The Milky Way may be surrounded by 'too many' mini galaxies, new discoveries reveal

A photo of the Subaru Telescope against the night sky with the Milky Way clearly visible
The potential dwarf galaxies, Sextans II and Virgo III, were spotted by the Hyper Suprime-Cam (HSC) attached to Japan's Subaru Telescope. (Image credit: Hideaki Fujiwara, Credit:NAOJ)

Astronomers have discovered a previously unknown pair of potential satellite galaxies orbiting the Milky Way. The location of these dwarf galaxies suggests that hundreds of other unknown mini-galaxies could be hiding around us — potentially challenging our understanding of the galaxy's edges. 

A satellite galaxy is a clump of stars, either in a circular blob or a halo-like shape, that orbit the Milky Way independently from the rest of the galaxy. Our galaxy's largest-known satellite is the Large Magellanic Cloud, which holds around 30 billion stars and can be viewed with the naked eye. Other known satellites only contain a few hundred thousand or a couple of million stars.

A 2020 census of known satellite galaxies suggests scientists have found a maximum of around 60 satellite galaxies orbiting the Milky Way. However, there is some uncertainty about the true number of satellite galaxies, largely because scientists disagree over how large these star clusters should be and how far they should lie from the galactic center to be counted as true satellites, according to NASA

Related: The Milky Way will be visible without a telescope this summer. Here are the key nights to watch for.

But  most astronomers agree that there should be many more, currently unknown, satellite galaxies out there. Based on our current understanding of dark matter — which doesn't react with light but interacts gravitationally with visible matter and makes up around 27% of the universe's mass — researchers have long assumed that the Milky Way should have around 220 satellite galaxies. Our inability to spot many more of these is often referred to as the "missing satellites problem." 

In a new study, published June 8 in the journal Publications of the Astronomical Society of Japan, researchers discovered two new potential satellite galaxies, Sextans II and Virgo III. The satellites are located around 411,000 and 492,000 light-years from Earth respectively and are likely both ultra-compact dwarf galaxies (UCDs) — collections of old stars clumped tightly together, making them brighter than other satellite galaxies. 

However, the discoveries do not help solve the missing satellites problem. Instead, the location and orientation of these potential satellites hints that there are even more satellite galaxies than scientists initially realized. This raises a new problem, which researchers have dubbed the "too many satellites problem." 

Related: Does the Milky Way orbit anything?

Satellite galaxies vary widely in terms of size and distance from the galactic center. (Image credit: ESA/Gaia/DPAC)

Researchers discovered the UCDs using the Hyper Suprime-Cam (HSC) attached to Japan's Subaru Telescope at Mauna Kea Observatories in Hawaii. This instrument has spent the last few years searching a region of space around 33 light-years across. Based on the estimated 220 satellite galaxies postulated by the missing satellites problem, an area of this size should have around four satellite galaxies on average, Universe Today recently reported.

However, the latest discoveries bring the total number of satellites found by HSC in this area to nine. If this concentration of satellites is consistent around the Milky Way, it would mean that there could be at least 500 satellite galaxies around the Milky Way, researchers wrote in a statement

In the past, scientists have proposed multiple solutions to the missing satellites problem, including that some satellite galaxies are hiding behind larger satellites and that others are so diffuse they are almost impossible to detect with current technology. However, these factors are unlikely to be able to explain an overabundance of stars, leaving scientists with no real way to explain the new results.

"The next step is to use a more powerful telescope that captures a wider view of the sky," study co-author Masahi Chiba, an astronomer at Tohoku University in Japan, said in the statement. This should help clarify how common satellite galaxies really are, he added.

One such telescope is the upcoming Vera C. Rubin Observatory — a state-of-the-art facility equipped with the world's largest digital camera, which is expected to come online in 2025, Chiba said. "I hope that many new satellite galaxies will be discovered."

Harry Baker
Senior Staff Writer

Harry is a U.K.-based senior staff writer at Live Science. He studied marine biology at the University of Exeter before training to become a journalist. He covers a wide range of topics including space exploration, planetary science, space weather, climate change, animal behavior, evolution and paleontology. His feature on the upcoming solar maximum was shortlisted in the "top scoop" category at the National Council for the Training of Journalists (NCTJ) Awards for Excellence in 2023.