※何謂太陽系 SOLAR SYSTEM, stellar-planetary unit consisting of the sun; the nine planets and their satellites; the asteroids, comets, and meteoroids; and interplanetary dust and gas. The dimensions of this system are specified in terms of the mean distance from the earth to the sun, called the astronomical unit (AU). One AU is about 150 million km (about 93 million mi). The most distant known planet, Pluto, has an orbit at 39.44 AU from the sun. The boundary between the solar system and interstellar space—called the heliopause—is estimated to occur near 100 AU. The comets, however, achieve the greatest distance from the sun; they have highly eccentric orbits ranging out to 50,000 AU or more. This solar system is the only planetary system known to exist, although in the 1980s a number of relatively nearby stars were found to be encircled by swarms of orbiting material of indeterminate size or to be accompanied by suspected brown dwarfs. Many astronomers think it likely that solar systems of some sort are numerous throughout the universe.

※認識太陽系