The mystery of Milky Way’s satellite galaxies solved after more than 50 years

Astronomers took advantage of new data from the GAIA space observatory.
Nergis Firtina
One of the new high-resolution simulations of the dark matter enveloping the Milky Way and its neighbour, the Andromeda galaxy.
One of the new high-resolution simulations of the dark matter enveloping the Milky Way and its neighbour, the Andromeda galaxy.

Till Sawala/Sibelius collaboration 

Astronomers claim to have solved a long-standing mystery about how the Universe evolved: the spatial distribution of weak satellite galaxies around the Milky Way. The strange arrangement of these satellite galaxies, sometimes known as the plane of satellites, makes them appear to be situated on a vast, narrow, spinning plane.

According to a recent study conducted in collaboration with the Universities of Durham and Helsinki, the plane of satellites is a cosmic anomaly that will disintegrate over time like star constellations. The study was published in Nature Astronomy.

As stated in the release, these satellite galaxies have an unusual alignment in which they appear to reside on an immense thin rotating plane known as the plane of satellites. For more than 50 years, astronomers have been perplexed by this seemingly improbable arrangement, causing many to question the validity of the mainstream cosmological model, which aims to explain how the Universe came to look the way it does today.

The mystery of Milky Way’s satellite galaxies solved after more than 50 years
Positions and orbits of the 11 classical satellite galaxies of the Milky Way, projected “face-on” (top) and “edge-on” (bottom), integrated for 1 billion years into the past and future.

Their work resolves the problem that the plane of satellites presents for the cosmological standard model. This theory explains how the galaxies we see today gradually developed within clumps of cold dark matter, a mysterious substance that accounts for around 27 percent of the Universe.

The Milky Way's satellites seem to be arrayed in an implausibly thin plane piercing through the galaxy, and bizarrely, they are also rotating in a coherent and long-lived disk. There is no known physical mechanism capable of transforming satellites into planes. Instead, satellite galaxies were expected to be grouped in a nearly circular pattern, tracking the dark matter.

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"Satellites were mind-boggling"

Since the 1970s, when the plane of satellites was first identified, astronomers have made numerous unsuccessful attempts to locate comparable structures in accurate supercomputer models that chart the development of the Universe from the Big Bang to the present.

On the other hand, in this most recent study, astronomers took advantage of new data from the GAIA space observatory operated by the European Space Agency. A six-dimensional map of the Milky Way is being created by GAIA, which also provides exact locations and velocity measurements for around a billion stars in our galaxy.

With these data, researchers could predict the satellite galaxies' future and previous orbits and observe how the plane formed and disintegrated in a matter of a few hundred million years or a mere blink of an eye in cosmic time.

"The strange alignment of the Milky Way's satellite galaxies in the sky had perplexed astronomers for decades, so much so that it was deemed to pose a profound challenge to cosmological orthodoxy," said study co-author Professor Carlos Frenk, Ogden Professor of Fundamental Physics in the Institute for Computational Cosmology, at Durham University.

“But thanks to the amazing data from the GAIA satellite and the laws of Physics, we now know that the plane is just a chance alignment, a matter of being in the right place at the right time, just as the constellations of stars in the sky," he also added.

Study lead author Dr. Till Sawala of the University of Helsinki said: “The plane of satellites was truly mind-boggling.

“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.”