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IBEX measures
interstellar matter

KEITH COOPER
ASTRONOMY NOW
Posted: 01 February 2012


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NASA’s Interstellar Boundary Explorer (IBEX) has detected neutral atoms of hydrogen, helium, oxygen and neon breaching the Solar System’s magnetic barrier and reaching Earth. The new results show that the Local Cloud, which is a thin cloud of dust and gas that the Solar System is passing through, has a shortage of oxygen.


A diagram showing the Sun’s passage through the Local Cloud. Alpha Centauri, the nearest star to the Sun, lies on the other side of a second cloud called the G Cloud. Image: IBEX Team/M Paternostro (Adler Planetarium)/P Frisch (University of Chicago)/S Redfield (Wesleyan University).

The heliosphere is a magnetic bubble blown by the Sun’s solar wind that wraps around the Solar System and protects it from the high energy charged particles of interstellar space. The heliosphere collides with the interstellar medium as becomes squashed in the Sun’s direction of travel. The magnetic field deflects the charged particles but neutral atoms can pass clean through. In the 1990s NASA’s Ulysses probe detected neutral helium atoms coming from interstellar space at a velocity 11,200 kilometres per hour faster than the velocity of 83,700 kilometres per hour detected by IBEX.

“This is really alien matter from outside the Solar System, from other parts of the Galaxy,” says IBEX’s principal investigator, David McComas. ”This is the stuff that we are all made of.”


Astrospheres around other stars, including Mira, LL Orionis and BZ Camelopardalis, interacting with the interstellar medium. Image: NASA/ESA/JPL–Caltech/Goddard/SwRI.

It’s also puzzling. IBEX has shown that the Local Cloud, which the Sun is nearing the edge of and will emerge on the other side in a few thousand years, has a lower abundance of oxygen atoms relative to neon atoms compared to the Sun’s composition or even the average levels of oxygen in the Galaxy as a whole. Maybe the Galaxy has regional variations in chemistry, so that the gas cloud that formed the Sun 4.6 billion years ago had a slightly different composition to the Local Cloud. Alternatively, the oxygen may be locked up within dust grains. This is a problem that has faced astronomers before – star and planet forming regions are often found to be depleted in oxygen, where it is believed to be bonded to dust particles.

Understanding how the heliosphere interacts with the wider galactic environment will also help teach us about the analogues of the heliosphere around other stars. These analogues are termed ‘astrospheres’, according to Seth Redfield of Wesleyan University, USA. “A different environment would create a different heliospheric structure. In a dense cloud it will be squashed and in a thin cloud it will expand out. Our heliosphere is now the gold standard for studying these,” he says. Astrospheres may also be important when it comes to extraterrestrial life, protecting habitable planets from harmful interstellar cosmic rays.