How Many Galaxies Orbit Our Milky Way?

In outer space, the strong gravity of large bodies exerts an unyielding force over smaller entities. Satellites maintain their paths circling planets. Various celestial bodies like worlds, minor planets, and icy wanderers move around larger suns, with these luminous cores gathering alongside enormous singularities to create vast star systems known as galaxies.

Large galaxies, like the Milky Way , draw in smaller galaxies. The celestial vicinity of our solar system extends across 100,000 light-years and includes approximately 100 billion to 400 billion stars . The Milky Way is so big that, over billions of years, its mass has captured numerous dwarf galaxies, which contain no more than a few billion stars, as satellites.

However, how many satellite galaxies does the Milky Way possess?

The count keeps evolving due to the discovery of fainter galaxies through advanced telescopes and sky surveys. However, let’s begin with those we can readily observe. The two most notable satellite galaxies of the Milky Way are the Large Magellanic Cloud and the Small Magellanic Cloud. These galaxies circle the Milky Way at approximately 160,000 light-years away and are observable from the Southern Hemisphere without the aid of a telescope, as stated. NASA's Goddard Space Flight Center .

Nevertheless, these easily observable satellites are rare occurrences; for the most part, satellite galaxies tend to be tiny and faint, undetectable except through advanced telescope systems. Researchers locate dwarf galaxies primarily by employing equipment capable of scanning extensive areas of the sky comprehensively, noted Or Graur , an associate professor of astrophysics at the University of Portsmouth in the UK.

"As telescopes grow larger and our equipment becomes more advanced, we can delve deeper into detecting dimmer and dimmer dwarf galaxies, including those now referred to as ultra-faint dwarfs," which contain merely a few hundred thousand stars, according to Graur’s statement for Live Science.

Related: What is the number of galaxies in the universe?

Determining whether a neighboring dwarf galaxy is a satellite of the Milky Way requires spectroscopy — examining the light it emits — to ascertain its movement and trajectory, as mentioned. Marla Geha a professor of astronomy and physics at Yale University.

You can determine if the object is gravitationally bound to itself, and whether this group revolves around the Milky Way," Geha explained to Live Science. "A satellite galaxy continuously—and will continue—to orbit the bigger galaxy.

A recently released census from 2020 revealed The Astrophysical Journal , estimates suggest that approximately 60 satellites could be found circling the Milky Way up to a distance of around 1.4 million light-years away. However, determining an exact count for the Milky Way’s satellite galaxies remains challenging, partly due to the fact that some potential satellite galaxies haven’t yet undergone spectroscopic confirmation proving they indeed revolve around our galaxy.

"Most likely, there are between five and eight objects that lack clear spectroscopy or have uncertain spectroscopic data," Geha stated. Additionally, she mentioned that new satellite candidates continue to be identified.

When Geha embarked on her research over twenty years ago, the Milky Way was believed to have just eleven satellite galaxies. However, this number shifted with the commencement of the Sloan Digital Sky Survey in the early 2000s, as noted by Geha. This survey created the initial digital map encompassing more than a third of the celestial sphere, significantly enhancing astronomers’ ability to detect elusive dwarf galaxies thanks to its advanced digital imaging technology. These small galaxies frequently go unnoticed due to their low luminosity being overshadowed by nearer, more brilliant stars.

By employing Sloan’s digital imagery, scientists managed to algorithmically remove foreground stars—a task that was significantly more challenging when using traditional analog photos and photographic plates, according to Geha. This process uncovered dim dwarf galaxies that had been obscured until then.

Geha stated, "Every one of these major new imaging surveys has transformed the field." She added, "It’s technology that's truly behind all these advancements and the increase in the number of known satellites."

From Sloan in the early 2000s through the Dark Energy Survey in the late 2010s, every survey uncovered additional several dozen dwarf galaxies circling our galaxy. Vera C. Rubin Observatory In Chile, experts like Geha anticipate the discovery of hundreds more satellites; however, this could be contingent upon the Milky Way not devouring these galaxies first.

Graur explained that satellite galaxies are gravitationally tied to the Milky Way. The Milky Way continuously exerts gravitational pull on these satellites, gradually drawing them closer over time. As they get drawn in, their structure begins to break down and they become incorporated into the Milky Way itself.

One such casualty was a dwarf galaxy currently referred to as Gaia Enceladus , which was torn apart and consumed by the Milky Way, with its stars now twinkling within the Milky Way’s halo, according to Graur. In time, the satellite galaxies we see today will probably meet the same end, Geha noted.

If we were to wait for an extraordinarily long period, say billions upon billions of years," she explained, "the satellite galaxies would eventually be drawn into the parent galaxy and merge with it, resulting in an even larger central galaxy.