientific english (it's a multiple of million, of course). Commented Nov 16, 2015 at 9:04
• @Luaan You mean it's a power of a million since a thousand million (US billion) is by definition a multiple of a million. Commented Nov 16, 2015 at 12:21

They do! There's an entire class of galaxy, called a 'satellite galaxy' which is defined entirely based on them orbiting a larger galaxy (which would be called a 'central galaxy'). Our own milky-way is known to have many orbiting satellite galaxies, or at least 'dwarf-galaxies'. If dwarf-galaxies aren't enough, the milky-way itself is gravitationally bound to the andromeda galaxy, and they are effectively orbitting eachother. Because of the tremendous size-scales, however, the orbital period is billions of years --- in many cases, far longer than the age of the universe, so that a pair like the milky-way---andromeda 'local group' actually hasn't completed a single complete-orbit in the history of the universe. That's why we can definitely never (even hope to) see galaxies orbit in real-time.

• Can we apply Newton's laws to galaxies? "the velocities of stars in galaxies were observed to be larger than expected based on Newtonian mechanics."Source Commented Nov 14, 2015 at 23:42
• @Energizer777 yes, absolutely. The problem which motivated MOND has been solved by 'Dark Matter' which, on galactic scales, is well described by Newtonian Gravity. Commented Nov 14, 2015 at 23:46
• The dark matter has not been experimentally observed and the MOND model is much younger than the dark matter theory. Commented Nov 15, 2015 at 0:02
• @Energizer777 if you want to address MOND, that's fine -- but it should be done in its own question, instead of here. See, however, physics.stackexchange.com/a/5778/8521, physics.stackexchange.com/q/6561, physics.stackexchange.com/q/123661 Commented Nov 15, 2015 at 0:44
• @Source: that is exactly what led to the theory of dark matter. We assume Newtonian dynamics holds and use the observed velocities to determine the mass inside (at smaller distance from the galactic center). We discover that the mass inside stars is higher than we can account for by the observed light, hence the theory of dark matter. Commented Nov 17, 2015 at 6:06

galaxy come in many different sizes: some of the small-er ones do rotate ["orbit"] around the edge of a large galaxy ... one can also visualize galaxy-clusters, in which the entire cluster rotates .....