The new data could have implications for future astronauts on long interplanetary voyages, and could make scientists rethink which extrasolar planets could support life.

IBEX images the boundary of the solar system by capturing high-speed neutral atoms bouncing off the bubble that stands between our star and all the others. This bubble, called the heliosphere, is inflated by the solar wind, and acts as a protective shield from a downpour of charged particles that would otherwise bombard planets and damage life.

Last year, IBEX’s first map shocked scientists by showing an unexpected tight ribbon of neutral particles circling the solar system. Astronomers are still unsure what caused the ribbon, although they believe it is related to the galactic magnetic field that hugs the solar bubble like a wire mesh cage.

Now, the second map brings further surprises. In just six months, a bright “knot” at the tip of the ribbon appears to have relaxed, or “untied.” This means the force field that guards the solar system from radiation changes on a much shorter timescale than anyone had anticipated.

“We didn’t understand where the ribbon came from in the first place,” McComas said. “It’s even more confounding now, to know the structure can change on such very short timescales.” The results are in the Journal of Geophysical Research.

Compared to the forces acting on the heliosphere, McComas said, six months is an eyeblink. The inside of the bubble, which is shaped by the sun, should change only with the 11-year solar cycle. The forces acting on the outside change on timescales of tens of thousands of years.

The culprit could be a weakening solar wind. Measurements from two other solar observers, ACE and Ulysses, show that the force exerted by the solar wind — and therefore the size of the bubble — has dropped off by a factor of two over the past 18 years.

“The overall inflation, the blowing up of this bubble, has reduced by a factor of two, and the heliosphere surely has shrunk over this time,” McComas said. “Maybe this is part of what’s going on with the changes we’re seeing.”

A smaller bubble makes a worse shield, McComas added. As the heliosphere shrinks, more galactic cosmic rays can make their way into the inner solar system (see video above).

That could prove critical for astronauts spending months in transit to other parts of the solar system, like Mars or a nearby asteroid, he said.

“The fact that this radiation can change on the timescales on which we plan and execute manned missions to other places in the solar system is very important,” he said. “We have to get that right before we use this to rely on this for things as critical as planning manned missions.”

IBEX’s results could also provide insight into what would make an extrasolar planet a good place for life, said stellar physicist Merav Opher of  George Mason University.

“By understanding how astrospheres protect other solar system environments, we will be able to predict in a better way the habitable zones  for extrasolar planets,” she said. “For us it’s not just looking for the right temperature for water. If this planet has been bombarded by a huge dose of radiation, life cannot live there. It’s a key component that needs to be folded in when you are describing habitable zones.”

Video: Goddard Conceptual Image Lab/Walt Feimer

See Also:

• Edge of Solar System Is Not What We Expected
• Top 10 Things Launched Into Space in 2008
• Solar Wind Getting Weaker, Says NASA
• IBEX Launches to Study Solar System Force Field

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Authors: Lisa Grossman

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