Spacecraft probes gas cloud swaddling the solar system

David Shiga, reporter

Like a plane flying through fog, the solar system is pushing its way out of a cloud of interstellar matter enveloping it. It will take a while yet to escape, but new observations show what the cloud is made of ? and it is mysteriously different from the composition of the sun.

Previous observations suggested the sun was in a no man's land between two prominent gas clouds in interstellar space ? the Local Cloud and the G Cloud. The new measurements place the solar system firmly inside the Local Cloud, a bubble of gas thought to have been blown by a group of giant stars long ago.

What is this cloud made of? It's hard to tell, since particles blown out by the sun in the solar wind tend to insulate the solar system from outside influences, but occasionally particles get through these defences and reveal themselves as outsiders by telltale signs such as their direction. NASA's Ulysses spacecraft had previously tasted helium that made its way into the solar system this way.

Now NASA's Interstellar Boundary Explorer, which orbits Earth, has captured heavier alien elements for the first time. It finds that the interstellar matter has a lower abundance of oxygen relative to neon than the sun does.

That could mean that some of the interstellar oxygen is hidden ? perhaps bound up in dust grains. Alternatively, the sun may have formed in a region with a different composition than the Local Cloud, which may have only been enshrouding the solar system for tens of thousands of years.

So when will the sun escape? "Sometime in the next hundred to few thousand years", says the mission's chief scientist, David McComas of the Southwest Research Institute in San Antonio, Texas.

Journal reference: Astrophysical Journal Supplement Series, DOI: 10.1088/0067-0049/198/2/13

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