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A gas torus is a toroidal cloud of gas or plasma that encircles a planet. In the Solar System, gas tori tend to be produced by the interaction of a satellite's atmosphere with the magnetic field of a planet. The most famous example of this is the Io plasma torus, which is produced by the ionization of roughly 1 ton per second of oxygen and sulfur from the tenuous atmosphere of Jupiter's volcanic moon Io. Other examples include the largely neutral torus of oxygen and hydrogen produced by Saturn's moon Enceladus, and the proposed (though not observationally supported) torus of nitrogen produced by Saturn's moon Titan.
A fictional gas torus is the setting for Larry Niven's novels The Integral Trees and The Smoke Ring, in which a gas giant in orbit around a neutron star generates a gas torus of sufficient density and free oxygen to support life (including humans).