Milky Way's mystery solved: the plane of satellite galaxies disperses over time
A study led by researchers at the Universities of Helsinki and Durham reveals the curious structure surrounding our galaxy to be short-lived, and consistent with dark matter theory. This breakthrough was possible thanks to new supercomputer simulations and observations by ESA's GAIA space telescope.

The patterns traced by constellations of stars have fascinated stargazers since prehistoric times. Stories and myths, and even the ability to predict our future, were built around these evocative stellar arrangements.

Modern astronomers punctured the magic of constellations when they showed that they are mere visual, chance alignments of stars who are often at great distances from one another. Since stars are moving, wait long enough and new constellations will appear while old ones will dissolve.

More recently, another strange pattern, this time traced by bright satellite galaxies that orbit our own Milky Way galaxy, has gripped the imagination of modern astronomers and physicists. In the 1970s, the great Cambridge astronomer, the late Professor Donald Lynden-Bell, noted that these satellites seem to be arranged in an implausibly thin plane piercing through our galaxy - the Milky Way’s “plane of satellites”. To add to the mystery, it was later argued that these galaxies are circling the Galaxy in a coherent and long-lived disk.

The Milky Way’s plane of satellites is so striking and yet so bizarre that astronomers have tried to look for similar structures in large cosmological simulations, which track the evolution of the universe from the Big Bang to the present with an impressive degree of realism. When they repeatedly failed to find them, they questioned the validity of the standard cosmological model, and some even questioned the very existence of dark matter that underpins it.

In a billion years the plane of satellite galaxies will have disintegrated, as will today's constellations

Now, a group of astronomers including at the Universities of Helsinki and Durham, have shown that the Milky Way plane of satellites is, like the constellations of yore, nothing more than a chance alignment – and like the constellations, it is bound to dissolve.

“The plane of satellites was truly mind boggling,” says the study’s lead author, Till Sawala. “It is perhaps unsurprising that a puzzle which has endured for almost fifty years required a combination of methods to solve it - and an international team to come together.” 

This breakthrough was made possible, in part, by new data from the European Space Agency's GAIA space observatory. GAIA is charting a six-dimensional map of the Milky Way, providing precise positions and motion measurements for about one billion stars in our Galaxy (about 1 percent of the total) and its companion systems. This allowed them to project the orbits of the satellites into the past and future, and see the plane form and dissolve in a few hundred million years – a mere blink of an eye in cosmic time.

To close the loop, astronomers also searched new, tailor-made cosmological simulations for evidence of planes of satellites. They realised that previous studies based on simulations had been misled by failing to take into account the distances of satellites from the centre of the Galaxy, which made the virtual satellite systems appear much rounder than the real one.

Taking this into account, they found several virtual Milky Ways which boast a plane very similar to what is observed. This removes one of the main objections to the validity of the standard model of cosmology, and means that the concept of dark matter remains the cornerstone of our understanding of the Universe.

“The standard Lambda cold dark matter model is currently the best model we have for our universe and solving the plane of satellites problem resolves another challenge to the model”, says co-author, professor Peter H. Johansson.

"We have been able to remove one of the main outstanding challenges to the cold dark matter theory: it continues to provide a remarkably faithful description of the evolution of our Universe", comments Durham University’s Carlos Frenk.