Shockingly Tiny Galaxy Near Andromeda Is Just One-Millionth the Size of the Milky Way

A group of astronomers has discovered the smallest, dimmest satellite galaxy bordering the Milky Way’s nearest galactic neighbor, the Andromeda Galaxy.

The itsy-bitsy satellite galaxy is named Andromeda XXXV and sits about 3 million light-years from Earth. The discovery of the galaxy gives astronomers a useful comparative tool for studying satellite galaxies on the outskirts of our own galaxy, the Milky Way. The team’s findings were published this week in The Astrophysical Journal Letters.

“These are fully functional galaxies, but they’re about a millionth of the size of the Milky Way,” said Eric Bell, an astronomer at the University of Michigan and senior author of the study, in a university release. “It’s like having a perfectly functional human being that’s the size of a grain of rice.”

Andromeda XXXV is only about 20,000 times more massive than our Sun—very small, even for a satellite galaxy. For comparison, the Milky Way’s mass is about 1.5 trillion solar masses, and the beefiest galaxies can be up to 30 trillion solar masses.

Though it is a full-fledged galaxy, Andromeda XXXV is small enough to be ensnared by the gravitational pull of Andromeda—much like the satellite galaxies of the Milky Way. The researchers observed Andromeda XXXV with the Hubble Space Telescope.

“This type of galaxy was only discoverable around one system, the Milky Way, in the past,” Bell said. “Now we’re able to look at one around Andromeda and it’s the first time we’ve done that outside our system.”

The Hubble observations revealed that not only was Andromeda XXXV a satellite galaxy, but it is small enough to raise questions about how such satellites even form stars.

“Most of the Milky Way satellites have very ancient star populations. They stopped forming stars about 10 billion years ago,” said Marco Arias, the lead author of the study, in the same release. “What we’re seeing is that similar satellites in Andromeda can form stars up to a few billion years ago—around 6 billion years.”

The finding is usable in differentiating satellite galaxy formation and star formation in the Milky Way from the conditions in other galaxies. There are anywhere between 100 billion to 2 trillion galaxies in the observable universe, but such small, faint galaxies are hard to see—which is why you’re hearing about the Hubble observations of Andromeda XXXV.

There are still outstanding questions about the little galaxy—including how it survived the universe heating up nearly 13 billion years ago. The “entire universe turned into a vat of boiling oil,” Bell said, and Andromeda XXXV is so small it could’ve lost all of its gas. But for several billion years thereafter, the galaxy continued to form stars.

More observations could clarify the nature of this industrious, persevering satellite—and by proxy, could shed light on the satellite galaxies swirling around the periphery of our own cosmic neighborhood.