Read more: “Mysteries of the Milky Way“
Among the many faint companions of the Milky Way (see “Milky Way mysteries: Disappearing dwarf galaxies“) are two shining exceptions. The large and small Magellanic clouds are by far the largest of our dwarf-galaxy entourage. They are complex and active places where bright young stars are being born and others die, as the most recent nearby supernova, which occurred in the Large Magellanic Cloud in 1987, revealed.
That is all well and good – except that these galaxies might not be our satellites at all.
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Observations in 2006 by the Hubble Space Telescope showed that these two dwarfs are crawling across the sky at a little shy of a millionth of a degree per year. That might not sound speedy, but the galaxies are more than 150,000 light years away, so it translates to a velocity of more than 100 kilometres per second. That might be too fast for the clouds to be held by the Milky Way’s gravity.
Whether it is too fast depends on our galaxy’s total mass. Most of the Milky Way’s mass is thought to reside in its surrounding halo of dark matter, which stretches far beyond the bright disc of stars. The best way to estimate the mass of the whole galaxy is by following the motions of other, smaller satellite galaxies that are more conclusively in orbit, to see how they respond to our gravity. But these objects are fainter and slower than the Magellanic clouds, making them even harder to catch in motion. Our best estimates of the Milky Way’s mass are somewhere between one trillion and three trillion times that of the sun.
That allows three possibilities. If the mass is near the top end, then we should easily hold onto the Magellanic clouds, and they have probably orbited the Milky Way a couple of times since it formed. If it is somewhere in the middle, the clouds are likely to be making their first close approach to us. Over a few hundred million years they will begin to head further away, but they will return to us eventually, like a comet on an elongated orbit. If the Milky Way is right at the lightweight end of the scale, however, the Magellanic clouds are just passing, and we will have to wave goodbye to this charismatic pair of space tourists.