Probably the most surprising objects observed in X-rays are comets. Comets were
described by Fred Whipple as being "dirty snowballs". So how does a snowball
make X-rays? Through a phenomenon called charge exchange. The Sun is constantly
streaming ions into the Solar system. Also called the Solar wind, the ions are so highly charged that they will steal electrons from cold
gas if possible. A comet becomes active when it gets close to the Sun and
it starts putting cold gas out into space. This is when charge exchange can occur.
This charge exchange takes place every time an ion gets close to a cold gas molecule,
and each time this happens, an X-ray is emitted. This makes comets very bright
in X-rays. Comets are so bright that some scientists think we will be able to
see comets around other stars in X-rays.
Planets have also been detected in X-rays, including the Earth. In addition to charge exchange, which was used to deduce that Mars is
losing its atmosphere to space, the most common mechanism is fluorescence. Venus, Jupiter and Saturn have all been detected in X-rays and the physics surrounding the generation of X-rays on each planet
has proven to be unique
HEA: Scott Wolk
AMP: Alexander Dalgarno, Vasili Kharchenko
Parts of our solar system in X-rays. (Top) The disk of Jupiter reflects X-rays from the Sun, while strong magnetic fields focus ions on the poles. (Middle) The Comet C/1999 S4 (LINEAR). Ions in the solar wind exchange their charge with neutral molecules in the comet. The process makes comets bright X-ray emitters. (Bottom) Venus emits X-rays by fluorescent radiation giving it phases, much as it appears in optical light.