Galactic tide

A galactic tide is a tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way.

Such an external force upon a satellite can produce ordered motions within it, leading to large-scale observable effects: the interior structure and motions of a dwarf satellite galaxy may be severely affected by a galactic tide, inducing rotation (as with the tides of the Earth's oceans) or an anomalous mass-to-luminosity ratio.

The dwarf galaxy M32, a satellite galaxy of Andromeda, may have lost its spiral arms to tidal stripping, while a high star formation rate in the remaining core may be the result of tidally-induced motions of the remaining molecular clouds[4] (Because tidal forces can knead and compress the interstellar gas clouds inside galaxies, they induce large amounts of star formation in small satellites.)

In the Solar System, the theoretical Oort cloud, source of most long-period comets, lies in this transitional region.

The Oort cloud is a vast shell surrounding the Solar System, possibly over a light-year in radius.

It has been suggested that the galactic tide may also contribute to the formation of an Oort cloud, by increasing the perihelia of planetesimals with large aphelia.

[9] This shows that the effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system.

However, cumulatively, the effect can be quite significant; up to 90% of all comets originating from an Oort cloud may be the result of the galactic tide.