Every Planet in the solar system, including the Earth, emits x-rays.
The European Space Agency's XMM-Newton X-Ray Observatory took this recent image of Mars in the x-ray spectrum. Every planet in the solar system, including the Earth, emits x-rays, but scientists aren't completely sure why. One reason is thought to be "florescence emission", when x-rays from the Sun hit atoms in our atmosphere and then get re-emitted with a characteristic signature. The bulk of the x-rays in the picture are coming from oxygen in the Martian atmosphere.
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