By Alan Boyle, Science Editor, NBC News

The search for Planet X may be problematic, but astronomers believe there's a "Galaxy X" lurking on the other side of the Milky Way.

Based on an analysis of our home galaxy's distribution of cold atomic hydrogen gas, two astronomers at the University of California at Berkeley are predicting that a previously undetected dwarf galaxy, about 1 percent the mass of the total Milky Way, should lie about 300,000 light-years out from the center of the Milky Way ... in an area that's obscured by intervening gas and dust.

Such a galaxy hasn't been detected directly — yet — but postdoctoral fellow Sukanya Chakrabarti and Leo Blitz are betting that it's out there nevertheless.

"This is the first time in my profession that I'm really going out on a limb and making a very specific prediction," Chakrabarti told me.

This month, a request is being put in for observing time on the infrared-sensitive Spitzer Space Telescope, as part of its GLIMPSE survey. If the request is approved, Chakrabarti and Blitz should find out sometime this year whether their bet has paid off. The payoff could be big: If Galaxy X is found, its existence could explain a curious discrepancy in the current theoretical model for dark matter distribution in the universe.

And if the bet goes bad? "Even if we're wrong, we'll learn something significant, because it could be due to the shape of the dark matter halo," she said.

Chakrabarti discussed her research, which has been submitted to the Astrophysical Journal, at the American Astronomical Society's winter meeting in Seattle today.

Reading the ripples

Our Milky Way is surrounded by about 80 known or suspected dwarf galaxies, known as satellite galaxies. Some of them are well-known in their own right, such as the Large and Small Magellanic Clouds. The problem is that there don't seem to be enough of them. The best model for dark matter distribution at large scales predicts that, at the scale of the Milky Way, there should be hundreds or perhaps even thousands of dwarf galaxies.

Chakrabarti and Blitz suggest that the "missing" dwarf galaxies may be so small and dim that they're obscured by the dust of the bigger galaxies in their neighborhood. The dwarfs may even be made primarily of invisible dark matter. But even then, they should create disturbances, or ripples, in the much larger pool of hydrogen gas within the disk of a galaxy. The gas is gravitationally bound to the galaxy, but extends much farther out than the galaxy's visible stars — sometimes as much as five times farther out.

That cold hydrogen gas can be mapped by radio telescopes.

"The method is like inferring the size and speed of a ship by looking at its wake," Blitz explained in a news release. "You see the waves from a lot of boats, but you have to be able to separate out the wake of a medium or small ship from that of an ocean liner."

To test the idea, the researchers looked at high-resolution radio data for the Whirlpool Galaxy and another galaxy known as NGC 1512, collected as part of the Very Large Array's THINGS and THINGS-SOUTH surveys. When Chakrabarti fed the data into her mathematical model, the results accurately predicted the locations of two known satellite galaxies. The Whirlpool satellite was a third the mass of the primary galaxy, while the NGC 1512 satellite was one-hundredth the mass of the big galaxy.

Chakrabarti said her model should work with galaxies as small as a thousandth of the primary galaxy's mass.

When she and Blitz checked radio data for the Milky Way, the model pointed to a dwarf satellite galaxy that's around 3 billion to 10 billion solar masses, or roughly one-hundredth the mass of the Milky Way. The galaxy should be on the opposite side of the Milky Way, somewhere in the constellations of Norma or Circinus, just west of the galactic center in Sagittarius when viewed from Earth.

NASA's Spitzer Space Telescope would be well-suited for spotting such a galaxy, because its infrared camera can see objects that are obscured by dust in visible-light wavelengths. While Chakrabarti and Blitz wait to find out whether Spitzer will be able to look for Galaxy X, they're also checking dozens of other galaxies to make sure their mathematical model holds up.

Planet X vs. Galaxy X

Chakrabarti likes to compare her quest to the search for unseen planets in the 19th century. Most famously, the French mathematician Urbain Le Verrier predicted the position of a new planet in 1846 purely by analyzing its gravitational effect on other celestial bodies. When the German astronomer Johann Gottfried Galle looked where Le Verrier predicted, he found the planet Neptune almost immediately.

That success sparked a search for a supposed planet within the orbit of Mercury, called Vulcan. (It turned out, however, that Mercury's perturbations were explained instead as a consequence of Albert Einstein's general relativity theory.) The discovery of Neptune also led Percival Lowell to start searching for a planet even farther out, which he called "Planet X." That quest eventually resulted in the discovery of Pluto in 1930. (You can read all about the quest for Planet X in my book, "The Case for Pluto.")

Some observers suggest that there could be yet another Planet X still out there — perhaps a giant planet on the far edge of the solar system. And some have even claimed that an as-yet-undetected planet is heading our way for a close encounter in (gasp!) 2012. There's no chance of that happening, but NASA's Wide-field Infrared Survey Explorer, or WISE, would be capable of seeing a distant Planet X if it's the size of Jupiter or bigger.

The WISE mission's principal investigator, Edward Wright of the University of California at Los Angeles, told me that such a Planet X would be detected as a brown dwarf candidate, moving across the background of stars. More than 1,000 brown dwarf candidates are being checked right now, and if any of them "are moving a lot, then you'll hear from us," Wright said.

Chakrabarti marveled at the difference between the 19th-century search for Planet X and the 21st-century search for Galaxy X.

"In the 1800s, all Le Verrier had was one solar system," she said, "whereas we can now go and test the statistical viability of this method on a very large sample of galaxies."

More about dark matter and Planet X:

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Alan Boyle, Science Editor, NBC News