Fantastically detailed, visually arresting photographs of Andromeda, a spiral galaxy that lies 2.5 million light-years from Earth, have been available for years. But getting a full panorama of our own Milky Way Galaxy is considerably more difficult. Nestled in the thick of the galaxy, we are unable to see it from the outside. For astronomers trying to map out the Milky Way's structure in detail, the exercise is a bit like trying to figure out what one's own face looks like—without the aid of a mirror.

That means that new discoveries are still possible in our own cosmic backyard, a fact made apparent by a new study that identifies a previously unseen spiral arm of the Milky Way. The newfound structure, some 70,000 light-years away, may be the continuation of a major, previously known spiral arm, part of which is visible much closer to Earth. Thomas Dame and Patrick Thaddeus of the Harvard–Smithsonian Center for Astrophysics announce the finding in a study that is set to appear in The Astrophysical Journal Letters.

Astronomers can map distant galactic structures using telescope dishes operating in the microwave or radio bands that can identify spectral signatures of specific atoms or molecules. Dame and Thaddeus found the new appendage by tracing the well-known Scutum–Centaurus arm to where it ought to extend on the far side of the galaxy. Something tantalizing showed up in telescope surveys that had scanned the galaxy for microwave emissions from hydrogen atoms. But with so much hydrogen in the galaxy, discrete structures can be hard to identify, and false positives abound. "You can pick up all kinds of patterns in the wallpaper" with hydrogen, Dame says.

So he and Thaddeus went looking for carbon monoxide, which is thought to be a reliable tracer of the kind of molecular gas clouds that form stars. Finding molecular clouds strung along the purported arm would verify that it was a genuine piece of the Milky Way's structure and not a mere pattern in the wallpaper. And indeed the researchers did find several molecular clouds along the arm, one of which they mapped out in detail. It is roughly 300 light-years in diameter, with the mass of 50,000 suns. Mapping the entire arm will take years, Dame says.

The location of the arm matches where the Scutum–Centaurus arm would emerge from behind the galactic center. That region that is difficult to observe; for one thing, the galactic center is dense and roiling with activity. It is nearly impossible to see the middle span of Scutum–Centaurus behind the center that would connect the inner portion of the arm to the newfound outer portion. "We've apparently found it much farther out than anyone has ever traced it before," Dame says. "If our proposition is correct, it means that the Scutum–Centaurus arm goes almost all the way around the galaxy."

It makes sense that Scutum–Centaurus would continue outward to where the newfound structure lies; indeed, such an extension of Scutum–Centaurus would preserve the galaxy's general symmetry. The Perseus arm, a sort of mirror image of Scutum–Centaurus whose arc carries it past the sun's neighborhood, wraps around the galaxy in just the way it now appears Scutum–Centaurus does on the opposite side of the galaxy.

So it is not entirely unexpected that such a structure would exist; some artists' renderings have shown Scutum–Centaurus encircling the galaxy in just the way that the new observations suggest. But seeing is believing, and the new arm has not been convincingly spotted before. "I would say that it's been scarcely noticed," Dame says. "A couple other people sort of drew a line through it and didn't even mention it."

Part of the reason the newfound spiral arm was easy to miss is that it does not sit neatly in the plane of the Milky Way. "It's following the warp in the galaxy," Dame says. "The galaxy is sort of—roughly speaking—like a sombrero hat." Such warping is not uncommon on the outer fringes of spiral galaxies. "When the star density gets low, there's less to hold it in place," he says, and the tug of dwarf galaxies or of other perturbers can bend the disk out of shape. The new structure is at the very edge of the galaxy, where stars become scarce, but there may nonetheless be some stars there to complement the gas astronomers have detected. "It's interesting, of course, that there are molecular clouds there," Dame says. "It's hard to prevent molecular clouds from forming stars."