Surprise: Solar System "Force Field" Shrinks Fast

It's cold, dusty, and bereft of planets, but the outskirts of our solar system are anything but dull, according to increasing evidence from NASA's Interstellar Boundary Explorer (IBEX) craft.

As charged particles flow out from the sun, they eventually bump up against interstellar medium—the relatively empty areas between stars. These interactions "inflate" a protective bubble that shields Earth and the entire solar system from potentially harmful cosmic rays (solar system pictures).

Now IBEX has surprised astronomers by showing that this force field-like structure, the heliosphere, is an unexpectedly dynamic, unpredictable boundary.

"If we've learned anything from IBEX so far, it is that the models that we're using for interaction of the solar wind with the galaxy were just dead wrong," David McComas, principal investigator for the IBEX program, said during a NASA press conference Thursday.

Heliosphere Changes Fast

For starters, it's been assumed that the heliosphere's expansion and contraction follows the sun's roughly 11-year activity cycle, during which the flow rate of charged particles, or solar wind, fluctuates.

But when scientists compared IBEX maps of the heliosphere taken just six months apart, the researchers found that it had shrunk to a much greater extent than expected. (See "Sun's Power Hits New Low, May Endanger Earth?")

This quick shrinkage could be a concern for astronauts, said McComas, of the Southwest research Institute in San Antonio, Texas. That's because, as the heliosphere shrinks, it lets in more cosmic radiation, which can compromise the body's immune system.

(Related: "Solar System Is 'Bullet Shaped.'")

Among IBEX's other new surprises: a long, mysterious ribbon of uncharged particles found last year that has apparently lost its brightest, most energetic region since its 2009 discovery.

"What we're seeing is the knot pull apart as it spreads across a region of the ribbon," McComas said in a press statement.

"To this day the science team can't agree on exactly what causes the knot or the ribbon, but by comparing different sky maps, we find the surprising result that the region is changing over relatively short time periods. Now we have to figure out why."

IBEX's new cosmic-ribbon findings published September 29 in the Journal of Geophysical Research.