Astronomers have discovered that the Milky Way may have an up-to-this-point unseen next-door neighbor that is about one percent of our galaxy’s mass—or 10 billion times the mass of our Sun.

A news paper by University of California-Berkeley astronomers Sukanya Chakrabarti and Leo Blitz proposes that perturbations in gas on the edges of our galaxy are being caused by previously undiscovered galaxy about 300,000 light years away that is orbiting the Milky Way in an elongated elliptical path.

The paper has been accepted for publication by Monthly Notices of the Royal Astronomical Society.

New Scientist was the first media outlet to report on Chakrabarti’s and Blitz’s findings. Chakrabarti told the publication’s web site that the newly-discovered satellite galaxy swept through the Milky Way about 300 million years ago, intersecting about 16,000 light years from our galaxy’s center—closer than our Sun, which is roughly 26,000 light years away.

The scientists believe that we can’t see this galaxy because it is not as bright as another, readily visible Milky Way satellite, the Large Magellanic Cloud, even though it may be roughly the same size. Unlike the LMC—which is composed of young stars and the gaseous raw materials that spawned them—the invisible galaxy may contain only old, burned-out stars.

Additionally, Chakrabarti told New Scientist, the simulations suggest that the unseen satellite galaxy orbits ours in the same plane as our galaxy’s disc, so that if it is on the opposite side of the Milky Way from Earth, it may be hidden by thick gas and dust in the galactic plane.

"It’s very likely to be in a region of very high obscuration,” says Chakrabarti.

New Scientist notes that the astronomers’ discovery of the unseen galaxy parallels the manner in which early 19th-century mathematicians John Couch Adams and Urbain Le Verrier deduced the existence of the planet Neptune.

They studied data on perturbations in the movement of Uranus, and used it to calculate that the disturbance was being caused by the gravity of another planet.

Le Verrier sent his predictions to German astronomer Johan Gottfried Galle, who then visually sighted Neptune in 1846.

If Chakrabarti’s and Blitz’s calculations prove to be correct, the unseen galaxy will be the first nearby galaxy discovered through its gravity rather than by its light.

Chakrabarti told New Scientist she hopes to pinpoint the galaxy’s location by studying the distribution of gas on the Milky Way’s edge, so that astronomers will be able to look for it.

Science Channel