Killer cosmic clouds are large areas in outer space, probably bigger than our entire solar system, that have much higher concentrations of hydrogen than normal. For the past five million years, our planet has been traveling in a relatively empty, typical region of space, with a density of less than one particle (mostly hydrogen) per cubic inch. Killer clouds are found where new stars are being formed and have much higher densities of hydrogen on the order of hundreds of particles per cubic inch.

Some scientists believe that a killer cloud could collapse the solar system’s heliosphere—a bubble of space produced by the solar wind that partially protects our planet (and the other planets and satellites in our solar system) from cosmic rays. Cosmic rays are high-speed particles from outer space that constantly hit the heliosphere, but most are deflected by this shield. This is good because exposure to the powerful radiation from these rays could fry a human being. If the heliosphere around our planet collapsed from the introduction of a cosmic cloud, much higher levels of cosmic radiation would strike Earth, dramatically altering life, though scientists are not sure how much or in what ways.