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Meet the Milky Way鈥檚 elderly neighbours. It turns out that the faint galaxies orbiting our own are among the oldest in our universe 鈥� a discovery hailed as the equivalent to finding the remains of the first humans that inhabited Earth.

These satellite galaxies, including Ursa Major and Bootes I, are thought to be more than 13 billion years old.聽 The closest is probably Segue-1, some 75,000 light-years away, says Sownak Bose of the Harvard-Smithsonian Centre for Astrophysics.

Most attempts to find older galaxies have looked very deep into the universe, which makes sense. Since light takes a long time to travel to us from the far reaches of the cosmos, this is the equivalent to looking back in time. Now it turns out they are right on our doorstep.

Old galaxies

鈥淭he nice thing about that is that, because they are relatively near, we can actually see them,鈥� says Bose. 鈥淭heoretical models predict that very very old galaxies should be sprinkled throughout the cosmos.鈥�

The discovery was made as part of research into how galaxies grew from very small ones to the large ones we see today. These old galaxies formed in the 鈥渃osmic dark ages鈥�, a period about 380,000 years after the big bang in which the early, hot universe had cooled down and become transparent for the first time.

As part of this, the researchers studied satellite galaxies orbiting the Milky Way. They found two different populations: very faint ones which they think formed are among the first that formed, and brighter ones which formed later. The Milky Way itself also formed later, because the smaller, earlier galaxies inhibited the growth of our larger galaxy.

The observation of these very faint galaxies has only been possible in the past decade, thanks to increasingly sophisticated sky maps from the Sloan Digital Sky Survey and the Dark Energy Survey, which have so far detected 54 satellites around the Milky Way.

However, both surveys only cover a fraction of total area of the sky. 鈥淭here could be between 100-150 satellites in total surrounding the Milky Way鈥攖he vast majority of which is comprised of this very old population of galaxies,鈥� says Bose.