The upper levels of the Sun's atmosphere are of very low density, but the solar activity heats the gases there to very high temperatures. Here the electrons are stripped from atoms to form ions, and the two types of particles together form a plasma. The gravitational field of the Sun is unable to retain this superhot plasma, and it streams outward into space as the solar wind. Measurements of the properties of the solar wind are routinely carried out by U.S. spacecraft at many different locations within the solar system.
Most of the mass (99.86 percent) of the solar system is concentrated in the Sun, which thus exerts the gravitational force that holds the scattered members of the system together. There is a remarkable degree of orderliness in the motions of the members of the solar system under the influence of the Sun's gravity. With the exception of the comets, some of the asteroids, and Pluto, the motions of the bodies in the solar system are confined to approximately the same plane, called the plane of the ecliptic. There is a striking similarity in the way in which these bodies revolve and rotate. The planets all revolve around the Sun in the same direction, and the Sun rotates in this direction as well. With only two exceptions, Venus and Uranus, the planets also rotate in this common direction. Many of the planets, particularly in the outer solar system, are accompanied by swarms of satellites, and again, with a few exceptions, these also tend to revolve in a plane close to the plane of the ecliptic and with the same sense of motion. All of these tendencies can be summarized by saying that the angular momentum vectors of the bodies in the solar system are for the most part aligned.
